# Minerals/Silicates

Silicate minerals are those with more atomic percent silicon oxide than other constituent elements.
For example, kaolinite is Al$_2$Si$_2$O$_5$(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.

## Metasilicates

Def. the "oxyanion of silicon SiO$_3$$^{2-}$ or any salt or mineral containing this ion" is called a metasilicate.

## Orthosilicates

Def.
* "any salt or ester of orthosilicic acid, (M$^+$)$_4$SiO$_4$$^{4−}$ or Si(OR)$_4$" or
* "any silicate mineral, such as garnet or olivine, in which the SiO$_4$ tetrahedra do not share oxygen atoms with each other"
is called an orthosilicate.
On the right is an image of andradite variety demantoid. Andradite has the formula Ca$_3$Fe$^{3+}$$_3$Si$_3$O$_{12}$. Demantoid andradite is green.

## Nesosilicates

Def. "any simple silicate mineral in which the SiO$_4$ tetrahedra are isolated and have metal ions as neighbours" is called a neosilicate.
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.
The second nesosilicate image down 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."

## Sorosilicates

Def. any group of silicates that have structurally isolated double tetrahedra (the dimeric anion Si$_2$O$_7$$^{6-}$) is called a sorosilicate.

## Cyclosilicates

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."

## Abenakiite-(Ce)

Abenakiite-(Ce) has the chemical formula Na $_{26}$Ce $_6$(SiO $_3$) $_6$(PO $_4$) $_6$(CO $_3$) $_6$(S$^{4+}$ O)O. Abenakiite-(Ce) (IMA1991-054; IMA Symbol Abk-Ce) is a mineral of sodium, cerium, neodymium, lanthanum, praseodymium, thorium, samarium, oxygen, sulfur, carbon, phosphorus, and silicon. The silicate groups may be given as the cyclic Si $_6$O $_{18}$ grouping. Its Mohs scale rating is 4 to 5.
Abenakiite-(Ce) was discovered in a sodalite syenite xenolith at Mont Saint-Hilaire, Québec, Canada, together with aegirine, eudialyte, manganoneptunite, polylithionite, serandite, and steenstrupine-(Ce).
Combination of elements in abenakiite-(Ce) is unique. Somewhat chemically similar mineral is steenstrupine-(Ce). The hyper-sodium abenakiite-(Ce) is also unique in supposed presence of sulfur dioxide ligand. With a single grain (originally) found, abenakiite-(Ce) is extremely rare.
In the crystal structure, described as a hexagonal net, of abenakiite-(Ce) there are:
* chains of NaO $_7$ polyhedra, connected with PO $_4$ groups
* columns with six-membered rings of NaO $_7$, and NaO $_7$-REEO $_6$, and SiO $_4$ polyhedra (REE - rare earth elements)
* CO $_3$ groups, NaO $_6$ octahedra, and disordered SO $_2$ ligands within the columns

## Inosilicates

Def. "any silicate having interlocking chains of silicate tetrahedra" is called an inosilicate.
To form these chains, each silica tetrahedron shares two oxygens with neighboring tetrahedra.
Single chain tetrahedra are the pyroxenes. Double chains of tetrahedra are the amphiboles.

## Aegirines

Numerous black prismatic terminated crystals of aegirine are shown in the image on the right with analcite and smaller colorless prismatic terminated crystals of natrolite, which are from 3 mm to 10 mm in length. Aegirine is a pyroxene.

## 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.

## Amphiboles

Anthophyllite is a double-chain inosilicate, or amphibole.

## Phyllosilicates

Def. any "silicate mineral having a crystal structure of parallel sheets of silicate tetrahedra" is called a phyllosilicate.
Phyllosilicate tetrahedra share three oxygens with other silica tetrahedra to form two-dimensional sheets.

## Clay minerals

Def. a "mineral substance made up of small crystals of silica and alumina, that is ductile when moist" is called a clay mineral.
Clay minerals can be classified as 1:1 or 2:1: a 1:1 clay consists of one tetrahedral sheet of corner-sharing SiO $_4$ tetrahedra, three vertex oxygen ions with other tetrahedra, the unshared vertex forms part of one side of the octahedral sheet, where the octahedral sheets form from small cations, such as aluminum or magnesium, and are coordinated by six oxygen atoms, all of the tetrahedra "point" in the same direction, but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedra, bonded to a hydrogen atom forming an OH group, and one octahedral sheet, of AlO $_4$ octahedra, where the sheet units have the chemical composition (Al,Si) $_3$O $_4$, examples are kaolinite and serpentinite; a 2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, and examples are talc, vermiculite, and montmorillonite.

## Kaolinites

Kaolinite has the chemical formula Al $_2$Si $_2$O $_5$(OH) $_4$.
Kaolinite is a clay mineral, a layered silicate mineral, with one tetrahedral sheet of silica (SiO $_4$) linked through oxygen atoms to one octahedral sheet of alumina (AlO $_6$) octahedra. Rocks that are rich in kaolinite are known as kaolin or porcelain (china) clay.
The chemical formula for kaolinite as used in mineralogy is Al $_2$Si $_2$O $_5$(OH) $_4$, however, in ceramics applications the formula is typically written in terms of oxides, thus the formula for kaolinite is Al $_2$O $_3$*2SiO $_2$*2H $_2$O.
As the most numerous element is oxygen at 9 for 52.9 at %, kaolin is an oxide.

## Tektosilicates

Def. a type "of silicate crystal structure characterized by the sharing of all SiO$_4$ tetrahedral oxygens resulting in three-dimensional framework structures" is called a tektosilicate.
Def. any "of various silicate minerals ... with a three-dimensional framework of silicate tetrahedra" is called a tectosilicate.

## Feldspars

Def. a group of "aluminum silicates [aluminosilicates] of the alkali metals sodium, potassium, calcium and barium" are called feldspars, or feldspar.
"The mineralogical composition of most feldspars can be expressed in terms of the ternary system Orthoclase (KAlSi$_3$O$_8$), Albite (NaAlSi$_3$O$_8$) and Anorthite (CaAl$_2$Si$_2$O$_8$)."
"The minerals of which the composition is comprised between Albite and Anorthite are known as the plagioclase feldspars, while those comprised between Albite and Orthoclase are called the alkali feldspars due to the presence of alkali metals sodium and potassium."

## Feldspathoids

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

## Analcites

The image down on the right contains analcime, or analcite, as colorless sharply formed undamaged crystals to 25 mm in diameter on a 78 mm x 65 mm x 53 mm matrix. They are associated with numerous black prismatic terminated crystals of aegirine, as well as smaller colorless prismatic terminated crystals of natrolite, these from 3 mm to 10 mm in length. Aegirine is a pyroxene. Natrolite is another feldspathoid like analcime of the zeolite group.

## Earth

"Feldspar is by far the most abundant group of minerals in the earth's crust, forming about 60% of terrestrial rocks."

## Hypotheses

* Most minerals on Earth are oxides.