Mineralogy

File:Blue hibonite.jpeg

Mineralogy is the scientific study of minerals.

Minerals are solid crystalline substances of natural occurrence.

Def. "a solid, homogeneous, crystalline chemical element or compound that results from natural usually inorganic processes" or any "naturally occurring usually inorganic material that has a (more or less) definite chemical composition and characteristic physical properties"^[1] is called a mineral.

Def.

1. the scientific study of minerals or
2. "a science dealing with minerals, their crystallography, physical and chemical properties, classification, and the ways of distinguishing them"^[2]

is called mineralogy.

Def. any "of a number of chemical elements in the periodic table that form a metallic bond with other metal atoms; generally shiny, somewhat malleable and hard, often a conductor of heat and electricity"^[3] is called a metal.

Metals can be divided into alkali metals; alkaline, or alkali, earth metals; metalloids such as boron, gallium, and mercury; transition metals like iron, niobium, and tungsten; precious metals such as palladium, silver, rhenium, osmium, iridium, platinum, and gold; and post-transition metals like germanium, antimony, and bismuth. Some metals with higher atomic numbers and weights are considered heavy metals: cadmium, indium, tin, mercury, thallium, lead, and bismuth.

Metals constitute a large portion of the Periodic Table of elements. Their combinations with each other and the other elements result in the minerals that compose rocky-objects.

Generally, the transition metals constitute the periodic table of elements from groups 3-12, beginning with scandium (Sc) and ending with element number 112 Copernicium.

Usually, these and the other metals are divided into several subgroups for specific purposes:

1. Metalloids - gallium (Ga) through selenium (Se) and cadmium (Cd) through tellurium (Te).
2. Alkaline earth metals - group 2: beryllium (Be) through radium (Ra).
3. Alkali metals - group 1: lithium (Li) through francium (Fr).
4. Body-centered cubic metals - titanium (Ti) through chromium (Cr), zirconium (Zr) through molybdenum (Mo), and hafnium (Hf) through tungsten (W).
5. Heavy metals - mercury (Hg) through polonium (Po).
6. Lanthanides - lanthanum (La) through lutetium (Lu), also called the rare-earths.
7. Precious metals - ruthenium (Ru) through silver (Ag) and rhenium (Re) through gold (Au).
8. Siliconides.
9. Transition metals are often restricted to manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), and zinc (Zn).
10. Transuranics - neptunium (Np) through Roentgenium (Rg) and Copernicium (Cn).

The category of metals also includes aluminum, scandium (Sc), yttrium (Y), technetium (Tc), and the actinides.

The alkalis, or alkali metals, are the group 1 elements of the Periodic Table. In addition to the true metals: lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr), hydrogen (H) is usually included.

Each of the elements in group 1 has or may have native mineral occurrences on Earth or elsewhere nearby.

Various minerals contain significant amounts of the alkali metals: Li, Na, K, Rb, and Cs. Trace amounts of Fr occur.

Tarapacaite is a natural mineral pigment composed of potassium chromate which is a likely source of yellow.

File:Beryllium-Chromium phase diagram.png

The alkaline earth metals are the elements in Group 2 of the Periodic Table: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).

Beryllium occurs in a hexgonal close-packed (hcp) crystal structure at room temperature (α-Be).

As indicated in the phase diagram on the left beryllium occurs as (β-Be) which is bcc at higher temperatures up to melting.

Native beryllium is not known to occur on the surface of the Earth, but may eventually be found among beryllium-bearing minerals in small amounts.

The elements Be, Mg, Ca, Sr, Ba, and Ra are common in minerals such as calcite which makes up rocks such as limestone and dolomite.

Calcite on the right is a common calcium carbonate mineral that occurs in orange.

For its calcium content it is an alkaline earth mineral, for its carbon and carbonate content it is a carbonide, and is a chalcogen as an oxide.

Def. an "element, such as silicon or germanium, intermediate in properties between that of a metal and a nonmetal; especially one that exhibits the external characteristics of a metal, but behaves chemically more as a nonmetal"^[4] is called a metalloid.

Metalloids include boron, carbon, aluminum, silicon, phosphorus, and arsenic.

Metalloids are elements whose properties are intermediate between metals and solid nonmetals or semiconductors.

A variety of elements are often considered metalloids:

1. boron, considered here in the boronides,
2. aluminum, a face-centered cubic metal, considered in the aluminides,
3. silicon, here in the siliconides,
4. gallium (it can occur in the liquid state as a mineraloid),
5. germanium,
6. arsenic,
7. selenium, also included in the chalcogens,
8. indium,
9. tin,
10. antimony,
11. tellurium, also included in the chalcogens,
12. polonium, considered as among the heavy metals and
13. astatine, here is with the halogens.

Boron occurs primarily on Earth as an oxide, or borate, such as borax, imaged on the top right.

"Mineral subclasses apply to the borate and silicate classes, where the configuration and bonding of tetrahedra are used to group structurally similar minerals. The subclasses are: neso-, soro-, cyclo-, ino-, phyllo- and tectosilicates(borates). Traditionally the borates are divided into monoborates, diborates, triborates, tetraborates etc. (e.g. Strunz & Nickel, 2001), however, enough structural data is known to base classification of borates on the polymerisation of the borate anion."^[5]

File:Lonsdaleite.jpg

Although carbonide is an archaic term for carbide, here it may serve to classify carbon based minerals including carbonates, carbides, and hydrocarbons.

Carbonides are naturally occurring minerals composed of 50 atomic percent, or more, carbon. Carbonide-like minerals with greater than 25 at % carbon are also included. This separates carbon containing minerals from carbonates which are at most 25 at % carbon.

"The mineral Lonsdaleite [on the right] is a translucent, brownish yellow and is made from the atoms of carbon but the arrangement of these atoms is different from the arrangement of carbon atoms in a diamond. [...] The mineral is very rare and is formed naturally whenever [...] graphite containing meteorites fall on the earth and hit the surface."^[6]

Def. a "naturally occurring, glimmering glass-like allotrope of carbon in which each atom is surrounded by four others in the form of a tetrahedron"^[7] is called a diamond.

Def. a solid, homogeneous, monoclinic (space group P2/c, no. 13, or P2₁/c, no. 14), naturally occurring, chemical compound with the formula C₂₄H₁₂ that results from natural inorganic processes is called a carpathite.

Malachite is a mineral that occurs in rocks at or near the interface between Earth's atmosphere and crust. It is a copper carbonate hydroxide mineral.

File:Aluminum1.jpg

Def. any "intermetallic compound of aluminium and a more electropositive element"^[9] is called an aluminide.

The aluminides are those naturally occurring minerals with a high atomic % aluminum.

In the image on the right of a flake of native aluminum, the scale bar = 1 mm.

Aluminium is the third most abundant element (after oxygen and silicon) in the Earth's crust, and the most abundant metal there. It makes up about 8% by mass of the crust, though it is less common in the mantle below.

As a mineral occurrence, aluminum is mostly an oxide.

On the right is an example of bauxite, which is the main rock (ore) from which aluminum is recovered.

"The major aluminum phases recognised in bauxites and laterites are gibbsite also known as hydrargillite (γ-Al(OH)₃), and boehmite (γ-AlO(OH))."^[10]

"Gibbsite is the main mineral in bauxites formed in areas characterized by a tropical climate with alternating rainy and dry periods (monsoon). Bauxites with primarily boehmite appear to be more constrained to the subtropical areas (Mediterranean type bauxite). Thermal action or low-grade metamorphism mostly favors diaspore formation. Furthermore, diaspore is formed as a minor constituent in many types of bauxite in addition to gibbsite and boehmite ^[1-3]."^[10]

Perhaps the most common violet mineral is sapphire. A sample of uncut natural sapphire is at lowest right. Sapphires may be found naturally, by searching through certain sediments (due to their resistance to being eroded compared to softer stones) or rock formations.

Def. any "element from group 15 of the periodic table; nitrogen, phosphorus, arsenic, antimony and bismuth"^[11] is called a pnictogen.

Some of the pnictogens like phosphorus, arsenic, antimony and bismuth, occur as metalloids.

Arsenopyrite on the right is 33.3 at % arsenic.

Fluorapatite, a sample of which is shown at right, is a mineral with the formula Ca₅(PO₄)₃F (calcium fluorophosphate) that is the most common phosphate mineral occurring widely as an accessory mineral in igneous rocks, in calcium rich metamorphic rocks, as a detrital or diagenic mineral in sedimentary rocks, is an essential component of phosphorite ore deposits and occurs as a residual mineral in lateritic soils.^[12]

At left is another fluorapatite example that is violet in color on quartz crystals.

Realgar an arsenic sulfide mineral of 1.5-2.5 Mohs hardness is used to make red-orange pigment.

Realgar is a sulfide mineral. But, with equal atomic numbers of sulfur and arsenic, it may act as a pnictide.

This piece on the right is from the less well-known Royal Reward Mine of Washington.

Satterlyite is a hydroxyl bearing iron phosphate mineral. The mineral can be found in phosphatic shales. Satterlyite is part of the phosphate mineral group. Satterlyite is a transparent, light brown to light yellow mineral. Satterlyite has a formula of (Fe²⁺,Mg,Fe³⁺)₂(PO₄)(OH). Satterlyite occurs in nodules in shale in the Big Fish River (Mandarino, 1978). These nodules were about 10 cm in diameter, some would consist of satterlyite only and others would show satterlyite with quartz, pyrite, wolfeite or maricite.

Holtedahlite, a mineral that was found in Tingelstadtjern quarry in Norway, with the formula (Mg₁₂PO₄)₅(PO₃OH,CO₃)(OH,O)₆ is isostructural with satterlyite (Raade, 1979). Infrared absorption powder spectra show that satterlyite is different than natural haltedahlite in that there is no carbonate for phosphate substitution (Kolitsch, 2002). Satterlyite is also structurally related to phosphoellenbergerite, a mineral that was discovered in Modum, Norway; near San Giocomo Vallone Di Gilba, in Western Alps of Italy (Palache, 1951); the minerals formula is Mg₁₄(PO₄)₅(PO₃OH)₂(OH)₆ (Kolitsch, 2002).

Turquoise at right is an opaque, blue-to-green mineral that is a hydrous phosphate of copper and aluminium, with the chemical formula CuAl₆(PO₄)₄(OH)₈·4H₂O.

The chalcogen minerals contain significant quantities of oxygen, sulfur, selenium, tellurium and polonium. Quartz is composed of SiO₂.

Two atoms of oxygen are present for one atom of silicon. This yields 66.7 at % oxygen.

Chalcogens also include oxides, sulfides (sulfur compounds), selenides, tellurides, and polonides.

Native sulfur is a naturally occurring chalcogen mineral. On Io as shown second down on the right, native sulfur occurs in yellow and red allotropes.

Covellite is a copper sulfide (CuS) mineral.

Covellite has been found in veins at depths of 1,150 meters, as the primary mineral. Covellite formed as clusters in these veins reaching one meter across.

Cubanite is a yellow mineral of copper, iron, and sulfur, CuFe₂S₃.^[13] Cubanite occurs in high temperature hydrothermal deposits with pyrrhotite and pentlandite as intergrowths with chalcopyrite. It results from exsolution from chalcopyrite at temperatures below 200 to 210 °C.^[14] It has also been reported from carbonaceous chondrite meteorites.^[14]

The mineral orpiment at right is a source of yellow and orange pigments.

The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS₂. This mineral's metallic luster and pale brass-yellow hue have earned it the nickname fool's gold because of its superficial resemblance to gold. Pyrite is the most common of the sulfide minerals [on Earth]. Pyrite is usually found associated with other sulfides or oxides in quartz veins, sedimentary rock, and metamorphic rock, as well as in coal beds, and as a replacement mineral in fossils. Despite being nicknamed fool's gold, pyrite is sometimes found in association with small quantities of gold. Gold and arsenic occur as a coupled substitution in the pyrite structure. In the Carlin–type gold deposits, arsenian pyrite contains up to 0.37 wt% gold.^[15]

"Frozen sulfuric acid on Jupiter's moon Europa is depicted in this image produced from data gathered by NASA's Galileo spacecraft. The brightest areas, where the yellow is most intense, represent regions of high frozen sulfuric acid concentration. Sulfuric acid is found in battery acid and in Earth's acid rain."^[16]

Def. any "element of group 7, i.e. fluorine, chlorine, bromine, iodine and astatine, which form a salt by direct union with a metal"^[17] is called a halogen.

The mineral on the right is a KCl mineral called sylvite.

Fluorite is a mineral composed of NaF.

Although fluorite usually appears violet or purple in color, the crystals at left are cyan with some blue or violet fluorite mixed in suggesting slight variations in composition.

Def. "any of the group or metallic elements that have an incomplete inner (d) electron shell; they have multiple valencies and form coloured compounds and stable complex ions; the elements scandium to copper and those lying below them in the periodic table"^[18] is called a transition element, or transition metal.

Transition metals are often restricted to manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), and zinc (Zn), but generally include Sc through zinc (Zn), Y through cadmium (Cd), lanthanum through mercury, actinium through copernicium (Cn).

Kamacite is an alloy of iron and nickel, which is only found on earth in meteorites. The proportion iron:nickel is between 90:10 to 95:5; small quantities of other elements, such as cobalt or carbon may also be present. The mineral has a metallic luster, is gray and has no clear cleavage although the structure is isometric-hexoctahedral. Its density is around 8 g/cm³ and its hardness is 4 on the Mohs scale. It is also sometimes called balkeneisen.

The Nantan irons, a piece is on the right, a witnessed fall in 1516, have a composition of 92.35% iron and 6.96% nickel.

Taenite (Fe,Ni) is a mineral found naturally on Earth mostly in iron meteorites. It is an alloy of iron and nickel, with[nickel proportions of 20% up to 65%. Taenite is one of four known Fe-Ni meteorite minerals: The others are kamacite, tetrataenite, and antitaenite. It is opaque with a metallic grayish to white color. The structure is isometric-hexoctahedral. Its density is around 8 g/cm³ and hardness is 5 to 5.5 on the Mohs scale. Taenite is magnetic. The crystal lattice has the c≈a= 3.582ű0.002Å.^[19] The Strunz classification is I/A.08-20, while the Dana classification is 1.1.11.2. It is a Hexoctahedral (cubic) in structure.

Tetrataenite is a native metal found in meteorites with the composition FeNi. It is one of the mineral phases found in meteoric iron.^[20][21][22]

Antitaenite is a meteoritic metal alloy mineral composed of iron and nickel, 20-40% Ni (and traces of other elements) that has a face centered cubic crystal structure. ... [It exists] as a new mineral species occurring in both iron meteorites and in chondrites^[23] ... The pair of minerals antitaenite and taenite constitute the first example in nature of two minerals that have the same crystal structure (face centered cubic) and can have the same chemical composition (same proportions of Fe and Ni) - but differ in their electronic structures: taenite has a high magnetic moment whereas antitaenite has a low magnetic moment.^[24] [This difference] arises from a high-magnetic-moment to low-magnetic-moment transition occurring in the Fe-Ni bi-metallic alloy series.^[25]

Hematite is the mineral form of iron(III) oxide (Fe₂O₃), one of several iron oxides. Hematite is colored black to steel or silver-gray, brown to reddish brown, or red. Huge deposits of hematite are found in banded iron formations.

Microlite is composed of sodium calcium tantalum oxide with a small amount of fluorine (Na,Ca)₂Ta₂O₆(O,OH,F). Microlite is a mineral in the pyrochlore group that occurs in pegmatites and constitutes an ore of tantalum. It has a Mohs hardness of 5.5 and a variable specific gravity of 4.2 to 6.4. It occurs as disseminated microscopic subtranslucent to opaque octahedral crystals with a refractive index of 2.0 to 2.2. Microlite is also called djalmaite. Microlite occurs as a primary mineral in lithium-bearing granite pegmatites, and in miarolitic cavities in granites.

Def. a "rare metallic chemical element of high economic value"^[26] is called a precious metal.

Def. any "metal that has a specific gravity greater than about 5"^[27] is called a heavy metal.

Heavy metals usually includes cadmium, indium, tin, antimony, mercury, thallium, lead, and bismuth.

Heavy metals are mercury (Hg) through polonium (Po), copernicium (Cn) through livermorium (Lv).

The cadmoindite specimen on the right has about 28.6 at % cadmium.

Breithauptite is a nickel antimonide mineral with the simple formula NiSb. Breithauptite is a metallic opaque copper-red mineral crystallizing in the hexagonal - dihexagonal dipyramidal crystal system. It is typically massive to reniform in habit, but is observed as tabular crystals. It has a Mohs hardness of 3.5 to 4 and a specific gravity of 8.23.

It occurs in hydrothermal calcite veins associated with cobalt–nickel–silver ores.

Cinnabar or cinnabarite (red mercury(II) sulfide (HgS), native vermilion), is the common ore of mercury. Its color is cochineal-red, towards brownish red and lead-gray. Cinnabar [may be] found in a massive, granular or earthy form and is bright scarlet to brick-red in color.^[28] Generally cinnabar occurs as a vein-filling mineral associated with recent volcanic activity and alkaline hot springs. Cinnabar is deposited by epithermal ascending aqueous solutions (those near surface and not too hot) far removed from their igneous source.

"Crocoite is a mineral consisting of lead chromate, PbCrO₄. Crystals are of a bright hyacinth-red color. Relative rarity of crocoite is connected with specific conditions required for its formation: an oxidation zone of lead ore bed and presence of ultramafic rocks serving as the source of chromium (in chromite).

The second crocoite on the right from Dundas extended mine in Tasmania is used to make the first synthetic orange pigment, chrome orange.

"Indium minerals are very rare ; only 7 species have been defined so far : roquesite, CuInS₂ (Picot & Pierrot, 1963) ; indite, FeIn₂S₄, and dzhalindite, In(OH)₃ (Genkin & Murav'eva, 1963) ; sakuraiite, (Cu,Fe,Zn)₃(In,Sn)S₄ (Kato, 1965) ; native indium (Ivanov, 1966b) ; yixunite, PtIn (Yu Tsu-Hsiang et al., 1976) ; petrukite, (Cu,Fe,Zn,Ag)₃(Sn,In)S₄ (Kissin & Owens, 1989)."^[29]

Def. "naturally occurring oxides of the lanthanide metals"^[30] are called rare earths.

Gadolinite on the right often contains a variety of rare earth elements.

Thorianite is a rare thorium oxide mineral, ThO₂.^[31] It has a high percentage of thorium; it also contains the oxides of uranium, lanthanum, cerium, praseodymium and neodymium. The mineral is slightly less radioactive than pitchblende, but is harder to shield due to its high energy gamma rays. It is common in the alluvial gem-gravels of Sri Lanka, where it occurs mostly as water worn, small, heavy, black, cubic crystals.

Elements usually emit a gamma-ray during nuclear decay or fission. The gamma-ray spectrum at right shows typical peaks for ²²⁶Ra, ²¹⁴Pb, and ²¹⁴Bi. These isotopes are part of the uranium-radium decay line. As ²³⁸U is an alpha-ray emitter, it is not shown. The peak at 40 keV is not from the mineral. From the color of the rock shown the yellowish mineral is likely to be autunite.

Autunite occurs as an oxidizing product of uranium minerals in granite pegmatites and hydrothermal deposits.

Carnotite is a potassium uranium vanadate radioactive mineral with chemical formula: K₂(UO₂)₂(VO₄)₂·3H₂O. The water content can vary and small amounts of calcium, barium, magnesium, iron, and sodium are often present. Carnotite is a bright to greenish yellow mineral that occurs typically as crusts and flakes in sandstones. Amounts as low as one percent will color the sandstone a bright yellow. The high uranium content makes carnotite an important uranium ore and also radioactive. It is a secondary vanadium and uranium mineral usually found in sedimentary rocks in arid climates. It is an important ore of uranium in the Colorado Plateau region of the United States where it occurs as disseminations in sandstone and concentrations around petrified logs.

Uraninite is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely UO₂, but also contains UO₃ and oxides of lead, thorium, and rare earth elements. It is most commonly known as pitchblende (from pitch, because of its black color. All uraninite minerals contain a small amount of radium as a radioactive decay product of uranium. Uraninite also always contains small amounts of the lead isotopes ²⁰⁶Pb and ²⁰⁷Pb, the end products of the decay series of the uranium isotopes ²³⁸U and ²³⁵U respectively. The extremely rare element technetium can be found in uraninite in very small quantities (about 0.2 ng/kg), produced by the spontaneous fission of uranium-238.

The image at left shows well-formed crystals of uraninite. The image at right shows botryoidal unraninite. Because of the uranium decay products, both sources are gamma-ray emitters.

Torbernite is a radioactive, hydrated green copper uranyl phosphate mineral, found in granites and other uranium-bearing deposits as a secondary mineral. Torbernite is isostructural with the related uranium mineral, autunite. The chemical formula of torbenite is similar to that of autunite in which a Cu²⁺ cation replaces a Ca²⁺. The number of water hydration molecules can vary between 12 and 8, giving rise to the variety of metatorbernite when torbernite spontaneously dehydrates.

Uranophane Ca(UO₂)₂(SiO₃OH)₂·5H₂O is a rare calcium uranium nesosilicate hydrate mineral that forms from the oxidation of uranium bearing minerals. Uranophane is also known as uranotile. It has a yellow color and is radioactive.

File:Neptunium Aeschynite.png

Def. any naturally occurring element "lying beyond uranium in the periodic table; having an atomic number greater than 92"^[32] is called a transuranic.

Minerals containing transuranics are not rare. But, the stability of such isotopes makes it unlikely to find large concentrations of specific elements.

The transuranics start after uranium (U) in the periodic table. They include the named elements: plutonium (Pu), americium (Am), curium (Cm), berkelium (Bk), californium (Cf), einsteinium (Es), fermium (Fm), mendelevium (Md), nobelium (No), lawrencium (Lr), rutherfordium (Rf), dubnium (Db), seaborgium (Sg), bohrium (Bh), hassium (Hs), meitnerium (Mt), darmstadtium (Ds), roentgenium (Rg), copernicium (Cn), flerovium (Fl), and livermorium (Lv).

Def. any "salt of silica or of one of the silicic acids"^[33] is called a silicate.

Mineralogically, silicate minerals are divided according to structure of their silicate anion into the following groups:^[34][35]

Silicate minerals are those with more atomic percent silicon oxide than other constituent elements.

For example, kaolinite is Al₂Si₂O₅(OH)₄. If aluminum (Al) at two atoms per molecular unit were the most numerous element, kaolinite would be an aluminide.

As the most numerous element is oxygen at 9 for 52.9 at %, kaolin is an oxide. The crystal structure consists of a sheet of interconnected silica tetrahedra. So, kaolin is a phyllosilicate.

Def. the "oxyanion of silicon SiO₃^2- or any salt or mineral containing this ion"^[36] is called a metasilicate.

File:Sphene (titanite) 7.jpg

Def. "any simple silicate mineral in which the SiO₄ tetrahedra are isolated and have metal ions as neighbours"^[37] is called a neosilicate.

Def. a "type of silicate crystal structure characterized by the linking of SiO₄ tetrahedra through other cations rather than the sharing of oxygens among SiO₄ tetrahedra"^[38] is called a nesosilicate.

• Nesosilicates (lone tetrahedron) - [SiO₄]⁴⁻, e.g. olivine, tephroite.

File:Andradite Dermantoid Garnet.jpg

Def. "any salt or ester of orthosilicic acid, (M⁺)₄SiO₄⁴⁻ or Si(OR)₄"^[39] or "any silicate mineral, such as garnet or olivine, in which the SiO₄ tetrahedra do not share oxygen atoms with each other"^[39] is called an orthosilicate.

Def. any "of a group of olive green magnesium-iron" orthosilicates "that crystallize in the orthorhombic system"^[40] is called an olivine.

At right is a visual close up of green sand which is actually olivine crystals that have been eroded from lava rocks. Some olivine crystals are still inside the lava rock.

Forsterite (Mg₂SiO₄) is the magnesium rich end-member of the olivine solid solution series.

Forsterite is associated with igneous and metamorphic rocks and has also been found in meteorites. In 2005 it was also found in cometary dust returned by the Stardust probe.^[41] In 2011 it was observed as tiny crystals in the dusty clouds of gas around a forming star.^[42]

Two polymorphs of forsterite are known: wadsleyite (also orthorhombic) and ringwoodite (isometric). Both are mainly known from meteorites.

Rhodolite is a varietal name for rose-pink to red mineral pyrope, a species in the garnet group.

Chemically, rhodolite is an iron-magnesium-aluminium silicate, [(Mg,Fe)₃Al₂(SiO₄)₃,] part of the pyrope-almandine solid-solution series, with an approximate garnet composition of Py₇₀Al₃₀.

Spurrite is a nesosilicate that can occur naturally as a yellow mineral. Its chemical formula is Ca₅(SiO₄)₂CO₃.^[43] Spurrite is generally formed in contact metamorphism zones as mafic magmas are intruded into carbonate rocks.^[44]

Def. any group of silicates that have structurally isolated double tetrahedra is called a sorosilicate.

Def. any group of silicates that have structurally isolated double tetrahedra (the dimeric anion Si₂O₇^6-)^[45] is called a sorosilicate.

• Sorosilicates (double tetrahedra) - [Si₂O₇]⁶⁻, e.g. epidote, melilite group, leucophanite.

Axinite-(Mg) or magnesioaxinite, Ca₂MgAl₂BOSi₄O₁₅(OH) magnesium rich, can be pale blue to pale violet^[46]

On the right is both a rough stone and a cut stone of tanzanite. Tanzanite is the blue/purple variety of the mineral zoisite (a calcium aluminium hydroxy silicate) with the formula (Ca₂Al₃(SiO₄)(Si₂O₇)O(OH))]. Tanzanite is noted for its remarkably strong trichroism, appearing alternately sapphire blue, violet and burgundy depending on crystal orientation.^[47] Tanzanite can also appear differently when viewed under alternate lighting conditions. The blues appear more evident when subjected to fluorescent light and the violet hues can be seen readily when viewed under incandescent illumination. A rough violet sample of tanzanite is third down at left.

Tanzanite in its rough state is usually a reddish brown color. It requires artificial heat treatment to 600 °C in a gemological oven to bring out the blue violet of the stone.^[48]

Tanzanite is found only in the foothills of Mount Kilimanjaro.

Tanzanite is universally heat treated in a furnace, with a temperature between 550 and 700 degrees Celsius, to produce a range of hues between bluish-violet to violetish-blue. Some stones found close to the surface in the early days of the discovery were gem-quality blue without the need for heat treatment.

Def. any group of silicates that have a ring of linked tetrahedra is called a cyclosilicate.

Beryl of various colors is found most commonly in granitic pegmatites, but also occurs in mica schists. Goshenite a beryl clear to white cyclosilicate is found to some extent in almost all beryl localities.

• Cyclosilicates (rings) - [Si_nO_3n]²ⁿ⁻, e.g. tourmaline group.

The gem-gravel placer deposits of Sri Lanka contain aquamarine.

The deep blue version of aquamarine is called maxixe. Maxixe is commonly found in the country of Madagascar. Its color fades to white when exposed to sunlight or is subjected to heat treatment, though the color returns with irradiation.

The pale blue color of aquamarine is attributed to Fe²⁺. The Fe³⁺ ions produce golden-yellow color, and when both Fe²⁺ and Fe³⁺ are present, the color is a darker blue as in maxixe. Decoloration of maxixe by light or heat thus may be due to the charge transfer Fe³⁺ and Fe²⁺.^{[49][50][51][52]} Dark-blue maxixe color can be produced in green, pink or yellow beryl by irradiating it with high-energy particles (gamma rays, neutrons or even X-rays).^[53]

Eudialyte is a somewhat rare, red silicate mineral, which forms in alkaline igneous rocks, such as nepheline syenites.

Def. "any silicate having interlocking chains of silicate tetrahedra"^[54] is called an inosilicate.

• Inosilicates (single chain) - [Si_nO_3n]²ⁿ⁻, e.g. pyroxene group.

Def. a group of monoclinic or orthorhombic, single chain inosilicates with the general formula of X Y(Si,Al)₂O₆, where

X is calcium, sodium, ferrous iron (Fe²⁺), magnesium, zinc, manganese and lithium;

Y is chromium, aluminum, ferric iron (Fe³⁺), magnesium, manganese, scandium, titanium, vanadium, and ferrous iron (Fe²⁺)

is called a pyroxene.

The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems. Pyroxenes have the general formula XY(Si,Al)₂O₆ (where X represents calcium, sodium, iron⁺² and magnesium and more rarely zinc, manganese and lithium and Y represents ions of smaller size, such as chromium, aluminium, iron⁺³, magnesium, manganese, scandium, titanium, vanadium and even iron⁺²). Although aluminium substitutes extensively for silicon in silicates such as feldspars and amphiboles, the substitution occurs only to a limited extent in most pyroxenes.

At right is an image of a very rare, sharp, complete-all-around pyroxene is from Ducktown District, Polk County, Tennessee, USA, circa mid to late 1800s.

Often a mineral appears blue due to the presence of copper or sulfur. Glaucophane is a blue silicate that owes its color to its characteristic formation.

• Inosilicates(double chain) - [Si_4nO_11n]⁶ⁿ⁻, e.g. amphibole group.

Def. a group of monoclinic or orthorhombic double chain inosilicates with the general formula of

X₂Y₅Z₈O₂₂(OH)₂ where

X is magnesium, ferrous iron (Fe²⁺), calcium, lithium, sodium, and ferric iron (Fe³⁺)

Y is Al, Mg, or Fe or less commonly Mn, Cr, Ti, Li, etc.

Z is chiefly Si or Al

is called an amphibole.

Often a mineral appears blue due to the presence of copper or sulfur. Glaucophane is a blue silicate that owes its color to its characteristic formation.

Glaucophane is a mineral belonging to the amphibole group, chemical formula Na₂Mg₃Al₂Si₈O₂₂(OH)₂. The blue color is very diagnostic for this species. It, along with the closely related mineral riebeckite are the only common amphibole minerals that are typically blue. Glaucophane forms in metamorphic rocks that are either particularly rich in sodium or that have experienced low temperature-high pressure metamorphism such as would occur along a subduction zone. This material has undergone intense pressure and moderate heat as it was subducted downward toward the mantle. It is glaucophane's color that gives the blueschist facies its name. Glaucophane is also found in eclogites that have undergone retrograde metamorphism.^[55]

Def. any "silicate mineral having a crystal structure of parallel sheets of silicate tetrahedra"^[56] is called a phyllosilicate.

• Phyllosilicates (sheets) - [Si_2nO_5n]²ⁿ⁻, e.g. micas and clays like Kaolin.

Lepidolite (KLi₂Al(Al,Si)₃O₁₀(F,OH)₂ is a lilac-gray or rose-colored member of the mica group that is a secondary source of lithium. It is a phyllosilicate mineral^[35] and a member of the polylithionite-trilithionite series.^[57]

It is associated with other lithium-bearing minerals like spodumene in pegmatite bodies. It is one of the major sources of the rare alkali metals rubidium and caesium.^[58]

It occurs in granite pegmatites, in some high-temperature quartz veins, greisens and granites. Associated minerals include quartz, feldspar, spodumene, amblygonite, tourmaline, columbite, cassiterite, topaz and beryl.

Def. a group of monoclinic phyllosilicates with the general formula^[59]

X₂Y_4–6Z₈O₂₀(OH,F)₄

in which X is K, Na, or Ca or less commonly Ba, Rb, or Cs;

Y is Al, Mg, or Fe or less commonly Mn, Cr, Ti, Li, etc.;

Z is chiefly Si or Al, but also may include Fe³⁺ or Ti;

dioctahedral (Y = 4) and trioctahedral (Y = 6)

is called a mica.

Def. type "of silicate crystal structure characterized by the sharing of all SiO₄ tetrahedral oxygens resulting in three-dimensional framework structures"^[38] is called a tektosilicate.

Def. any "of various silicate minerals ... with a three-dimensional framework of silicate tetrahedra"^[60] is called a tectosilicate.

• Tectosilicates (3D framework) - [Al_xSi_yO_2(x+y)]^x−, e.g. quartz, feldspars, zeolites.

Def. a continuous framework, tectosilicate of SiO₄ silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO₂ of trigonal trapezohedral class 3 2, usually with some substitutional or interstitial impurities, is called α-quartz.

When the concentration of interstitial or substitutional impurities becomes sufficient to change the space group of a mineral such as α-quartz, the result is another mineral. When the physical conditions are sufficient to change the solid space group of α-quartz without changing the chemical composition or formula, another mineral results.

Shocked quartz is associated with two high pressure polymorphs of silicon dioxide: coesite and stishovite. These polymorphs have a crystal structure different from standard quartz. Again, this structure can only be formed by intense pressure, but moderate temperatures. High temperatures would anneal the quartz back to its standard form.

Beta quartz (β-Quartz) is stable "between 573° and 870°C".^[38]

Def. a polymorph of α-quartz formed at an estimated minimum pressure of 35 GPa up to pressures above 40 GPa with a orthorhombic space group Pmmm no. 47 is called seifertite.

Def. a polymorph of α-quartz formed at temperatures from 22-460°C with at least seven space groups for its forms with tabular crystals, or a "rare mineral of volcanic rocks that solidified at a high temperature, with the chemical composition silicon dioxide, SiO₂, [...] chemically identical to quartz, but has a different crystal structure"^[61] is called tridymite.

Alpha-quartz (space group P3₁21, no. 152, or P3₂21, no. 154) under a high pressure of 2-3 gigapascals and a moderately high temperature of 700°C changes space group to monoclinic C2/c, no. 15, and becomes the mineral coesite.

Def. a "high-pressure polymorph of silica found in extreme conditions such as the impact craters of meteorites, with the chemical composition of silicon dioxide, SiO₂"^[62] is called coesite.

Def. a polymorph of α-quartz formed by pressures > 100 kbar or 10 GPa and temperatures > 1200 °C, or a "fine-grained mineral, SiO₂, that is a polymorphic form of quartz formed under very high pressure"^[63] is called stishovite.

Stishovite may be formed by an instantaneous over pressure such as by an impact or nuclear explosion type event.

Minute amounts of stishovite has been found within diamonds.^[64]

Def. a high-temperature (above 1470°C) polymorph of α-quartz with cubic, Fd3m, space group no. 227, and a tetragonal form (P41212, space group no. 92), or a mineral of volcanic rocks that solidified at a high temperature, chemically identical to quartz, with the chemical formula SiO₂ but a different crystal structure,^[65] is called cristobalite

Sodalite is a rich royal blue mineral. Massive sodalite samples are opaque, crystals are usually transparent to translucent. Occurring typically in massive form, sodalite is found as vein fillings in plutonic igneous rocks such as nepheline syenites.

Lazurite is a tectosilicate mineral with sulfate, sulfur and chloride with formula: (Na,Ca)₈[(S,Cl,SO₄,OH)₂|(Al₆Si₆O₂₄)]. It is a feldspathoid and a member of the sodalite group. The colour is due to the presence of S^3- anions. Lazurite is a product of contact metamorphism of limestone.

Def. any "of a large group of ... aluminum [tectosilicates] of the alkali metals sodium, potassium, calcium and barium"^[66] is called feldspar.

Def. any of a group of silicates "that did not contain enough silica to satisfy all the chemical bonds"^[67] of the framework is called a feldspathoid.

Plagioclase minerals and their compositions
% An = % CaAl₂Si₂O₈
% Ab = % NaAlSi₃O₈

Name	% An	% Ab
Anorthite || 90–100 || 10–0
Bytownite || 70–90 || 30–10
Labradorite || 50–70 || 50–30
Andesine || 30–50 || 70–50
Oligoclase || 10–30 || 90–70
Albite || 0–10 || 100–90

Def. any "of a group of aluminum silicate feldspathic minerals ranging in their ratio of calcium to sodium"^[68] is called plagioclase.

Hauyne, haüyne or hauynite occurs in Vesuvian lavas in Monte Somma, Italy.^[69] It is a tectosilicate mineral with sulfate, with endmember formula Na₃Ca(Si₃Al₃)O₁₂(SO₄).^[70] It is a feldspathoid and a member of the sodalite group.^[71][72] Haüyne occurs in phonolites and related leucite- or nepheline-rich, silica-poor, igneous rocks; less commonly in nepheline-free extrusives^{[73][71][72][74]} and metamorphic rocks (marble).^[71]

Def. any frozen "volatile chemical, such as water, ammonia, or carbon dioxide"^[75] is called an ice.

A megacryometeor is a very large chunk of ice sometimes called huge hailstones, but do not need to form in thunderstorms.

Def. "a very large water ice object that falls from the sky, similar in composition to hailstones"^[76] is called a megacryometeor.

A megacryometeor is a very large chunk of ice which, despite sharing many textural, hydro-chemical and isotopic features detected in large hailstones, is formed under unusual atmospheric conditions which clearly differ from those of the cumulonimbus cloud scenario (i.e. clear-sky conditions). They are sometimes called huge hailstones, but do not need to form in thunderstorms. Jesus Martinez-Frias, a planetary geologist at the Center for Astrobiology in Madrid, pioneered research into megacryometeors in January 2000 after ice chunks weighing up to 6.6 pounds (3.0 kg) rained on Spain out of cloudless skies for ten days.

Def. "pieces of ice falling as precipitation"^[77] are called hail.

Def. a "single ball of hail"^[78] is called a hailstone.

Def. water ice crystals falling as light white flakes are called snow.

Blue ice occurs when snow falls on a glacier, is compressed, and becomes part of a glacier ... blue ice was observed in Tasman Glacier, New Zealand in January 2011.^[79] Ice is blue for the same reason water is blue: it is a result of an overtone of an oxygen-hydrogen (O-H) bond stretch in water which absorbs light at the red end of the visible spectrum.^[80]

Common dodecahedra

Ih, order 120
Regular-	Small stellated-	Great-	Great stellated-
Th, order 24	T, order 12	Oh, order 48	Td, order 24
Pyritohedron	Tetartoid	Rhombic-	Trapezoidal-
D4h, order 16		D3h, order 12
Rhombo-hexagonal-	Rhombo-square-	Trapezo-rhombic-	Rhombo-triangular-

Def. an "experimental science of determining the arrangement of atoms in solids"^[81], or the "study of crystals"^[81] is called crystallography.

Many minerals occur in large enough crystals to be considered gems.

Def. a "precious stone, usually of substantial monetary value or prized for its beauty or shine"^[82] is called a gem.

Def. the "art and science of gems"^[83] is called gemology.

Def. a "substance that resembles a mineral but does not exhibit crystallinity"^[84] is called a mineraloid.

Def. a "chemical element existing in an uncombined state in nature"^[38] is called a native element.

Def. a naturally occurring, silvery-colored, metallic liquid, composed primarily of the chemical element mercury, is called mercury, or native mercury.

Def. a "small, round, dark glassy object, composed of silicates"^[85] is called a tektite.

Def. a "hard, generally yellow to brown translucent fossil resin"^[86] is called an amber.

Def. a naturally occurring black glass is called an obsidian.

Def. a naturally occurring, hydrous, noncrystalline or amorphous form of silica, where 3% to 21% of the total weight is water, of the chemical formula SiO₂·nH₂O is called an opal.

Def. a "flammable liquid ranging in color from clear to very dark brown and black, consisting mainly of hydrocarbons, occurring naturally in deposits under the Earth's surface"^[87] is called petroleum.

Limonite is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as FeO(OH)·nH₂O, although this is not entirely accurate as the ratio of oxide to hydroxide can vary quite widely. Limonite is one of the two principle iron ores, the other being hematite, and has been mined for the production of iron since at least 2500 BCE.^[88][89] Although originally defined as a single mineral, limonite is now recognized as a mixture of related hydrated iron oxide minerals, among them goethite, akaganeite, lepidocrocite, and jarosite. Individual minerals in limonite may form crystals, but limonite does not, although specimens may show a fibrous or microcrystalline structure,^[90] and limonite often occurs in concretionary forms or in compact and earthy masses; sometimes mammillary, botryoidal, reniform or stalactitic. Because of its amorphous nature, and occurrence in hydrated areas limonite often presents as a clay or mudstone. However there are limonite pseudomorphs after other minerals such as pyrite.^[91] This means that chemical weathering transforms the crystals of pyrite into limonite by hydrating the molecules, but the external shape of the pyrite crystal remains. Limonite pseudomorphs have also been formed from other iron oxides, hematite and magnetite; from the carbonate siderite and from iron rich silicates such as almandine garnets. Limonite usually forms from the hydration of hematite and magnetite, from the oxidation and hydration of iron rich sulfide minerals, and chemical weathering of other iron rich minerals such as olivine, pyroxene, amphibole, and biotite. It is often the major iron component in lateritic soils. One of the first uses was as a pigment. The yellow form produced yellow ochre for which Cyprus was famous.^[92]

Def.

1. a "naturally occurring aggregate of solid mineral matter that constitutes a significant part of the earth's crust",^[93]
2. any "natural material with a distinctive composition of minerals",^[93] or
3. a "naturally occurring aggregate of minerals"^[94]

is called a rock.

1. Determining mineral crystal structures requires much less expertise than large organic molecular crystals.

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