# Areiominerals

Areiominerals are minerals found on and apparently native to the planet Mars[citation needed]. The initial minerals determined to be from Mars were found in meteorites (Areiometeorites) on Earth. Subsequent exploration of the surface of Mars by probes sent from Earth have detected many minerals common to Earth and Mars with variations in compositions.

## Allan Hills 84001

Composition of the ALH84001 meteorite includes Low-Ca Orthopyroxene, Chromite, Maskelynite, and Fe-rich carbonate.
Allan Hills 84001 (ALH84001) is a fragment of a Martian meteorite that was found in the Allan Hills in Antarctica on December 27, 1984, by a team of American meteorite hunters from the ANSMET project. Like other members of the shergottite–nakhlite–chassignite (SNC) group of meteorites, ALH84001 is thought to have originated on Mars. However, it does not fit into any of the previously discovered SNC groups. Its mass upon discovery was 1.93 kilograms (4.3 lb).
ALH 84001 was found on the Allan Hills Far Western Icefield during the 1984–85 season, by Roberta Score, Lab Manager of the Antarctic Meteorite Laboratory at the Johnson Space Center.
ALH84001 is thought to be one of the oldest Martian meteorites, proposed to have crystallized from molten rock 4.091 billion years ago. Chemical analysis suggests that it originated on Mars when there was liquid water on the planet's surface.
In September 2005, Vicky Hamilton, of the University of Hawaii at Manoa, presented an analysis of the origin of ALH84001 using data from the Mars Global Surveyor and 2001 Mars Odyssey spacecraft orbiting Mars. According to the analysis, Eos Chasma in the Valles Marineris canyon appears to be the source of the meteorite. The analysis was not conclusive, partly because it was limited to areas of Mars not obscured by dust.
The theory holds that ALH84001 was blasted away from the surface of Mars by the impact of a meteor about 17 million years ago, and fell on Earth about 13,000 years ago. These dates were established by a variety of radiometric dating techniques, including samarium–neodymium (Sm–Nd), rubidium–strontium (Rb–Sr), potassium–argon (K–Ar), and carbon-14 dating. Other meteorites that have potential biological markings have generated less interest because they do not contain rock from a "wet" Mars; ALH84001 is the only meteorite originating when Mars may have had liquid surface water.
In October 2011, it was reported that isotopic analysis indicated that the carbonates in ALH84001 were precipitated at a temperature of 18 °C (64 °F) with water and carbon dioxide from the Martian atmosphere. The carbonate carbon and oxygen isotope ratios imply deposition of the carbonates from a gradually evaporating subsurface water body, probably a shallow aquifer meters or tens of meters below the surface.
In April 2020, researchers reported discovering nitrogen-bearing organics in Allan Hills 84001.

## EETA 79001

Martian meteorite, EETA 79001, has been analyzed with the Raman point-count procedure. "Raman spectra [occur] for pyroxene, olivine, maskelynite (shocked, isotropized feldspar), chromite, magnetite, ilmenite, ulvöspinel, pyroxferroite, merrillite, apatite, anatase, an Fe sulfide, calcite and hematite."
"The major Raman peak of chromite from this meteorite occurs in a range from 679 to 699 cm$^{-1}$ [...], which corresponds to a (Cr + Fe$^{3+}$)/(Cr + Fe$^{3+}$ + Al) ratio of 0.75-1.0.$^8$ Although not reported in previous studies on EETA79001, magnetite was detected in Raman point-counting measurements on EETA79001,482 rock chip [...] Magnetite and ulvöspinel [...] found in this meteorite have smaller grain sizes, are always observed in multi-phase spectra and only rarely appear insequential spectra."
Approximate area percentages can be estimated using the thin section at right. The penetration depth of a scanning electron beam is usually ≤ 1 µm. Most of the backscattered electrons come from a shallower depth. Using the mineral determinations, the approximate mineral composition is
* merrillite 35 %,
* maskelynite 20 %,
* pyroxene 20 %,
* ulvöspinel 12 %,
* symplectite 5 %,
* olivine 1 %, and the remainder
* about 4 %.
Of the minerals present in this meteorite, only merrillite has apparently never been found in terrestrial rocks.

## NWA 2373

Roughly three-quarters of all Martian meteorites can be classified as shergottites. "[T]he most frequent type of rock (basaltic lithologies) among all known Martian meteorites is the basaltic shergottites."
"The dominant group of Martian meteorites, shergottites, are divided into two subgroups consisting of basalts and lherzolites.
Almost 100 rocks are known that demonstrably come from the Planet Mars. Meteorite researchers and collectors generally refer to the Martian rocks as the SNC meteorites - the shergottites, the nakhlites, and the chassignites. Most of these Martian rocks are shergottites.
Shergottites are a group of Martian rocks named after the Shergotty Meteorite, the type example. The Shergotty Meteorite is a shergottite that was found and identified in 2004.
The first image at right shows a small sample 6 mm from the NWA 2373 Meteorite (NWA = "Northwest Africa"). The light brown-colored material is the outer weathered surface of the rock. The greenish and black speckled surface shows the crystal & mineral make-up of the rock itself. Mineral analysis performed by Theodore Bunch and James Wittke at Northern Arizona University has shown that NWA 2373 is composed principally of olivine, pigeonite & augite pyroxene, plagioclase feldspar glass (maskelynite), chromite, Ti-magnetite, chlorapatite, and trace amounts of other minerals. It looks like an ultramafic rock, but it's apparently a basaltic shergottite (also regarded as a picritic shergottite).
NWA 2373 is reportedly paired with the NWA 1068 Meteorite. Available isotopic dates on the NWA 1068 Meteorite show it formed 185 million years ago (late Amazonian, equivalent to Earth's Early Jurassic), and was ejected from the Martian surface about 2.2 million years ago (information based on cosmogenic isotope analysis).
Very light snow is known to occur at high latitudes on Mars.

## Tissint meteorites

The Tissint meteorite is a Martian meteorite that fell in Tata Province in the Guelmim-Es Semara region of Morocco on July 18, 2011. Tissint is the fifth Martian meteorite that people have witnessed falling to Earth, and the first since 1962. Pieces of the meteorite are on display at several museums, including the Museum of Natural History of Vienna and the Natural History Museum in London.
On July 18, 2011, around 2 AM local time, a bright fireball was observed by several people in the Oued Drâa valley, east of Tata, Morocco. One observer reported that the fireball was initially yellow in color, then turned green, illuminating the entire area before it appeared to break into two pieces; two sonic booms were heard over the valley.
In October 2011, nomads began to find very fresh, fusion-crusted stones in a remote area of the Oued Drâa intermittent watershed, centered about 50 kilometres (31 mi) ESE of Tata and 48 kilometres (30 mi) SSW of Tissint village, near the Oued El Gsaïb drainage and also near El Ga’ïdat plateau known as Hmadat Boû Rba’ine. The largest pieces were recovered in the El Ga’ïdat plateau, and the smallest ones (a few grams) were found closer to the El Aglâb mountains. One 47 g (1.7 oz) crusted stone was documented as found at 29°28.917’ N, 7°36.674’ W.
Tissint was named after the town of Tissint, 48 kilometres (30 mi) away from the fall site.
Up until 1990, only five meteorites had been found in Morocco, but since then, more meteorites have landed in the area. Current as of 2012 records show that meteorite hunters have discovered 754 at specific sites in Morocco as well as thousands of others from uncertain locations. After the increases in meteorite falls, a market for meteorites drove the emergence of a meteorite prospecting industry in northwestern Africa and Oman. The rocks have been quickly brought out of the country into collections abroad because the significant discoveries resulted in high prices for the rocks (an auction on October 14, 2012, included fragments of the Tissent meteorite). This made it difficult for researchers such as Hasnaa Chennaoui-Aoudjehane, the only one who has studied the meteorite, to have access to samples for her research and leaves Morocco with few remains of the meteorites that fell there.
Dozens of fragments with masses ranging from 0.2 to 1,282 grams (0.0071 to 45.2212 oz) were collected, totaling roughly 12–15 kilograms (26–33 lb). The rocks are variably coated by a shining black fusion crust, characterized by thicker layers on exterior ridges and glossy regions above interior olivine phenocrysts and impact melt pockets. Some stones have a thinner secondary fusion crust on some surfaces, and some are broken in places, revealing the interior. The exposed interior of the stones appears pale green-grey in color, with mm-sized, pale yellow olivine phenocrysts with sparse vesicular pockets and thin veins of black glass.
The meteorite was ejected from the surface of Mars between 700,000 and 1,1 million years ago. Tissint appears to be derived from a deep mantle source region that was unlike any of the other known Martian shergottite meteorites.
The material is highly shocked and indicates it was ejected during the largest impact excavation in record. Given the widely dispersed shock melting observed in Tissint, alteration of other soft minerals (carbonates, halides, sulfates and even organics), especially along grain boundaries, might have occurred. This may in part explain the lack of such minerals in Tissint, but it is unknown if it is of biotic origin.
The meteorite fragments were recovered within days after the fall, so it is considered an "uncontaminated" meteorite. The meteorite displays evidence of water weathering, and there are signs of elements being carried into cracks in the rocks by water or fluid, which is something never seen before in a Martian meteorite. Specifically, scientists found carbon and nitrogen-containing compounds associated with hydrothermal mineral inclusions. One team reported measuring an elevated carbon-13 ($^{13}$C) ratio, while another team reported a low $^{13}$C ratio as compared to the content in Mars' atmosphere and crust, and suggested that it may be of biological origin, but the researchers also noted that there are several geological processes that could explain that without invoking complex life-processes; for example, it could be of meteoritic origin and would have been mixed with Martian soil when meteorites and comets impact the surface of Mars, or of volcanic origin.
An analysis by Hasnaa Chennaoui-Aoudjehane, a Moroccan meteoriticist of Hassan II University in Casablanca, determined that the meteorite is a depleted picritic ,shergottite similar to EETA79001A. The internal structure of the meteorite includes olivine macrocrysts (or nodules) embedded into a fine-grained matrix made of pyroxene and feldspar glass. The matrix has numerous cracks filled with black glassy material. Like other shergottites Tissint meteorite is enriched in magnesium oxide and other compatible elements such as nickel and cobalt. The bulk composition is also depleted in light rare earths and other incompatible elements such as beryllium, lithium and uranium. However the glassy material is enriched in these elements.
The data on refractory trace elements, sulfur and fluorine as well as the data on the isotopic composition of nitrogen, argon and carbon released upon heating from the matrix and glass veins in the meteorite unambiguously indicate the presence of a Martian surface component including trapped atmospheric gases. So, the influence of in situ Martian weathering can be distinguished from terrestrial contamination in the meteorite. The Martian weathering features in Tissint are compatible with the results of spacecraft observations of Mars, and Tissint has a cosmic ray dating exposure age of 0.7 ± 0.3 Ma—consistent with the reading of many other shergottites, notably EETA79001, suggesting that they were ejected from Mars during the same event.
The overall composition of the Tissint meteorite corresponds to that of aluminium-poor ferrous iron (ferroan) basaltic rock, which likely originated as a result of magmatic activity at the surface of Mars. These basalt then underwent weathering by fluids, which deposited minerals enriched in incompatible elements in fissures and cracks. A later impact on the surface of Mars melted the leached material forming black glassy veins. Finally shergottites were ejected from Mars about 0.7 million years ago.

## Heat Shield Rock

NASA's Mars Exploration Rover Opportunity has found an iron meteorite on Mars (now known as Heat Shield Rock), the first meteorite of any type ever identified on another planet. The pitted, basketball-size object is mostly made of iron and nickel. Readings from spectrometers on the rover determined that composition. Opportunity used its panoramic camera to take the images used in this approximately true-color composite on the rover's 339th martian day, or sol (Jan. 6, 2005). This composite combines images taken through the panoramic camera's 600-nanometer (red), 530-nanometer (green), and 480-nanometer (blue) filters.

## Maskelynites

Maskelynite was first identified in the Shergotty meteorite by Gustav Tschermak von Seysenegg (1872) as an isotropic glass of an unknown origin with near labradorite composition. Similar phases were found in chondrites and Martian meteorites. In 1963, D. J. Milton and P. S. de Carli produced a maskelynite-like glass by subjecting gabbro to an explosive shock wave.
"Maskelynite occurs in the rocks of the central peaks of Clearwater West and Manicouagan craters, Quebec, Canada."
"A clear, glassy pseudomorph of plagioclase was described in the Shergotty basaltic achondrite by Tschermak (1872) and named maskelynite."
Maskelynite is "a transformation of plagioclase produced by remelting or by mechanical means."
"A mechanical transformation of Stillwater gabbro to a rock resembling shergottite was achieved by Milton and DeCarli (1963) by shock-loading the gabbro at 250 to 350 kb peak pressure and a calculated temperature of no more than 350°C. They advocated restricting the term maskelynite to a non-crystalline phase clearly pseudomorphous after crystalline feldspar. Duke (1963) restudied the Shergotty meteorite in detail and suggested the maskelynite resulted from a passage of a strong shock wave through the meteorite."
"Terrestrial samples containing isotropic material conforming to Milton and DeCarli's definition have been collected from two large probable ancient meteorite craters in the Canadian Shield (Beals, et al. 1963)."
Plagioclase is a series of tectosilicate minerals within the feldspar group with a specific chemical composition: NaAlSi $_3$O $_8$ – CaAl $_2$Si $_2$O $_8$. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or 'record-groove' effect.
Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase is the most abundant mineral in the crust of Mars.

## Symplectites

A symplectite (or symplektite) is a material texture: a micrometre-scale or submicrometre-scale intergrowth of two or more crystals. Symplectites form from the breakdown of unstable phases, and may be composed of minerals, ceramics, or metals. Fundamentally, their formation is the result of slow grain-boundary diffusion relative to interface propagation rate.
A cellular precipitation reaction, in which a reactant phase decomposes to a product phase with the same structure as the parent phase and a second phase with a different structure, can form a symplectite. Eutectoid reactions, involving the breakdown of a single phase to two or more phases, neither of which is structurally or compositionally identical to the parent phase, can also form symplectites.

## Akimotoites

Formula: (Mg,Fe)SiO $_3$.
Akimotoite is a rare silicate mineral in the ilmenite group of minerals, polymorphous with pyroxene and bridgmanite, a natural silicate perovskite that is the most abundant mineral in Earth's silicate mantle. Akimotoite has a vitreous luster, is colorless, and has a white or colorless streak, crystallizes in the trigonal crystal system in space group R3, is the silicon analogue of geikielite (MgTiO $_3$).
The crystal structure is similar to that of ilmenite (FeTiO $_3$) with Si and Mg in regular octahedral coordination with oxygen, the octahedra align in discrete layers alternating up the c-axis, space group is R3 (trigonal) with a = 4.7284 Å; c = 13.5591 Å; V = 262.94 Å$^3$; Z = 6.
Akimotoite was found in the Tenham meteorites in Queensland, Australia, is believed to have formed as the result of an extraterrestrial shock event, is the silicon analogue of geikielite (MgTiO$_3$), was named after physicist Syun-iti Akimoto (also known as Shun'ichi Akimoto (秋本 俊一)) (1925–2004), University of Tokyo.
It has also been reported from the Sixiangkou meteorite in the Gaogang District, Jiangsu Province, Taizhou Prefecture, China; the Zagami Martian meteorite, Katsina State, Nigeria and from the Umbarger meteorite, Randall County, Texas.
Akimotoite is believed to be a significant mineral in the Earth's mantle at depths of 600–800 kilometres (370–500 mi) in cooler regions of the mantle such as where a subducted slab enters into the lower mantle, as Akimotoite is elastically anisotropic and has been suggested as a cause of seismic anisotropy in the lower transition zone and uppermost lower mantle.
Molar volume is a function of pressure for akimotoite MgSiO $_3$ at room temperature, experimental data (N=26).
The EH70 (Sanloup et al., 1999) model for the bulk chemical composition of the Martian mantle is constrained by oxygen isotope signatures of the Martian meteorites and has a systematically lower Mg/Si ratio.
The "mineralogy of the EH70 composition [was studied] through in-situ laser-heated diamond anvil cell (LHDAC) and large volume press (LVP) experiments. [...] Garnet-dominated mineralogy has been observed up to 25 GPa. Bridgmanite begins to appear from 25.2 GPa and continues in a mixed phase with garnet up to 27 GPa at which point only bridgmanite and calcium perovskite remain. [...] Akimotoite is stable 300 K higher in temperature than Perple_X results. [A] significant amount of the skiagite component in the majoritic garnet [is] stable at the conditions of the Martian core-mantle boundary in EH70."

## Bridgmanites

The magnesium end-member of the silicate perovskite (Mg,Fe)SiO $_3$ is called bridgmanite
Bridgmanite, the most abundant mineral in Earth, was discovered in a shocked meteorite.
In 2014, the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA) approved the name bridgmanite for perovskite-structured (Mg,Fe)SiO $_3$, in honor of physicist Percy Williams Bridgman, who was awarded the Nobel Prize in Physics in 1946 for his high-pressure research.
Bridgmanite is a high-pressure polymorph of enstatite, but in the Earth predominantly forms, along with ferropericlase, from the decomposition of ringwoodite (a high-pressure form of olivine) at approximately 660 km depth, or a pressure of ~24 GPa. The depth of this transition depends on the mantle temperature; it occurs slightly deeper in colder regions of the mantle and shallower in warmer regions. The transition from ringwoodite to bridgmanite and ferropericlase marks the bottom of the mantle transition zone and the top of the lower mantle. Bridgmanite becomes unstable at a depth of approximately 2700 km, transforming isochemically to post-perovskite.
Bridgmanite is the most abundant mineral in the mantle. The proportions of bridgmanite and calcium perovskite depends on the overall lithology and bulk composition. In pyrolitic and harzburgitic lithogies, bridgmanite constitutes around 80% of the mineral assemblage, and calcium perovskite < 10%. In an eclogitic lithology, bridgmanite and calcium perovskite comprise ~30% each.

## Enstatites

Enstatite melts at a higher temperature than its high-calcium cousin, and is present in the highlands which indicates that older magmas on Mars had higher temperatures than younger ones.

## Garnets

"Abundances of rare earth elements, Hf, Sc, Co, Cr and Th in garnet megacrysts and their volcanic hosts or matrices are used to estimate garnet/liquid partition coefficients for these elements."
"The wide variation in garnet/liquid partition coefficients from kimberlites to rhyolites cannot be explained as an effect of temperature and we conclude that a major factor is the composition of the melt from which the garnet crystallized."
"Garnet = Mineral Group of general formula: A$_3$B$_2$(SiO$_4$)$_3$, A = Ca, Mg, Fe$^{2+}$, Mn$^{2+}$; B = Al, Fe$^{3+}$, Cr, V, Zr, Ti."
The garnets make up two solid solution series: pyrope-almandine-spessartine (pyralspite), with the composition range [Mg,Fe,Mn] $_3$Al $_2$(SiO $_4$) $_3$; and uvarovite-grossular-andradite (ugrandite), with the composition range Ca $_3$[Cr,Al,Fe] $_2$(SiO $_4$) $_3$.
Def. a "hard transparent mineral that is often used as gemstones and abrasives" is called a garnet.

## Goethites

The red/orange appearance of the dust is caused by iron(III) oxide (nanophase Fe$_2$O$_3$) and the iron(III) oxide-hydroxide mineral goethite.
Spirit rover discovered sulfates and goethite in the Columbia Hills.

## Hematites

Minerals produced through hydrothermal alteration and weathering of primary basaltic minerals are also present on Mars, including hematite, phyllosilicates (clay minerals), goethite, jarosite, iron sulfate minerals, opaline silica, and gypsum. Many of these secondary minerals require liquid water to form (aqueous minerals) such as those that contain fluvial landforms indicating that abundant water was once present.
Sulfate deposits preserve chemical and morphological fossils, and fossils of microorganisms form in iron oxides like hematite.

## Ilmenites

Ilmenite was detected as FeTiO $_3$ a magnetic mineral.

## Kaolinites

Mars Reconnaissance Orbiter, launched in 2005, carried multiple instruments which found the mineralogy to be dominated by mafic minerals such as olivine, mica, pyroxene and smectite clays such as kaolinite.

## Magnetites

The Mars Exploration Rovers identified magnetite as the mineral responsible for making the dust magnetic, where it probably also contains some titanium.

## Marjorites

Majorite is a type of garnet found in the mantle of the Earth, chemical formula is Mg $_3$(MgSi)(SiO $_4$) $_3$, distinguished from other garnets in having Si in octahedral as well as tetrahedral coordination, first described in 1970 from the Coorara Meteorite of Western Australia and has been reported from various other meteorites in which majorite is thought to result from an extraterrestrial high pressure shock event. Mantle derived xenoliths containing majorite have been reported from potassic ultramafic magmas on Malaita Island on the Ontong Java Plateau Southwest Pacific.

## Merrillites

Merrillite is a calcium phosphate mineral with the chemical formula Ca $_9$NaMg(P $_4$) $_7$, an anhydrous, sodic member of the whitlockite group.
The mineral is named after George P. Merrill (1854–1929) of the Smithsonian Institution, who had described the mineral from four meteorites in 1915: the Alfianello, Dhurmsala, Pultusk, and Rich Mountain meteorites.
Merrillite occurs in pallasites, lunar rocks, martian meteorites, and many other meteorite groups.

## Montmorillonites

Chemical formula detected on Mars: (Na,Ca)$_{0.33}$(Al,Mg)$_2$Si $_4$O $_{10}$(OH) $_2$*nH $_2$O. The Visible and Infrared Mineralogical Mapping Spectrometer (OMEGA) observed montmorillonite and localized phyllosilicate minerals.

## Olivines

On October 17, 2012, the Curiosity rover on the planet Mars at "Rocknest" performed the first X-ray diffraction analysis of Martian soil revealing the presence of several minerals, including feldspar, pyroxenes and olivine, and suggested that the Martian soil in the sample was similar to the "weathered basaltic soils" of Hawaiian volcanoes.

## Perovskites

Perovskite is a calcium titanium oxide mineral composed of calcium titanate (chemical formula CaTiO $_3$), also applied to the class of compounds which have the same type of crystal structure as CaTiO $_3$ ($^{XII}$A$^{2+}$$^{VI}$B$^{4+}$X$^{2−}$$_3$), known as the perovskite structure. Many different cations can be embedded in this structure, allowing the development of diverse engineered materials.
Found in the Earth's mantle, perovskite's occurrence at Khibina Massif is restricted to the silica under-saturated ultramafic rocks and foidolites, due to the instability in a paragenesis with feldspar. Perovskite occurs as small anhedral to subhedral crystals filling interstices between the rock-forming silicates.
Perovskite is found in contact metamorphism carbonate skarns at Magnet Cove igneous complex, Magnet Cove, Arkansas, in altered blocks of limestone ejected from Mount Vesuvius, in chlorite and talc schist in the Urals and Switzerland, and as an accessory mineral in alkaline and mafic igneous rocks, nepheline syenite, melilitite, kimberlites and rare carbonatites. Perovskite is a common mineral in the Ca-Al-rich inclusions found in some chondritic meteorites.
A rare-earth-bearing variety knopite with the chemical formula (Ca,Ce,Na)(Ti,Fe)O $_3$ is found in alkali intrusive rocks in the Kola Peninsula and near Alnö, Sweden. A niobium-bearing variety dysanalyte occurs in carbonatite near Schelingen, Kaiserstuhl (Baden-Württemberg), Germany.
In stars and brown dwarfs the formation of perovskite grains is responsible for the depletion of titanium(II) oxide in the photosphere. Stars with a low temperature have dominant bands of TiO in their spectrum; as the temperature gets lower for stars and brown dwarfs with an even lower mass, CaTiO $_3$ forms and at temperatures below 2000 K TiO is undetectable. The presence of TiO is used to define the transition between cool M dwarf stars and the colder L dwarfs.
The stability of perovskite in igneous rocks is limited by its reaction relation with sphene. In volcanic rocks perovskite and sphene are not found together, the only exception being an etindite from Cameroon.
Perovskites have a nearly cubic structure with the general formula ABO $_3$. In this structure the A-site ion, in the center of the lattice, is usually an alkaline earth or rare-earth element. B-site ions, on the corners of the lattice, are 3d, 4d, and 5d transition metal elements. A large number of metallic elements are stable in the perovskite structure if the Goldschmidt tolerance factor $t$ is in the range of 0.75–1.0
    $t={\frac {R_{A}+R_{\rm {O}}}{{\sqrt {2}}(R_{B}+R_{\rm {O}})}},$
where R$_A$, R$_B$ and R$_O$ are the ionic radii of A and B site elements and oxygen, respectively.
Perovskites have sub-metallic to metallic luster, colorless streak, and cube-like structure along with imperfect cleavage and brittle tenacity. Colors include black, brown, gray, orange to yellow. Perovskite crystals may appear to have the cubic crystal form, but are often pseudocubic and actually crystallize in the orthorhombic system, as is the case for CaTiO $_3$ (Strontium titanate, with the larger strontium cation in the A-site, is cubic). Perovskite crystals have been mistaken for galena; however, galena has a better metallic luster, greater density, perfect cleavage and true cubic symmetry.

## Pigeonites

In the image on the right is a polarized light microscope image of part of a grain of orthopyroxene containing exsolution lamellae of augite from the Bushveld intrusion. The texture documents a multistage history: (1) crystallization of twinned pigeonite, followed by exsolution of augite; (2) breakdown of pigeonite to orthopyroxene plus augite; (3) exsolution of augite parallel to the former twin plane of pigeonite.
Formula: (Ca,Mg,Fe)(Mg,Fe)Si $_2$O $_6$. The calcium cation fraction can vary from 5% to 25%, with iron and magnesium making up the rest of the cations.
Pigeonite is a mineral in the clinopyroxene subgroup of the pyroxene group.
Pigeonite is found as phenocrysts in volcanic rocks on Earth and as crystals in meteorites from Mars and the Moon. In slowly cooled intrusive igneous rocks, pigeonite is rarely preserved. Slow cooling gives the calcium the necessary time to separate itself from the structure to form exsolution lamellae of calcic clinopyroxene, leaving no pigeonite present. Textural evidence of its breakdown to orthopyroxene plus augite may be present, as shown in the accompanying microscopic image.
Pigeonite is named for its type locality on Lake Superior's shores at Pigeon Point, Minnesota, United States. It was first described in 1900.

## Plagioclases

The rocks on the plains of Gusev are a type of basalt that contain olivine, pyroxene, plagioclase, and magnetite, and they look like volcanic basalt as they are fine-grained with irregular holes (vesicles and vugs).

## Pyroxenes

Pyroxene minerals are also widespread across the surface. Both low-calcium (ortho-) and high-calcium (clino-) pyroxenes are present, with the high-calcium varieties associated with younger volcanic shields and the low-calcium forms (enstatite) more common in the old highland terrain, where its presence in the highlands indicates that older magmas on Mars had higher temperatures than younger ones.
Between 1997 and 2006, the Thermal Emission Spectrometer (TES) on the Mars Global Surveyor (MGS) spacecraft mapped the global mineral composition of the planet. TES identified two global-scale volcanic units on Mars: Surface Type 1 (ST1) characterises the Noachian-aged highlands and consists of unaltered plagioclase- and clinopyroxene-rich basalts, and Surface Type 2 (ST2) is common in the younger plains north of the dichotomy boundary and is more silica rich than ST1.

## Quartz

Only the Thermal Emission Imaging System of the Mars Odyssey was designed to look at minerals, and detected the presence of quartz, olivine, and hematite.

## Skiagites

Formula: Fe$^{2+}$ $_3$Fe$^{3+}$ $_2$Si $_3$O $_{12}$.
Skiagite is an iron garnet.
Member of: Garnet Supergroup.

## Smectites

Smectite group includes dioctahedral smectites, such as montmorillonite, nontronite and beidellite, and trioctahedral smectites, such as saponite. In 2013, analytical tests by the Curiosity rover found results consistent with the presence of smectite clay minerals on the planet Mars.