# Geominerals/Inosilicates

Def. "any silicate having interlocking chains of silicate tetrahedra" is called an inosilicate.

## Pyroxenes

Pyroxenes are a "large group of inosilicate (chain silicate) minerals with the general formula" ABSi $_2$O $_6$, "divided into the Clinopyroxene Subgroup (monoclinic) and the Orthopyroxene Subgroup (orthorhombic)."
Def. a group of monoclinic or orthorhombic, single chain inosilicates with the general formula of AB(Si,Al) $_2$O $_6$, where
    A = calcium, sodium, ferrous iron (Fe$^{2+}$ ), magnesium, zinc, manganese and lithium;
    B = chromium, aluminum, ferric iron (Fe$^{3+}$ ), magnesium, manganese, scandium, titanium, vanadium, and ferrous iron (Fe$^{2+}$ )
is called a pyroxene.
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.

## Isopyroxenes

A pyroxene that occurs as an isometric mineral or a mineral that is a member of the crystal class isometric.

## Davemaoites

Davemaoite is a high-pressure calcium silicate perovskite (CaSiO $_3$) mineral with a distinctive cubic crystal structure.
"Davemaoite is a vehicle for radioactive isotopes that help to heat the planet’s mantle."
"Davemaoite is mostly calcium silicate (CaSiO $_3$), but it can scavenge radioactive isotopes of uranium, thorium and potassium. These isotopes generate a lot of heat in the lower portion of Earth’s mantle — the layer that lies between the planet’s crust and core. That makes davemaoite an important player in managing how heat moves through the deep Earth and, in turn, how heat cycles between the mantle and crust to drive processes such as plate tectonics."
"The greenish, octahedral-shaped diamond was dug up decades ago in Botswana at the Orapa mine, the world’s largest opencast diamond mine. In 1987, a mineral dealer sold the diamond to George Rossman, a mineralogist at the California Institute of Technology in Pasadena. Tschauner, Rossman and their colleagues began studying it several years ago as part of an investigation into minerals trapped in deep-Earth diamonds."
The "diamond [was irradiated] with X-rays at the Advanced Photon Source [at Argonne National Laboratory], Lemont, Illinois, which revealed that the inclusions were rich in a calcium mineral," using a technique known as X-ray diffraction with the X-ray Synchrotron light source.
"It’s the strength of the diamond that keeps the inclusions at high pressure."
"The version in the diamond has a perovskite crystal structure that only forms at the temperatures and pressures found between 660 and 900 kilometres deep."
"Davemaoite is one of three main minerals in Earth’s lower mantle making up around 5–7% of the material there."
"The one found at Orapa is rich in potassium — so one way to find more davemaoite might be to look for deep diamonds in potassium-rich areas."

## Orthopyroxenes

Pyroxenes that crystallize in the orthorhombic system are known as orthopyroxenes.
* Enstatite, Mg $_2$Si $_2$O $_6$
* Bronzite, intermediate between enstatite and hypersthene
* Hypersthene, (Mg,Fe)SiO $_3$
* Eulite, intermediate between hypersthene and ferrosilite
* Ferrosilite, Fe $_2$Si $_2$O $_6$
* Donpeacorite, (MgMn)MgSi $_2$O $_6$
* Nchwaningite, Mn$^{2+}$ $_2$SiO $_3$(OH) $_2$⋅(H $_2$O)

## 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.

## Entatites

Formula: Mg $_2$Si $_2$O $_6$ Simplified: MgSiO $_3$.
Crystal System: Orthorhombic.
Common Impurities: Fe,Ca,Al,Co,Ni,Mn,Ti,Cr,Na,K.
Polymorph of Akimotoite, Bridgmanite, Clinoenstatite, Unnamed (Mg silicate tetragonal garnet).
Geological Setting: Magmatic mafic rocks.
Enstatite and the other orthorhombic pyroxenes are distinguished from those of the monoclinic series by their optical characteristics, such as straight extinction, much weaker double refraction and stronger pleochroism.
Enstatite is a common mineral in meteorites, where crystals have been found in stony and iron meteorites, including one that fell at Breitenbach in the Ore Mountains, Bohemia, in some meteorites, together with olivine it forms the bulk of the material; it can occur in small spherical masses, or chondrules, with an internal radiated structure.
There are a couple of different ways to organize different chondrules into textural types according to their appearance.
| Name | Abbreviation |
| Porphyritic olivine | PO |
| Porphyritic pyroxene | PP |
| Porphyritic olivine-pyroxene | POP |
| Radial pyroxene | RP |
| Barred olivine | BO |
| Cryptocrystalline | C |
| Granular olivine-pyroxene | GOP |
| Glassy chondrules | – |

## Chondrule sizings

"Disaggregation [...] and thin-section analyses [...] are two standard methods used to obtain statistical data on chondrule sizes. Disaggregation has the advantage that chondrule diameters and abundances can be measured directly, but information on chondrule textures, compositions, and rims is not readily obtainable without subsequent sectioning. In addition, most chondrites are not amenable to disaggregation. In thin section, the compositions, textures, and rim characteristics of chondrules can be determined at the time chondrule sizes are measured. However, the diameters and relative abundances determined from thin-section measurements must be corrected for several sources of bias."
The first diagram on the right shows one source of bias due to a large variation in actual chondrule diameters.
The top diagram at left illustrates that "random sectioning of chondrules produces apparent diameters less than or equal to the true diameters, D."
The second diagram at right shows apparent "chondrule diameters under various viewing conditions. In reflected light, the probability of observing an apparent diameter between d$_1$ and d$_2$, where d$_2$ < d$_1$ < D [varies]. In transmitted light when the matrix is transparent, the observed diameter is the largest diameter occurring in thin section. If the matrix is opaque, only chondrules 1 and 2 [ar]e observed in transmitted light, resulting in an underestimation of the abundance of small chondrules."

## Silicate perovskites

Silicate perovskite (Mg,Fe)SiO $_3$ is either bridgmanite or CaSiO $_3$ (calcium silicate) when arranged in a perovskite structure. Silicate perovskites are not stable at Earth's surface, and mainly exist in the lower part of Earth's mantle, between about 670 and 2,700 km (420 and 1,680 mi) depth. They are thought to form the main mineral phases, together with ferropericlase.
The existence of silicate perovskite in the mantle was first suggested in 1962, and both MgSiO $_3$ and CaSiO $_3$ had been synthesized experimentally before 1975. By the late 1970s, it had been proposed that the seismic discontinuity at about 660 km in the mantle represented a change from spinel structure minerals with an olivine composition to silicate perovskite with ferropericlase.
Natural silicate perovskite was discovered in the heavily shocked Tenham meteorite.
Calcium silicate perovskite is stable at slightly shallower depths than bridgmanite, becoming stable at approximately 500 km, and remains stable throughout the lower mantle.
Calcium silicate perovskite has been identified at Earth's surface as inclusions in diamonds. The diamonds are formed under high pressure deep in the mantle. With the great mechanical strength of the diamonds a large part of this pressure is retained inside the lattice, enabling inclusions such as the calcium silicate to be preserved in high-pressure form.
Experimental deformation of polycrystalline MgSiO $_3$ under the conditions of the uppermost part of the lower mantle suggests that silicate perovskite deforms by a dislocation creep mechanism. This may help explain the observed seismic anisotropy in the mantle.

## Clinopyroxenes

"A subgroup name for monoclinic Pyroxene Group minerals."
"The most widespread members include aegirine, augite, hedenbergite and diopside."
"In the Ca-Fe-Mg (diopside-hedenbergite-augite) quadrilateral, the ideal structural formula is M2M1[Si $_2$O $_6$], where M2 is a distorted octahedral site which preferentially accepts large cations such as alkaline earth/alkali atoms (Ca, but may accept transition metals) and where M1 is a smaller regular octahedral site preferentially accepting transition metals, etc. (most commonly Mg, Fe$^{2+}$ ). The substitutions containing no changes in valence within M2 (2+), Mi (2+), and tetrahedral sites, T (4+), is the normal scheme. A variety of additional substitution schemes, which require coupling of substituents, have been identified in these pyroxenes. Often these substitutions result in a change of species designation when they are dominant."
"Substitution 1 has Na substitution in M2 and trivalent ions of Al, Fe, Cr, and Sc substituting in M1."
"Substitution 2 involves Na substitution in M2 and mixed valance transition metals such as [Fe$^{2+}$ $_{0.5}$Ti$^{4+}$ $_{0.5}$] substituting in M1."
"Substitution 3, also called Tschermak's Component, involves coupled M1 octahedral and tetrahedral substitutions, such as CaAl[AlSiO $_6$] or CaTi$^{3+}$ [AlSiO $_6$]."
"Substitution 4 is a variant of Substitution 2 and has dominant Ca in M2 with mixed valence substitution in M1 involving divalent and tetravalent Ti and Al substitution in tetrahedral sites, e.g. Ca[(Mg, Fe$^{2+}$ $_{0.5}$Ti$^{4+}$ $_{0.5}$)][AlSiO $_6$]. In highly substituted Ca-Fe-Mg pyroxenes, several substitutional schemes may be identified."
"Substitution schemes 2 and 4 have not yet yielded named mineral species."
"Augite could be considered to be a sub-calcic member of the diopside-hedenbergite series but, as defined by Morimoto et al. (1988) may have Fe>Mg or Mg>Fe without a change in name. Because of the way augite has been defined (Morimoto et al, 1988), there is the implication that in rare cases, a variety of anions may be dominant in various the various sites, including M2, M1, and T. These extreme compositions are rare in the large majority of augites. The clinopyroxenes with Wo $_{(5-20)}$ are classed as pigeonite, another mineral that could be thought of as slightly alkalic clinoenstatite-clinoferrosilite."
"However, there is a structural discontinuity between these pyroxenes at low-medium temperatures, and detailed chemical analysis and even structural analysis should be obtained in order to properly classify these unusual compositions. Similarly, extensive substitution of Na, for example, as in scheme 1 (Morimoto et al., 1988) could lead to Aegirine-augite or aegirine compositions depending on the other elements involved."

## Aerinites

Aerinite (Ca $_4$(Al,Fe,Mg) $_{10}$Si $_{12}$O $_{35}$(OH $_{12}$CO $_3$·12H $_2$O) is a bluish-purple inosilicate mineral, that crystallizes in the monoclinic system and occurs as fibrous or compact masses and coatings, has a dark, vitreous luster, a specific gravity of 2.48 and a Mohs hardness of 3. The IMA symbol is Aer.
It is a low-temperature hydrothermal phase occurring in zeolite facies alteration of dolerites. Associated minerals include prehnite,scolecite and mesolite.
Its name comes from a Greek root "aerinos," meaning "atmosphere" or "sky blue". It was first described by Lasaulx (1876) from a specimen in the Wroclaw museum that was obtained in Aragon, Spain. In 1882, the geologist Luis Mariano Vidal found the mineral in situ in Caserras del Castillo, a locality that currently belongs to the municipality of Estopiñán del Castillo, in Huesca (Spain).
Aerinite is a rare mineral. It has been found in several deposits in the Spanish Pyrenees of Huesca and Lleida, as in Estopiñán del Castillo, Camporrells, Juseu and Tartareu. In France, the site is important from St. Pendelon, in the Landes. Aerinite was used as a blue pigment in Romanesque paintings in many churches in the Spain, and also in some French, including the most famous of them, the Pantocrator in the church of San Clemente de Tahull.

## Augites

Formula: (Ca $_x$Mg $_y$Fe $_z$)(Mg $_{y1}$Fe $_{z1}$)Si $_2$O $_6$, where 0.4 ≤ x ≤ 0.9, x+y+z=1 and y1+z1=1.
IMA Formula: (Ca,Mg,Fe) $_2$Si $_2$O $_6$.
Crystal System: Monoclinic, Clinopyroxene Subgroup > Pyroxene Group.
Common Impurities: Ti,Cr,Na,Mn,K.
Geological Setting: Major rock forming mineral in mafic igneous rocks, ultramafic rocks, and some high-grade metamorphic rocks.
Augite is an essential mineral in mafic igneous rocks; for example, gabbro and basalt and common in ultramafic rocks, occurs in relatively high-temperature metamorphic rocks such as mafic granulite and metamorphosed iron formations, commonly occurs in association with orthoclase, sanidine, labradorite, olivine, leucite, amphiboles and other pyroxenes.

## 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.

## Triclinopyroxenes

Pyroxenes crystallizing in the Triclinic system are called triclinopyroxenes.

## Marsturites

Marsturite has the formula NaCaMn$^{2+}$ $_3$Si $_5$O $_{14}$(OH).
Common impurities: Fe,Mg.
Crystal System: Triclinic.
Marsturite is an inosilicate with 5-periodic single chains.
Epitaxy comments: "Marsturite very frequently occurs grown epitaxially on bladed rhodonite crystals (Franklin, NJ)."
Geologic environment, paragenetic mode: metamorphosed Mn-Zn ore body.
Geologic environment of type material: "As a secondary mineral in cavities in fractures traversing massive -franklinite-willemite-zincite ore in a Precambrian Zn-Mn-Fe orebody hosted in the Franklin marble."

## Nambulites

Formula: LiMn$^{2+}$ $_4$Si $_5$O $_{14}$(OH).
"Often contains minor Na replacing Li."
Common impurities: "Ti,Al,Fe,Mg,Ca,K,H2O,C,P".
An incrediblely rare, 5 mm, cherry-red nambulite crystal on a bit of matrix from the one-time find at the Kombat Mine in the early 1970s. Ex. John Barlow Collection. John bought a huge lot of this material when it came out in the one time find. I got these thumbnail crystals from his grandson about 5-6 years ago.

## Natronambulites

Formula: (Na,Li)(Mn$^{2+}$ ,Ca) $_4$Si $_5$O $_{14}$(OH).
Natronambulite is a triclinic inosilicate.
Common impurities: "Fe,Mg,Ca,H $_2$O".

## Pyroxenoids

Def. "any of a large group of minerals physically resembling pyroxene" is called a pyroxenoid.

## Pectolites

Pectolite is a white to gray mineral, NaCa $_2$Si $_3$O $_8$(OH), sodium calcium hydroxide inosilicate that crystallizes in the triclinic system typically occurring in radiated or fibrous crystalline masses, has a Mohs hardness of 4.5 to 5 and a specific gravity of 2.7 to 2.9. The gemstone variety, larimar, is a pale to sky blue.
Occurrence: It was first described in 1828 at Mt. Baldo, Trento Province, Italy, and named from the Greek pektos – "compacted" and lithos – "stone".
Occurrence: as a primary mineral in nepheline syenites, within hydrothermal cavities in basalts and diabase and in serpentinites in association with zeolites, datolite, prehnite, calcite and serpentine. It is found in a wide variety of worldwide locations.
Pectolite is found in many locations, but larimar has a unique volcanic blue coloration, which is the result of copper substitution for calcium.
Miocene volcanic rocks, andesites and basalts, erupted within the limestones of the south coast of the island, contained cavities or vugs which were later filled with a variety of minerals, including the blue pectolite, which are a secondary occurrence within the volcanic flows, dikes, and plugs, which erode, the pectolite fillings are carried down the slope to end up in the alluvium and the beach gravels, and the Bahoruco River carried the pectolite-bearing sediments to the sea.
Larimar is a copper substituted for calcium NaCu $_2$Si $_3$O $_8$(OH) pectolite, a rare blue variety of the silicate mineral pectolite found only in the Dominican Republic, in the Caribbean]]. Its coloration varies from white, light-blue, green-blue to deep blue.

## Rhodonites

Group of transparent, sharp, well-defined, bright pink rhodonite crystals, associated with more than a dozen colorless and bright quartz crystals, are upon a matrix formed by brown to black sphalerite and minor pyrite. The entire specimen measures 53 mm x 52 mm x 40 mm, and the main rhodonite crystals are approximately 10 mm tall, 6 mm wide, and 1 mm thick. Mass: 112g. It comes from the 2007 find at the Chiurucu mine, where mining works stopped long ago, and not far from the famous Huanzala mine, in the district of Huallanca, Peru.
Rhodonite is a manganese pyroxenoid, (Mn, Fe, Mg, Ca)SiO $_3$, crystallizing in the triclinic system, occurring as cleavable to compact masses with a rose-red color (the name comes from the Greek ῥόδος rhodos, rosy), often tending to brown because of surface oxidation.
Rhodonite crystals often have a thick tabular habit, but are rare, a perfect, prismatic cleavage, almost at right angles, Mohs scale of mineral hardness of 5.5–6.5, and the specific gravity of 3.4–3.7; luster is vitreous, being less frequently pearly on cleavage surfaces. The manganese is often partly replaced by iron, magnesium, calcium, and sometimes zinc, which may sometimes be present in considerable amounts; a greyish-brown variety containing as much as 20% of calcium oxide is called bustamite; fowlerite is a zinciferous variety containing 7% of zinc oxide.
The inosilicate structure of rhodonite has a repeat unit of five silica tetrahedra. The rare polymorph pyroxmangite, formed at different conditions of pressure and temperature, has the same chemical composition but a repeat unit of seven tetrahedra.
Occurrence: In manganese-bearing deposits formed by hydrothermal, contact and regional metamorphic, and sedimentary processes.
Association: Calcite, willemite, franklinite (Franklin, New Jersey, USA); calcite, alleghanyite, tephroite, galaxite, grunerite, magnetite (Bald Knob, North Carolina, USA).

## Amphiboles

Def. a group of monoclinic or orthorhombic double chain of tetrahedra inosilicates with the general formula of
    X$_2$Y$_5$Z$_8$O$_{22}$(OH)$_2$ where
    X is magnesium, ferrous iron (Fe$^{2+}$), calcium, lithium, sodium, and ferric iron (Fe$^{3+}$)
    Y is Al, Mg, or Fe or less commonly Mn, Cr, Ti, Li, etc.
    Z is chiefly Si or Al
is called an amphibole.
Def. a "large group of structurally similar hydrated double silicate minerals, containing various combinations of sodium, calcium, magnesium, iron, and aluminium/aluminum" is called an amphibole.

## Agrellites

Agrellite (NaCa $_2$Si $_4$O $_{10}$F) is a mineral found in Quebec, Canada and a few other locations. The International Mineralogical Association (IMA) symbol is Are. Agrellite displays pink fluorescence under both shortwave and longwave ultraviolet light. It is named in honour of Stuart Olof Agrell (1913–1996).
"Agrellite is also a tubular structure with double chains periodically branching into loops(39)."

## Hornblendes

"Hornblende phenocrysts in recent andesites of the Soufrière Hills Volcano display reaction rims of microcrystalline plagioclase, pyroxene, Fe-oxides and interstitial glass, formed by decompression during magma ascent."
A chemical formula for hornblendes is (Ca, Na, K)$_{2-3}$(Mg, Fe$^{2+}$, Fe$^{3+}$, Al)$_5$(Si, Al)$_8$O$_{22}$(OH)$_2$.

## Hypotheses

* Most minerals on Earth are oxides.