# Geominerals/Nesosilicates

"Nesosilicates constitute an isolated silicon tetrahedra, in the form of orthosilicate ion (SiO$^{4−}$ $_4$), which is connected only by interstitial cations [54]."
"Zinc (ortho)germanate is a semiconductor ceramic material, with wide gap energy (4.7 eV) [, [3], [4], [5], [6]] and derived from the minerals phenakite (a fairly rare nesosilicate mineral consisting of beryllium orthosilicate, Be $_2$SiO $_4$) and willemite (a zinc silicate mineral, Zn $_2$SiO $_4$)."
The two terms "nesosilicate" and "orthosilicate" appear to refer to and often do refer to the same minerals, but there is a difference. The first refers to structure irrespective of chemistry of formation and the second refers to that chemistry which usually produces the structure upon crystallization.

## Theoretical nesosilicates

Def. a "type of silicate crystal structure characterized by the linking of SiO$_4$ tetrahedra through other cations rather than the sharing of oxygens among SiO$_4$ tetrahedra" is called a nesosilicate.
Def. "any salt or ester of orthosilicic acid, M$^+$ $_4$SiO$^{4−}$ $_4$ or Si(OR) $_4$" or "any silicate mineral, such as garnet or olivine, in which the SiO $_4$ tetrahedra do not share oxygen atoms with each other" is called an orthosilicate.

## Afwillites

The chemical formula is Ca $_3$(SiO $_3$OH) $_2$·2H $_2$O. Afwillite (IMA symbol: Afw) is a calcium hydroxide nesosilicate mineral that occurs as glassy, colorless to white prismatic monoclinic crystals, has a Mohs scale hardness between 3 and 4, occurs as an alteration mineral in contact metamorphism of limestone, in association with apophyllite, natrolite, thaumasite, merwinite, spurrite, gehlenite, ettringite, portlandite, hillebrandite, foshagite, brucite and calcite.
It was first described in 1925 for an occurrence in the Dutoitspan Mine, Kimberley, South Africa and was named for Alpheus Fuller Williams (1874–1953), a past official of the De Beers diamond company.
Afwillite has had a couple of formulas:
* Ca $_3$Si $_2$O $_4$(OH) $_6$, and
* Ca $_3$(HSiO $_4$) $_2$·2H $_2$O.
Afwillite may form in fractured veins of the mineral spurrite. Jennite, afwillite, oyelite and calcite are all minerals that form in layers within spurrite veins. Afwillite, and calcite, may form from precipitated fluids. Jennite is actually an alteration of afwillite, but both formed from calcium silicates through hydration. Laboratory studies determined that afwillite forms at a temperature below 200 °C (392 °F), usually around 100 °C. Afwillite and spurrite are formed through contact metamorphism of limestone. Contact metamorphism is caused by the interaction of rock with heat and/or fluids from a nearby crystallizing silicate magma.
Structurally, afwillite is a nesosilicate with isolated SiO$_4$ tetrahedra.
Afwillite has two oxides:
* 2SiO$_4$, Silicates, and
* 2H$_2$O, oxidanes, with each having about 50 molecular %.
Calcium does not occur as native calcium. Here it links to the oxygens in the silicate tetrahedra and the oxidanes. Afwillite is 47.6 at % silicate and 28.6 at % oxidane.
Afwillite has a complex monoclinic structure, and the silicon tetrahedra in the crystal structure are held together by hydrogen bonds. It has perfect cleavage]] parallel to its (101) and poor cleavage parallel to its (100) faces. It is biaxial and its 2V angle, the measurement from one optical axis to the other optical axis, is 50 – 56 degrees. When viewed under crossed polarizers in a petrographic microscope, it displays first-order orange colors, giving a maximum birefringence of 0.0167 (determined by using the Michel- Levy chart). Afwillite is optically positive. Additionally, it has a prismatic crystal habit. Under a microscope afwillite looks like wollastonite, which is in the same family as afwillite.
Afwillite is composed of double chains that consist of calcium and silicon polyhedral connected to each other by sharing corners and edges. This causes continuous sheets to form parallel to its miller index [-101] faces. The sheets are bonded together by hydrogen bonds and are all connected by Ca-Si-O bonds (Malik and Jeffery, 1976). Each calcium atom is in 6-fold octahedral coordination with the oxygen, and the silicon is in 4-fold tetrahedral coordination around the oxygen. Around each silicon there is one OH group and there are three oxygens that neighbor them. The silicon tetrahedra are arranged so that they share an edge with calcium(1), and silicon(2) shares edges with the calcium(2) and calcium(3) polyhedral. The silicon tetrahedra are held together by the OH group and hydrogen bonding occurs between the hydrogen in the OH and the silicon tetrahedra. Hydrogen bonding is caused because the positive ion, hydrogen, is attracted to negatively charge ions which, in this case, are the silicon tetrahedra.

## Alleghanyites

Alleghanyite (International Mineralogical Association (IMA) symbol: Alh) is a moderately rare humite mineral with the formula Mn$^{2+}$ $_5$(SiO $_4$) $_2$(OH) $_2$, belonging to the nesosilicates class. In general its occurrences are related with metamorphic (metamorphosed) manganese deposits. The mineral is named after Alleghany County, North Carolina, US.
Crystal system: Monoclinic.
Member of the Manganhumite subgroup > Humite group.
Dimorph of Ribbeite.
Isostructural with Chegemite, Chondrodite, Edgrewite, Humite, Hydroxylchondrodite, Hydroxylclinohumite, Jerrygibbsite, Kumtyubeite, Leucophoenicite, Manganhumite, Norbergite, Reinhardbraunsite, Ribbeite, Sonolite.
May occur as pink to red-brown fine-grained masses or rare crystals on vein fractures.
Geological Setting: in veins crosscutting franklinite ore near pegmatites in a metamorphosed stratiform Zn-Mn deposit.
Geological Setting of Type Material: Hydrothermally deposited, in lenses in a manganese-bearing vein.
Associated Minerals at Type Locality: Spessartine, Rhodonite and Galaxite.

## Almandines

"Almandine garnet-bearing andesites and dacites occur frequently in the Neogene calc-alkaline volcanic series of the northern Pannonian Basin (Hungary and Slovakia)."
"Coexisting phenocrysts of primary garnets include Ca-rich plagioclase, hornblende (magnesiohastingsite to tschermakite) and/or biotite. The primary garnets have high CaO (>4 wt %) and low MnO contents (<3 wt %). They have strongly light rare earth element depleted patterns and are enriched in heavy rare earth elements."
Almandine (or almandite) has the chemical formula Fe$^{2+}$$_3$Al$_2$Si$_3$O$_{12}$.

## Andalusites

Although andalusite is a metamorphic mineral, it also occurs as in the specimen on the right in volcanic rocks.
Def. an "aluminium nesosilicate mineral, Al$_2$SiO$_5$" is called an andalusite.

## Andradites

On the right is an image of andradite variety demantoid. Andradite has the formula Ca $_3$Fe$^{3+}$ $_3$Si $_3$O $_{12}$. Demantoid andradite is green.

## 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.

## Kyanites

"Volcanoes of the Eifel district have sampled the total crust. [...] Types with chlorite + clinozoisite + oligoclase, with chlorite + garnet + biotite + clinozoisite + oligoclase lead to types with garnet + clinozoisite within garnet but none outside, kyanite + staurolite + almandite + oligoclase."

## Olivines

Def. "[a]ny of a group of olive green magnesium-iron" orthosilicates "that crystallize in the orthorhombic system" 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 "(Mg$_2$SiO$_4$) 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. In 2011 it was observed as tiny crystals in the dusty clouds of gas around a forming star."
"Two polymorphs of forsterite are known: wadsleyite (also orthorhombic) and ringwoodite (isometric). Both are mainly known from meteorites."

## Rhodolites

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)$_3$Al$_2$(SiO$_4$)$_3$,] part of the pyrope-almandine solid-solution series, with an approximate garnet composition of Py$_{70}$Al$_{30}$.

## Sillimanites

"Ash-flow tuffs of Neogene age exposed over 2,500 km$^2$ in the Macusani region of southeastern Peru are the volcanic equivalent of S-type granites. The strongly peraluminous tuffs contain phenocrysts of andalusite, sillimanite, and muscovite and have high $^{87}$Sr/$^{86}$Sr$_i$ (0.7258 and 0.7226) and δ$^{18}$O (+11‰)."
Sillimanite is a nesosilicate.

## Sphenes

The image on the right is of the mineral sphene. Its molecular formula is CaTiSiO $_5$. Each crystallographic unit cell of sphene contains at least one molecular unit. The actual space group P 2$_1$/a has Z=4 (or four molecules per unit cell).
"Sphene is widely distributed as an accessory mineral in intermediate and felsic plutonic rocks, pegmatites and alpine veins, particularly in coarse-grained igneous rocks such as syenite, nepheline syenite, diorite and granodiorite. It occurs similarly in schists or gneisses and in some metamorphosed limestones."

## Staurolites

"Staurolite from the Dry River South volcanic-hosted massive sulfide deposit in northern Queensland, Australia is enriched in ZnO (2.5–6.8%) only within the massive sulfide lens or in highly pyritic biotite-chlorite schist just below the massive sulfide lens."
"Staurolite grains from the Dry River South and other massive sulfide lenses have low TiO$_2$ concentrations (mainly <0.4%) relative to staurolite grains from metasediments and alteration zones (mainly >0.4%). The low concentration of TiO$_2$ in staurolite from the massive sulfide lens results from the low initial Ti concentration in exhalative ores. High ZnO and low TiO$_2$ values are indicative of staurolite associated with Zn-rich massive sulfide."

## Uranophanes

Uranophane Ca(UO $_2$) $_2$(SiO $_3$OH) $_2$·5H $_2$O 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.

## Hypotheses

* Most minerals on Earth are oxides.