Ruby

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Ruby

Ruby
A ruby crystal from Dodoma Region, Tanzania

Main ruby producing countries

Ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum (aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called sapphires. Ruby is one of the traditional cardinal gems, alongside amethyst, sapphire, emerald, and diamond. The word ruby comes from ruber, Latin for red. The color of a ruby is due to the element chromium.

Some gemstones that are popularly or historically called rubies, such as the Black Prince's Ruby in the British Imperial State Crown, are actually spinels. These were once known as "Balas rubies".

The quality of a ruby is determined by its color, cut, and clarity, which, along with carat weight, affect its value. The brightest and most valuable shade of red, called blood-red or pigeon blood, commands a large premium over other rubies of similar quality. After color follows clarity: similar to diamonds, a clear stone will command a premium, but a ruby without any needle-like rutile inclusions may indicate that the stone has been treated. Ruby is the traditional birthstone for July and is usually pinker than garnet, although some rhodolite garnets have a similar pinkish hue to most rubies. The world's most valuable ruby to be sold at auction is the Sunrise Ruby, which sold for US$34.8 million.

Physical properties

Crystal structure of rubies

Rubies have a hardness of 9.0 on the Mohs scale of mineral hardness. Among the natural gems, only moissanite and diamond are harder, with diamond having a Mohs hardness of 10.0 and moissanite falling somewhere in between corundum (ruby) and diamond in hardness. Sapphire, ruby, and pure corundum are α-alumina, the most stable form of Al₂O₃, in which 3 electrons leave each aluminium ion to join the regular octahedral group of six nearby O²⁻ ions; in pure corundum this leaves all of the aluminium ions with a very stable configuration of no unpaired electrons or unfilled energy levels, and the crystal is perfectly colorless, and transparent except for flaws.

Crystal structure of ruby showing the substitution of Al³⁺ ions (blue) with Cr³⁺ (red). The substitution density of Cr³⁺ ions in this model is approximately 2%, approximating the maximum doping normally encountered.

When a chromium atom replaces an occasional aluminium atom, it too loses 3 electrons to become a chromium³⁺ ion to maintain the charge balance of the Al₂O₃ crystal. However, the Cr³⁺ ions are larger and have electron orbitals in different directions than aluminium. The octahedral arrangement of the O²⁻ ions is distorted, and the energy levels of the different orbitals of those Cr³⁺ ions are slightly altered because of the directions to the O²⁻ ions. Those energy differences correspond to absorption in the ultraviolet, violet, and yellow-green regions of the spectrum.

Transmittance of ruby in optical and near-IR spectra. Note the two broad violet and yellow-green absorption bands and one narrow absorption band at the wavelength of 694 nm, which is the wavelength of the ruby laser.

If one percent of the aluminium ions are replaced by chromium in ruby, the yellow-green absorption results in a red color for the gem. Additionally, absorption at any of the above wavelengths stimulates fluorescent emission of 694-nanometer-wavelength red light, which adds to its red color and perceived luster. The chromium concentration in artificial rubies can be adjusted (in the crystal growth process) to be ten to twenty times less than in the natural gemstones. Theodore Maiman says that "because of the low chromium level in these crystals they display a lighter red color than gemstone ruby and are referred to as pink ruby."

After absorbing short-wavelength light, there is a short interval of time when the crystal lattice of ruby is in an excited state before fluorescence occurs. If 694-nanometer photons pass through the crystal during that time, they can stimulate more fluorescent photons to be emitted in-phase with them, thus strengthening the intensity of that red light. By arranging mirrors or other means to pass emitted light repeatedly through the crystal, a ruby laser in this way produces a very high intensity of coherent red light.

All natural rubies have imperfections in them, including color impurities and inclusions of rutile needles known as "silk". Gemologists use these needle inclusions found in natural rubies to distinguish them from synthetics, simulants, or substitutes. Usually, the rough stone is heated before cutting. These days, almost all rubies are treated in some form, with heat treatment being the most common practice. Untreated rubies of high quality command a large premium.

Some rubies show a three-point or six-point asterism or "star". These rubies are cut into cabochons to display the effect properly. Asterisms are best visible with a single-light source and move across the stone as the light moves or the stone is rotated.

Maharlika Star Ruby 10,820 Carats. Largest Ruby with Star Asterism

Versus pink sapphire

Generally, gemstone-quality corundum in all shades of red, including pink, are called rubies. However, in the United States, a minimum color saturation must be met to be called a ruby; otherwise, the stone will be called a pink sapphire. Drawing a distinction between rubies and pink sapphires is relatively new, having arisen sometime in the 20th century. Often, the distinction between ruby and pink sapphire is not clear and can be debated. As a result of the difficulty and subjectiveness of such distinctions, trade organizations such as the International Colored Gemstone Association (ICGA) have adopted the broader definition for ruby which encompasses its lighter shades, including pink.

Occurrence and mining

Historically, rubies have been mined in Thailand, in the Pailin and Samlout District of Cambodia, as well as in Afghanistan, Australia, Brazil, Colombia, India, Namibia, Japan, and Scotland. After the Second World War, ruby deposits were found in Madagascar, Mozambique, Nepal, Pakistan, Tajikistan, Tanzania, and Vietnam.

The Republic of North Macedonia is the only country in mainland Europe to have naturally occurring rubies. They can mainly be found around the city of Prilep. Macedonian rubies have a unique raspberry color.

A few rubies have been found in the U.S. states of Montana, North Carolina, South Carolina and Wyoming.

Spinel, another red gemstone, is sometimes found along with rubies in the same gem gravel or marble. Red spinels may be mistaken for rubies by those lacking experience with gems. However, the finest red spinels, now heavily sought, can have values approaching all but the finest examples of ruby. The Mogok Valley in Upper Myanmar (Burma) was for centuries the world's main source for rubies. That region has produced some exceptional rubies; however, in recent years few good rubies have been found. In central Myanmar, the area of Mong Hsu began producing rubies during the 1990s and rapidly became the world's main ruby mining area. The most recently found ruby deposit in Myanmar is in Namya (Namyazeik) located in the northern state of Kachin.

In Pakistani Kashmir there are vast proven reserves of millions of rubies, worth up to half a billion dollars. However, as of 2017 there was only one mine (at Chitta Katha) due to lack of investment. In Afghanistan, rubies are mined at Jegdalek. In 2017 the Aappaluttoq mine in Greenland began running.

The rubies in Greenland are said to be among the oldest in the world at approximately 3 billion years old. The Aappaluttoq mine in Greenland is located 160 kilometers south of Nuuk, the capital of Greenland. The rubies are traceable from mine to market.

The Montepuez ruby mine in northeastern Mozambique is situated on one of the most significant ruby deposits in the world, although, rubies were only discovered here for the first time in 2009. In less than a decade, Mozambique has become the world's most productive source for gem-quality ruby.

Factors affecting value

Rubies, as with other gemstones, are graded using criteria known as the four Cs, namely color, cut, clarity and carat weight. Rubies are also evaluated on the basis of their geographic origin.

Color

In the evaluation of colored gemstones, color is the most important factor. Color divides into three components: hue, saturation and tone. Hue refers to color as we normally use the term. Transparent gemstones occur in the pure spectral hues of red, orange, yellow, green, blue, violet. In nature, there are rarely pure hues, so when speaking of the hue of a gemstone, we speak of primary and secondary and sometimes tertiary hues. Ruby is defined to be red. All other hues of the gem species corundum are called sapphire. Ruby may exhibit a range of secondary hues, including orange, purple, violet, and pink.

• 
  A naturally occurring ruby crystal
   
• 
  Natural ruby with inclusions
   
• 
  A cut pink ruby
   
• 
  Purple rubies

Clarity

Because rubies host many inclusions, their clarity is evaluated by the inclusions’ size, number, location, and visibility. Rubies with the highest clarity grades are known as “eye-clean,” because their inclusions are the least visible to the naked human eye. Rubies may also have thin, intersecting inclusions called silk. Silk can scatter light, brightening the gem's appearance, and the presence of silk can also show whether a ruby has been previously heat treated, since intense heat will degrade a ruby's silk.

Treatments and enhancements

Improving the quality of gemstones by treating them is common practice. Some treatments are used in almost all cases and are therefore considered acceptable. During the late 1990s, a large supply of low-cost materials caused a sudden surge in supply of heat-treated rubies, leading to a downward pressure on ruby prices.

Improvements used include color alteration, improving transparency by dissolving rutile inclusions, healing of fractures (cracks) or even completely filling them.

The most common treatment is the application of heat. Most rubies at the lower end of the market are heat treated to improve color, remove purple tinge, blue patches, and silk. These heat treatments typically occur around temperatures of 1800 °C (3300 °F). Some rubies undergo a process of low tube heat, when the stone is heated over charcoal of a temperature of about 1300 °C (2400 °F) for 20 to 30 minutes. The silk is partially broken, and the color is improved.

Another treatment, which has become more frequent in recent years, is lead glass filling. Filling the fractures inside the ruby with lead glass (or a similar material) dramatically improves the transparency of the stone, making previously unsuitable rubies fit for applications in jewelry. The process is done in four steps:

1. The rough stones are pre-polished to eradicate all surface impurities that may affect the process
2. The rough is cleaned with hydrogen fluoride
3. The first heating process during which no fillers are added. The heating process eradicates impurities inside the fractures. Although this can be done at temperatures up to 1400 °C (2500 °F) it most likely occurs at a temperature of around 900 °C (1600 °F) since the rutile silk is still intact.
4. The second heating process in an electrical oven with different chemical additives. Different solutions and mixes have shown to be successful; however, mostly lead-containing glass-powder is used at present. The ruby is dipped into oils, then covered with powder, embedded on a tile and placed in the oven where it is heated at around 900 °C (1600 °F) for one hour in an oxidizing atmosphere. The orange colored powder transforms upon heating into a transparent to yellow-colored paste, which fills all fractures. After cooling the color of the paste is fully transparent and dramatically improves the overall transparency of the ruby.

If a color needs to be added, the glass powder can be "enhanced" with copper or other metal oxides as well as elements such as sodium, calcium, potassium etc.

The second heating process can be repeated three to four times, even applying different mixtures. When jewelry containing rubies is heated (for repairs) it should not be coated with boracic acid or any other substance, as this can etch the surface; it does not have to be "protected" like a diamond.

The treatment can be identified by noting bubbles in cavities and fractures using a 10× loupe.

Synthesis and imitation

Artificial ruby under a normal light (top) and under a green laser light (bottom). Red light is emitted.

In 1837, Gaudin made the first synthetic rubies by fusing potash alum at a high temperature with a little chromium as a pigment. In 1847, Ebelmen made white sapphire by fusing alumina in boric acid. In 1877, Edmond Frémy and industrial glass-maker Charles Feil made crystal corundum from which small stones could be cut. In 1887, Fremy and Auguste Verneuil manufactured artificial ruby by fusing BaF₂ and Al₂O₃ with a little chromium at red heat.

In 1903, Verneuil announced he could produce synthetic rubies on a commercial scale using this flame fusion process, later also known as the Verneuil process. By 1910, Verneuil's laboratory had expanded into a 30 furnace production facility, with annual gemstone production having reached 1,000 kilograms (2,000 lb) in 1907.

Other processes in which synthetic rubies can be produced are through Czochralski's pulling process, flux process, and the hydrothermal process. Most synthetic rubies originate from flame fusion, due to the low costs involved. Synthetic rubies may have no imperfections visible to the naked eye but magnification may reveal curved striae and gas bubbles. The fewer the number and the less obvious the imperfections, the more valuable the ruby is; unless there are no imperfections (i.e., a perfect ruby), in which case it will be suspected of being artificial. Dopants are added to some manufactured rubies so they can be identified as synthetic, but most need gemological testing to determine their origin.

Synthetic rubies have technological uses as well as gemological ones. Rods of synthetic ruby are used to make ruby lasers and masers. The first working laser was made by Theodore H. Maiman in 1960. Maiman used a solid-state light-pumped synthetic ruby to produce red laser light at a wavelength of 694 nanometers (nm). Ruby lasers are still in use.

Rubies are also used in applications where high hardness is required such as at wear-exposed locations in mechanical clockworks, or as scanning probe tips in a coordinate measuring machine.

Imitation rubies are also marketed. Red spinels, red garnets, and colored glass have been falsely claimed to be rubies. Imitations go back to Roman times and already in the 17th century techniques were developed to color foil red—by burning scarlet wool in the bottom part of the furnace—which was then placed under the imitation stone. Trade terms such as balas ruby for red spinel and rubellite for red tourmaline can mislead unsuspecting buyers. Such terms are therefore discouraged from use by many gemological associations such as the Laboratory Manual Harmonisation Committee (LMHC).

Records and famous examples

Rubies at the National Museum of Natural History, Washington, D.C., USA

• The Smithsonian's National Museum of Natural History in Washington, D.C. has some of the world's largest and finest ruby gemstones. The 23.1-carat (4.62 g) Burmese ruby, set in a platinum ring with diamonds, was donated by businessman and philanthropist Peter Buck in memory of his late wife Carmen Lúcia. This gemstone displays a richly saturated red color combined with an exceptional transparency. The finely proportioned cut provides vivid red reflections. The stone was mined from the Mogok region of Burma (now Myanmar) in the 1930s.
• In 2007, the London jeweler Garrard & Co featured a heart-shaped 40.63-carat ruby on their website.
• On 13/14 December 2011, Elizabeth Taylor's complete jewelry collection was auctioned by Christie's. Several ruby-set pieces were included in the sale, notably a ring set with an 8.24 ct gem that broke the 'price-per-carat' record for rubies (US$512,925 per carat – i.e., over US$4.2 million in total), and a necklace that sold for over US$3.7 million.
• The Liberty Bell Ruby is the largest mined ruby in the world. It was stolen in a heist in 2011.
• The Sunrise Ruby is the world's most expensive ruby, most expensive colored gemstone, and most expensive gemstone other than a diamond. In May 2015, it sold at auction in Switzerland to an anonymous buyer for US$30 million.
• A synthetic ruby crystal became the gain medium in the world's first optical laser, conceived, designed and constructed by Theodore H. "Ted" Maiman, on 16 May 1961 at Hughes Research Laboratories

The concept of electromagnetic radiation amplification through the mechanism of stimulated emission had already been successfully demonstrated in the laboratory by way of the maser, using other materials such as ammonia and, later, ruby, but the ruby laser was the first device to work at optical (694.3 nm) wavelengths. Maiman's prototype laser is still in working order.

The Ruby Eye Amulet from Mesopotamia, Adilnor Collection, Sweden.

Historical and cultural references

• The Old Testament of the Bible mentions ruby many times in the Book of Exodus, and many times in the Book of Proverbs, as well as various other times. It is not certain that the Biblical words mean 'ruby' as distinct from other jewels.
• An early recorded transport and trading of rubies arises in the literature on the North Silk Road of China, wherein about 200 BC rubies were carried along this ancient trackway moving westward from China.
• Rubies have always been held in high esteem in Asian countries. They were used to ornament armor, scabbards, and harnesses of noblemen in India and China. Rubies were laid beneath the foundation of buildings to secure good fortune to the structure.
• A traditional Hindu astrological belief holds rubies as the "gemstone of the Sun and also the heavenly deity Surya, the leader of the nine heavenly bodies (Navagraha)." The belief is that worshiping and wearing rubies causes the Sun to be favorable to the wearer.
• In the Marvel comic books, the Godstone is a ruby that the son of J. Jonah Jameson, John Jameson found on the Moon that becomes activated by moonlight, grafts itself to his chest which turns him into the Man-Wolf.

Corundum

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Corundum

 

Corundum
General
Category	Oxide mineral – Hematite group
Formula (repeating unit)	Al2O3
IMA symbol	Crn
Strunz classification	4.CB.05
Dana classification	4.3.1.1
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol: (3 2/m)
Space group	R3c (No. 167)
Unit cell	a = 4.75 Å, c = 12.982 Å; Z = 6
Identification
Color	Colorless, gray, golden-brown, brown; purple, pink to red, orange, yellow, green, blue, violet; may be color zoned, asteriated mainly grey and brown
Crystal habit	Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular
Twinning	Polysynthetic twinning common
Cleavage	None – parting in 3 directions
Fracture	Conchoidal to uneven
Tenacity	Brittle
Mohs scale hardness	9 (defining mineral)
Luster	Adamantine to vitreous
Streak	Colorless
Diaphaneity	Transparent, translucent to opaque
Specific gravity	3.95–4.10
Optical properties	Uniaxial (−)
Refractive index	nω = 1.767–1.772 nε = 1.759–1.763
Pleochroism	None
Melting point	2,044 °C (3,711 °F)
Fusibility	Infusible
Solubility	Insoluble
Alters to	May alter to mica on surfaces causing a decrease in hardness
Other characteristics	May fluoresce or phosphoresce under UV light
Major varieties
Sapphire	Any color except red
Ruby	Red
Emery	Black granular corundum intimately mixed with magnetite, hematite, or hercynite

Corundum is a crystalline form of aluminium oxide (Al₂O₃) typically containing traces of iron, titanium, vanadium, and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure. Corundum has two primary gem varieties: ruby and sapphire. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present. A rare type of sapphire, padparadscha sapphire, is pink-orange.

The name "corundum" is derived from the Tamil-Dravidian word kurundam (ruby-sapphire) (appearing in Sanskrit as kuruvinda).

Because of corundum's hardness (pure corundum is defined to have 9.0 on the Mohs scale), it can scratch almost all other minerals. It is commonly used as an abrasive on sandpaper and on large tools used in machining metals, plastics, and wood. Emery, a variety of corundum with no value as a gemstone, is commonly used as an abrasive. It is a black granular form of corundum, in which the mineral is intimately mixed with magnetite, hematite, or hercynite.

In addition to its hardness, corundum has a density of 4.02 g/cm³ (251 lb/cu ft), which is unusually high for a transparent mineral composed of the low-atomic mass elements aluminium and oxygen.

Geology and occurrence

Corundum from Brazil, size about 2 cm × 3 cm (0.8 in × 1 in)

Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low-silica igneous syenite and nepheline syenite intrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites. It commonly occurs as a detrital mineral in stream and beach sands because of its hardness and resistance to weathering. The largest documented single crystal of corundum measured about 65 cm × 40 cm × 40 cm (26 in × 16 in × 16 in), and weighed 152 kg (335 lb). The record has since been surpassed by certain synthetic boules.

Corundum for abrasives is mined in Zimbabwe, Pakistan, Afghanistan, Russia, Sri Lanka, and India. Historically it was mined from deposits associated with dunites in North Carolina, US, and from a nepheline syenite in Craigmont, Ontario. Emery-grade corundum is found on the Greek island of Naxos and near Peekskill, New York, US. Abrasive corundum is synthetically manufactured from bauxite.

Four corundum axes dating to 2500 BC from the Liangzhu culture and Sanxingcun culture (the latter of which is located in Jintan District) have been discovered in China.

Synthetic corundum

• In 1837, Marc Antoine Gaudin made the first synthetic rubies by reacting alumina at a high temperature with a small amount of chromium as a colourant.
• In 1847, J. J. Ebelmen made white synthetic sapphires by reacting alumina in boric acid.
• In 1877, Frenic and Freil made crystal corundum from which small stones could be cut. Frimy and Auguste Verneuil manufactured artificial ruby by fusing BaF₂ and Al₂O₃ with a little chromium at temperatures above 2,000 °C (3,630 °F).
• In 1903, Verneuil announced that he could produce synthetic rubies on a commercial scale using this flame fusion process.

The Verneuil process allows the production of flawless single-crystal sapphire and ruby gems of much larger size than normally found in nature. It is also possible to grow gem-quality synthetic corundum by flux-growth and hydrothermal synthesis. Because of the simplicity of the methods involved in corundum synthesis, large quantities of these crystals have become available on the market at a fraction of the cost of natural stones.

Apart from ornamental uses, synthetic corundum is also used to produce mechanical parts (tubes, rods, bearings, and other machined parts), scratch-resistant optics, scratch-resistant watch crystals, instrument windows for satellites and spacecraft (because of its transparency in the ultraviolet to infrared range), and laser components. For example, the KAGRA gravitational wave detector's main mirrors are 23 kg (50 lb) sapphires, and Advanced LIGO considered 40 kg (88 lb) sapphire mirrors. Corundum has also found use in the development of ceramic armour thanks to its high hardiness.

Structure and physical properties

Crystal structure of corundum

Molar volume vs. pressure at room temperature

Corundum crystallizes with trigonal symmetry in the space group R3c and has the lattice parameters a = 4.75 Å and c = 12.982 Å at standard conditions. The unit cell contains six formula units.

The toughness of corundum is sensitive to surface roughness and crystallographic orientation. It may be 6–7 MPa·m^1/2 for synthetic crystals, and around 4 MPa·m^1/2 for natural.

In the lattice of corundum, the oxygen atoms form a slightly distorted hexagonal close packing, in which two-thirds of the octahedral sites between the oxygen ions are occupied by aluminium ions. The absence of aluminium ions from one of the three sites breaks the symmetry of the hexagonal close packing, reducing the space group symmetry to R3c and the crystal class to trigonal. The structure of corundum is sometimes described as a pseudohexagonal structure.

The Young’s modulus of corundum (sapphire) has been reported by many different sources with values varying between 300-500 GPa, but a commonly cited value used for calculations is 345 GPa. The Young’s modulus is temperature dependent, and has been reported in the [0001] direction as 435 GPa at 323 K and 386 GPa at 1,273 K. The shear modulus of corundum is 145 GPa, and the bulk modulus is 240 GPa.

Single crystal corundum fibers have potential applications in high temperature composites, and the Young’s modulus is highly dependent on the crystallographic orientation along the fiber axis. The fiber exhibits a max modulus of 461 GPa when the crystallographic c-axis [0001] is aligned with the fiber axis, and minimum moduli ~373 GPa when a direction 45° away from the c-axis is aligned with the fiber axis.

The hardness of corundum measured by indentation at low loads of 1-2 N has been reported as 22-23 GPa in major crystallographic planes: (0001) (basal plane), (1010) (rhombohedral plane), (1120) (prismatic plane), and (1012). The hardness can drop significantly under high indentation loads. The drop with respect to load varies with the crystallographic plane due to the difference in crack resistance and propagation between directions. One extreme case is seen in the (0001) plane, where the hardness under high load (~1kN) is nearly half the value under low load (1-2 N).

Polycrystalline corundum formed through sintering and treated with a hot isostatic press process can achieve grain sizes in the range of 0.55-0.7 μm, and has been measured to have four-point bending strength between 600-700 MPa and three-point bending strength between 750-900 Mpa.

Generalization

Main article: Corundum (structure)

 

Because of its prevalence, corundum has also become the name of a major structure type (corundum type) found in various binary and ternary compounds.

Chicxulub crater

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Chicxulub_crater

 

Chicxulub crater
Chicxulub impact structure
Imaging from NASA's Shuttle Radar Topography Mission STS-99 reveals part of the diameter ring of the crater in the form of a shallow circular trough. Numerous cenotes (sinkholes) cluster around the trough marking the inner crater rim.

The Chicxulub crater (IPA: [t͡ʃikʃuˈluɓ] ^ⓘ cheek-shoo-LOOB) is an impact crater buried underneath the Yucatán Peninsula in Mexico. Its center is offshore, but the crater is named after the onshore community of Chicxulub Pueblo (not the larger coastal town of Chicxulub Puerto). It was formed slightly over 66 million years ago when an asteroid, about ten kilometers (six miles) in diameter, struck Earth. The crater is estimated to be 200 kilometers (120 miles) in diameter and 20 kilometers (12 miles) in depth. It is believed to be the second largest impact structure on Earth, and the only one whose peak ring is intact and directly accessible for scientific research.

The crater was discovered by Antonio Camargo and Glen Penfield, geophysicists who had been looking for petroleum in the Yucatán Peninsula during the late 1970s. Penfield was initially unable to obtain evidence that the geological feature was a crater and gave up his search. Later, through contact with Alan R. Hildebrand in 1990, Penfield obtained samples that suggested it was an impact feature. Evidence for the crater's impact origin includes shocked quartz, a gravity anomaly, and tektites in surrounding areas.

The date of the impact coincides with the Cretaceous–Paleogene boundary (commonly known as the K–Pg or K–T boundary). It is now widely accepted that the devastation and climate disruption resulting from the impact was the primary cause of the Cretaceous–Paleogene extinction event, a mass extinction of 75% of plant and animal species on Earth, including all non-avian dinosaurs.

Discovery

In the late 1970s, geologist Walter Alvarez and his father, Nobel Prize-winning scientist Luis Walter Alvarez, put forth their theory that the Cretaceous–Paleogene extinction was caused by an impact event. The main evidence of such an impact was contained in a thin layer of clay present in the Cretaceous–Paleogene boundary (K–Pg boundary) in Gubbio, Italy. The Alvarezes and colleagues reported that it contained an abnormally high concentration of iridium, a chemical element rare on Earth but common in asteroids. Iridium levels in this layer were as much as 160 times above the background level. It was hypothesized that the iridium was spread into the atmosphere when the impactor was vaporized and settled across Earth's surface among other material thrown up by the impact, producing the layer of iridium-enriched clay. At the time, there was no consensus on what caused the Cretaceous–Paleogene extinction and the boundary layer, with theories including a nearby supernova, climate change, or a geomagnetic reversal. The Alvarezes' impact hypothesis was rejected by many paleontologists, who believed that the lack of fossils found close to the K–Pg boundary—the "three-meter problem"—suggested a more gradual die-off of fossil species.

The Alvarezes, joined by Frank Asaro and Helen Michel from University of California, Berkeley, published their paper on the iridium anomaly in Science in June 1980. Almost simultaneously Jan Smit and Jan Hertogen published their iridium findings from Caravaca, Spain in Nature in May 1980. These papers were followed by other reports of similar iridium spikes at the K–Pg boundary across the globe, and sparked wide interest in the cause of the K–Pg extinction; over 2,000 papers were published in the 1980s on the topic. There were no known impact craters that were the right age and size, spurring a search for a suitable candidate. Recognizing the scope of the work, Lee Hunt and Lee Silver organized a cross-discipline meeting in Snowbird, Utah, in 1981. Unknown to them, evidence of the crater they were looking for was being presented the same week, and would be largely missed by the scientific community.

Artist's impression of the asteroid slamming into tropical, shallow seas of the sulfur-rich Yucatán Peninsula in what is today Southeast Mexico. The aftermath of the asteroid collision, which occurred approximately 66 million years ago, is believed to have caused the mass extinction of non-avian dinosaurs and many other species on Earth. The impact spewed hundreds of billions of tons of sulfur into the atmosphere, producing a worldwide blackout and freezing temperatures which persisted for at least a decade.

In 1978, geophysicists Glen Penfield and Antonio Camargo were working for the Mexican state-owned oil company Petróleos Mexicanos (Pemex) as part of an airborne magnetic survey of the Gulf of Mexico north of the Yucatán Peninsula. Penfield's job was to use geophysical data to scout possible locations for oil drilling. In the offshore magnetic data, Penfield noted anomalies whose depth he estimated and mapped. He then obtained onshore gravity data from the 1940s. When the gravity maps and magnetic anomalies were compared, Penfield described a shallow "bullseye", 180 km (110 mi) in diameter, appearing on the otherwise non-magnetic and uniform surroundings—clear evidence to him of an impact feature. A decade earlier, the same map had suggested a crater to contractor Robert Baltosser, but Pemex corporate policy prevented him from publicizing his conclusion.

Penfield presented his findings to Pemex, who rejected the crater theory, instead deferring to findings that ascribed the feature to volcanic activity. Pemex disallowed release of specific data, but let Penfield and Camargo present the results at the 1981 Society of Exploration Geophysicists conference. That year's conference was under-attended and their report attracted scant attention, with many experts on impact craters and the K–Pg boundary attending the Snowbird conference instead. Carlos Byars, a Houston Chronicle journalist who was familiar with Penfield and had seen the gravitational and magnetic data himself, wrote a story on Penfield and Camargo's claim, but the news did not disseminate widely.

Although Penfield had plenty of geophysical data sets, he had no rock cores or other physical evidence of an impact. He knew Pemex had drilled exploratory wells in the region. In 1951, one bored into what was described as a thick layer of andesite about 1.3 kilometers (4,300 ft) down. This layer could have resulted from the intense heat and pressure of an Earth impact, but at the time of the borings it was dismissed as a lava dome—a feature uncharacteristic of the region's geology. Penfield was encouraged by William C. Phinney, curator of the lunar rocks at the Johnson Space Center, to find these samples to support his hypothesis. Penfield tried to secure site samples, but was told they had been lost or destroyed. When attempts to return to the drill sites to look for corroborating rocks proved fruitless, Penfield abandoned his search, published his findings and returned to his Pemex work. Seeing the 1980 Science paper, Penfield wrote to Walter Alvarez about the Yucatán structure, but received no response.

Alvarez and other scientists continued their search for the crater, although they were searching in oceans based on incorrect analysis of glassy spherules from the K–Pg boundary that suggested the impactor had landed in open water. Unaware of Penfield's discovery, University of Arizona graduate student Alan R. Hildebrand and faculty adviser William V. Boynton looked for a crater near the Brazos River in Texas. Their evidence included greenish-brown clay with surplus iridium, containing shocked quartz grains and small weathered glass beads that looked to be tektites. Thick, jumbled deposits of coarse rock fragments were also present, thought to have been scoured from one place and deposited elsewhere by an impact event. Such deposits occur in many locations but seemed concentrated in the Caribbean Basin at the K–Pg boundary. When Haitian professor Florentine Morás discovered what he thought to be evidence of an ancient volcano on Haiti, Hildebrand suggested it could be a telltale feature of a nearby impact. Tests on samples retrieved from the K–Pg boundary revealed more tektite glass, formed only in the heat of asteroid impacts and high-yield nuclear detonations.

In 1990, Carlos Byars told Hildebrand of Penfield's earlier discovery of a possible impact crater. Hildebrand contacted Penfield and the pair soon secured two drill samples from the Pemex wells, which had been stored in New Orleans for decades. Hildebrand's team tested the samples, which clearly showed shock-metamorphic materials. A team of California researchers surveying satellite images found a cenote (sinkhole) ring centered on the town of Chicxulub Pueblo that matched the one Penfield saw earlier; the cenotes were thought to be caused by subsidence of bolide-weakened lithostratigraphy around the impact crater wall. More recent evidence suggests the crater is 300 km (190 mi) wide, and the 180 km (110 mi) ring is an inner wall of it. Hildebrand, Penfield, Boynton, Camargo, and others published their paper identifying the crater in 1991. The crater was named for the nearby town of Chicxulub. Penfield also recalled that part of the motivation for the name was "to give the academics and NASA naysayers a challenging time pronouncing it" after years of dismissing its existence.

In March 2010, forty-one experts from many countries reviewed the available evidence: twenty years' worth of data spanning a variety of fields. They concluded that the impact at Chicxulub triggered the mass extinctions at the K–Pg boundary. Dissenters, notably Gerta Keller of Princeton University, have proposed an alternate culprit: the eruption of the Deccan Traps in what is now the Indian subcontinent. This period of intense volcanism occurred before and after the Chicxulub impact; dissenting studies argue that the worst of the volcanic activity occurred before the impact, and the role of the Deccan Traps was instead shaping the evolution of surviving species post-impact. A 2013 study compared isotopes in impact glass from the Chicxulub impact with isotopes in ash from the K–Pg boundary, concluding that they were dated almost exactly the same within experimental error.

Impact specifics

A 2013 study published in Science estimated the age of the impact as 66,043,000 ± 11,000 years ago (± 43,000 years ago considering systematic error), based on multiple lines of evidence, including argon–argon dating of tektites from Haiti and bentonite horizons overlying the impact horizon in northeastern Montana, United States. This date was supported by a 2015 study based on argon–argon dating of tephra found in lignite beds in the Hell Creek and overlying Fort Union formations in northeastern Montana. A 2018 study based on argon–argon dating of spherules from Gorgonilla Island, Colombia, obtained a slightly different result of 66,051,000 ± 31,000 years ago. The impact has been interpreted to have occurred in Northern Hemisphere spring based on annual isotope curves in sturgeon and paddlefish bones found in an ejecta-bearing sedimentary unit at the Tanis site in southwestern North Dakota. This sedimentary unit is thought to have formed within hours of impact. A 2020 study concluded that the Chicxulub crater was formed by an inclined (45–60° to horizontal) impact from the northeast. The site of the crater at the time of impact was a marine carbonate platform. The water depth at the impact site varied from 100 meters (330 ft) on the western edge of the crater to over 1,200 meters (3,900 ft) on the northeastern edge, with an estimated depth at the centre of the impact of approximately 650 meters (2,130 ft). The seafloor rocks consisted of a sequence of Jurassic–Cretaceous marine sediments, 3 kilometers (1.9 mi) thick. They were predominantly carbonate rock, including dolomite (35–40% of total sequence) and limestone (25–30%), along with evaporites (anhydrite 25–30%), and minor amounts of shale and sandstone (3–4%) underlain by approximately 35 kilometers (22 mi) of continental crust, composed of igneous crystalline basement including granite.

There is broad consensus that the Chicxulub impactor was a C-type asteroid with a carbonaceous chondrite-like composition, rather than a comet. In 1998, a meteorite, approximately 2.5 millimeters (1⁄8 in) across, was described from a deep sea sediment core from the North Pacific, from a sediment sequence spanning the Cretaceous–Paleogene boundary (when the site was located in the central Pacific), with the meteorite being found at the base of the K-Pg boundary iridium anomaly within the sediment core. The meteorite was suggested to represent a fragment of the Chicxulub impactor. Analysis suggested that it best fitted the criteria of the CV, CO and CR groups of carbonaceous chondrites. A 2021 paper suggested, based on geochemical evidence including the excess of chromium isotope ⁵⁴Cr and the ratios of platinum group metals found in marine impact layers, that the impactor matched the characteristics of CM or CR carbonaceous chondrites. The impactor was around 10 kilometers (6.2 miles) in diameter—large enough that, if set at sea level, it would have reached taller than Mount Everest.

Effects

An animation showing the Chicxulub impact and subsequent crater formation

The impactor's velocity was estimated at 20 kilometers per second (12 mi/s). The kinetic energy of the impact was estimated at 72 teratonnes of TNT (300 ZJ). The impact generated winds in excess of 1,000 kilometers per hour (620 mph) near the blast's center, and produced a transient cavity 100 kilometers (62 mi) wide and 30 kilometers (19 mi) deep that later collapsed. This formed a crater mainly under the sea and covered by 600 meters (2,000 ft) of sediment by the 21st century. The impact, expansion of water after filling the crater, and related seismic activity spawned megatsunamis over 100 meters (330 ft) tall, with one simulation suggesting the immediate waves from the impact may have reached up to 1.5 kilometers (0.93 mi) high. The waves scoured the sea floor, leaving ripples underneath what is now Louisiana with average wavelengths of 600 meters (2,000 ft) and average wave heights of 16 meters (52 ft), the largest ripples documented. Material shifted by subsequent earthquakes and the waves reached to what are now Texas and Florida, and may have disturbed sediments as far as 6,000 kilometers (3,700 mi) from the impact site. The impact triggered a seismic event with an estimated moment magnitude of 9–11 M_w .

A cloud of hot dust, ash and steam would have spread from the crater, with as much as 25 trillion metric tons of excavated material being ejected into the atmosphere by the blast. Some of this material escaped orbit, dispersing throughout the Solar System, while some of it fell back to Earth, heated to incandescence upon re-entry. The rock heated Earth's surface and ignited wildfires, estimated to have enveloped nearly 70% of the planet's forests. The devastation to living creatures even hundreds of kilometers away was immense, and much of present-day Mexico and the United States would have been devastated. Fossil evidence for an instantaneous extinction of diverse animals was found in a soil layer only 10 centimeters (3.9 in) thick in New Jersey, 2,500 kilometers (1,600 mi) away from the impact site, indicating that death and burial under debris occurred suddenly and quickly over wide distances on land. Field research from the Hell Creek Formation in North Dakota published in 2019 shows the simultaneous mass extinction of myriad species combined with geological and atmospheric features consistent with the impact event.

Due to the relatively shallow water, the rock that was vaporized included sulfur-rich gypsum from the lower part of the Cretaceous sequence, and this was injected into the atmosphere. This global dispersal of dust and sulfates would have led to a sudden and catastrophic effect on the climate worldwide, instigating large temperature drops and devastating the food chain. The researchers stated that the impact generated an environmental calamity that extinguished life, but it also induced a vast subsurface hydrothermal system that became an oasis for the recovery of life. Researchers using seismic images of the crater in 2008 determined that the impactor landed in deeper water than previously assumed, which may have resulted in increased sulfate aerosols in the atmosphere, due to more water vapor being available to react with the vaporized anhydrite. This could have made the impact even deadlier by cooling the climate and generating acid rain.

The emission of dust and particles could have covered the entire surface of Earth for several years, possibly up to a decade, creating a harsh environment for living things. Production of carbon dioxide caused by the destruction of carbonate rocks would have led to a sudden greenhouse effect. For over a decade or longer, sunlight would have been blocked from reaching the surface of Earth by the dust particles in the atmosphere, cooling the surface dramatically. Photosynthesis by plants would also have been interrupted, affecting the entire food chain. A model of the event developed by Lomax et al (2001) suggests that net primary productivity rates may have increased to higher than pre-impact levels over the long term because of the high carbon dioxide concentrations.

A long-term local effect of the impact was the creation of the Yucatán sedimentary basin which "ultimately produced favorable conditions for human settlement in a region where surface water is scarce".

Post-discovery investigations

Location of seismic surveys and boreholes

Geophysical data

Two seismic reflection datasets have been acquired over the offshore parts of the crater since its discovery. Older 2D seismic datasets have also been used that were originally acquired for hydrocarbon exploration. A set of three long-record 2D lines was acquired in October 1996, with a total length of 650 kilometers (400 mi), by the BIRPS group. The longest of the lines, Chicx-A, was shot parallel to the coast, while Chicx-B and Chicx-C were shot NW–SE and SSW–NNE respectively. In addition to the conventional seismic reflection imaging, data was recorded onshore to allow wide-angle refraction imaging.

In 2005, another set of profiles was acquired, bringing the total length of 2D deep-penetration seismic data up to 2,470 kilometers (1,530 mi). This survey also used ocean bottom seismometers and land stations to allow 3D travel time inversion to improve the understanding of the velocity structure of the crater. The data was concentrated around the interpreted offshore peak ring to help identify possible drilling locations. At the same time, gravity data were acquired along 7,638 kilometers (4,746 mi) of profiles. The acquisition was funded by the National Science Foundation (NSF), Natural Environment Research Council (NERC) with logistical assistance from the National Autonomous University of Mexico (UNAM) and the Centro de Investigación Científica de Yucatán (CICY – Yucatán Center for Scientific Investigation).

Borehole drilling

Intermittent core samples from hydrocarbon exploration boreholes drilled by Pemex on the Yucatán peninsula have provided some useful data. UNAM drilled a series of eight fully-cored boreholes in 1995, three of which penetrated deeply enough to reach the ejecta deposits outside the main crater rim, UNAM-5, 6 and 7. In 2001–2002, a scientific borehole was drilled near the Hacienda Yaxcopoil, known as Yaxcopoil-1 (or more commonly Yax-1), to a depth of 1,511 meters (4,957 ft) below the surface, as part of the International Continental Scientific Drilling Program. The borehole was cored continuously, passing through 100 meters (330 ft) of impactites. Three fully-cored boreholes were also drilled by the Comisión Federal de Electricidad (Federal Electricity Commission) with UNAM. One of them, (BEV-4), was deep enough to reach the ejecta deposits.

In 2016, a joint United Kingdom–United States team obtained the first offshore core samples, from the peak ring in the central zone of the crater with the drilling of the borehole known as M0077A, part of Expedition 364 of the International Ocean Discovery Program. The borehole reached 1,335 meters (4,380 ft) below the seafloor.

Morphology

Schematic cross-section over the Chicxulub impact structure

The form and structure (morphology) of the Chicxulub crater is known mainly from geophysical data. It has a well-defined concentric multi-ring structure. The outermost ring was identified using seismic reflection data. It is up to 130 kilometers (81 mi) from the crater center, and is a ring of normal faults, throwing down towards the crater center, marking the outer limit of significant crustal deformation. This makes it one of the three largest impact structures on Earth. Moving into the center, the next ring is the main crater rim, also known as the "inner rim" which correlates with a ring of cenotes onshore and a major circular Bouguer gravity gradient anomaly. This has a radius that varies between 70 and 85 kilometers (43 and 53 mi). The next ring structure, moving inwards, is the peak ring. The area between the inner rim and peak ring is described as the "terrace zone", characterized by a series of fault blocks defined by normal faults dipping towards the crater center, sometimes referred to as "slump blocks". The peak ring is about 80 km in diameter and of variable height, from 400 to 600 meters (1,300 to 2,000 ft) above the base of the crater in the west and northwest and 200 to 300 meters (660 to 980 ft) in the north, northeast and east. The central part of the crater lies above a zone where the mantle was uplifted such that the Moho is shallower by about 1–2 kilometers (0.62–1.24 mi) compared to regional values.

The ring structures are best developed to the south, west and northwest, becoming more indistinct towards the north and northeast of the structure. This is interpreted to be a result of variable water depth at the time of impact, with less well-defined rings resulting from the areas with water depths significantly deeper than 100 meters (330 ft).

Geology

Pre-impact geology

The center of the crater is near Chicxulub Puerto.

Stela in the main square of Chicxulub Puerto commemorating the impact

Before the impact, the geology of the Yucatán area, sometimes referred to as the "target rocks", consisted of a sequence of mainly Cretaceous limestones, overlying red beds of uncertain age above an unconformity with the dominantly granitic basement. The basement forms part of the Maya Block and information about its makeup and age in the Yucatán area has come only from drilling results around the Chicxulub crater and the analysis of basement material found as part of the ejecta at more distant K–Pg boundary sites. The Maya block is one of a group of crustal blocks found at the edge of the Gondwana continent. Zircon ages are consistent with the presence of an underlying Grenville age crust, with large amounts of late Ediacaran arc-related igneous rocks, interpreted to have formed in the Pan-African orogeny. Late Paleozoic granitoids (the distinctive "pink granite") were found in the peak ring borehole M0077A, with an estimated age of 326 ± 5 million years ago (Carboniferous). These have an adakitic composition and are interpreted to represent the effects of slab detachment during the Marathon-Ouachita orogeny, part of the collision between Laurentia and Gondwana that created the Pangaea supercontinent.

Red beds of variable thickness, up to 115 meters (377 ft), overlay the granitic basement, particularly in the southern part of the area. These continental clastic rocks are thought to be of Triassic-to-Jurassic age, although they may extend into the Lower Cretaceous. The lower part of the Lower Cretaceous sequence consists of dolomite with interbedded anhydrite and gypsum, with the upper part being limestone, with dolomite and anhydrite in part. The thickness of the Lower Cretaceous varies from 750 meters (2,460 ft) up to 1,675 meters (5,495 ft) in the boreholes. The Upper Cretaceous sequence is mainly platform limestone, with marl and interbedded anhydrite. It varies in thickness from 600 meters (2,000 ft) up to 1,200 meters (3,900 ft). There is evidence for a Cretaceous basin within the Yucatán area that has been named the Yucatán Trough, running approximately south–north, widening northwards, explaining the observed thickness variations.

Impact rocks

The most common observed impact rocks are suevites, found in many of the boreholes drilled around the Chicxulub crater. Most of the suevites were resedimented soon after the impact by the resurgence of oceanic water into the crater. This gave rise to a layer of suevite extending from the inner part of the crater out as far as the outer rim.

Impact melt rocks are thought to fill the central part of the crater, with a maximum thickness of 3 kilometers (1.9 mi). The samples of melt rock that have been studied have overall compositions similar to that of the basement rocks, with some indications of mixing with carbonate source, presumed to be derived from the Cretaceous carbonates. An analysis of melt rocks sampled by the M0077A borehole indicates two types of melt rock, an upper impact melt (UIM), which has a clear carbonate component as shown by its overall chemistry and the presence of rare limestone clasts and a lower impact melt-bearing unit (LIMB) that lacks any carbonate component. The difference between the two impact melts is interpreted to be a result of the upper part of the initial impact melt, represented by the LIMB in the borehole, becoming mixed with materials from the shallow part of the crust either falling back into the crater or being brought back by the resurgence forming the UIM.

The "pink granite", a granitoid rich in alkali feldspar found in the peak ring borehole shows many deformation features that record the extreme strains associated with the formation of the crater and the subsequent development of the peak ring. The granitoid has an unusually low density and P-wave velocity compared to typical granitic basement rocks. Study of the core from M0077A shows the following deformation features in apparent order of development: pervasive fracturing along and through grain boundaries, a high density of shear faults, bands of cataclasite and ultra-cataclasite and some ductile shear structures. This deformation sequence is interpreted to result from initial crater formation involving acoustic fluidization followed by shear faulting with the development of cataclasites with fault zones containing impact melts.

The peak ring drilling below the sea floor also discovered evidence of a massive hydrothermal system, which modified approximately 1.4 × 10⁵ km³ of Earth's crust and lasted for hundreds of thousands of years. These hydrothermal systems may provide support for the impact origin of life hypothesis for the Hadean eon, when the entire surface of Earth was affected by impactors much larger than the Chicxulub impactor.

Post-impact geology

After the immediate effects of the impact had stopped, sedimentation in the Chicxulub area returned to the shallow water platform carbonate depositional environment that characterised it before the impact. The sequence, which dates back as far as the Paleocene, consists of marl and limestone, reaching a thickness of about 1,000 m (3,300 ft). The K–Pg boundary inside the crater is significantly deeper than in the surrounding area.

On the Yucatán peninsula, the inner rim of the crater is marked by clusters of cenotes, which are the surface expression of a zone of preferential groundwater flow, moving water from a recharge zone in the south to the coast through a karstic aquifer system. From the cenote locations, the karstic aquifer is clearly related to the underlying crater rim, possibly through higher levels of fracturing, caused by differential compaction.

Astronomical origin and type of impactor

The original 1980 paper describing the crater suggested that it was created by an asteroid around 6.6 kilometers (4.1 mi) in diameter. In 2021, four independent laboratories reported elevated concentrations of iridium in the crater's peak ring, further corroborating the asteroid impact hypothesis.

A 2007 Nature report proposed a specific astronomical origin for the Chicxulub asteroid. The authors, William F. Bottke, David Vokrouhlický, and David Nesvorný, argued that a collision in the asteroid belt 160 million years ago between a 170 km (110 mi) diameter parent body and another 60 km (37 mi) diameter body resulted in the Baptistina family of asteroids, the largest surviving member of which is 298 Baptistina. They proposed that the Chicxulub asteroid was also a member of this group. Subsequent evidence has cast doubt on this theory. A 2009 spectrographic analysis revealed that 298 Baptistina has a different composition more typical of an S-type asteroid than the presumed carbonaceous chondrite composition of the Chicxulub impactor. In 2011, data from the Wide-field Infrared Survey Explorer revised the date of the collision which created the Baptistina family to about 80 million years ago, allowing only 15 million years for the process of resonance and collision, which takes many tens of millions of years. In 2010, another hypothesis implicated the newly discovered asteroid 354P/LINEAR, a member of the Flora family, as a possible remnant cohort of the K–Pg impactor. In 2021, a numerical simulation study argued that the impactor likely originated in the outer main part of the asteroid belt.

Some scholars have argued that the impactor was a comet, not an asteroid. Two papers in 1984 proposed it to be a comet originating from the Oort cloud, and it was proposed in 1992 that tidal disruption of comets could potentially increase impact rates. In 2021, Avi Loeb and a colleague suggested in Scientific Reports that the impactor was a fragment from a disrupted comet. A rebuttal in Astronomy & Geophysics countered that Loeb et al. had ignored that the amount of iridium deposited around the globe, 2.0×10⁸–2.8×10⁸ kg (4.4×10⁸–6.2×10⁸ lb), was too large for a comet of the size implied by the crater, and that they had overestimated likely comet impact rates. They concluded that all available evidence strongly favors an asteroid impactor, effectively ruling out a comet.