Minerals/Silicates

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.

Metasilicates
Def. the "oxyanion of silicon SiO32- or any salt or mineral containing this ion" is called a metasilicate.

Orthosilicates
Def. is called an orthosilicate.
 * 1) "any salt or ester of orthosilicic acid, (M+)4SiO44&minus; or Si(OR)4" or
 * 2) "any silicate mineral, such as garnet or olivine, in which the SiO4 tetrahedra do not share oxygen atoms with each other"

On the right is an image of andradite variety demantoid. Andradite has the formula Ca3Fe3+3Si3O12. Demantoid andradite is green.

Nesosilicates
Def. "any simple silicate mineral in which the SiO4 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 SiO4 tetrahedra through other cations rather than the sharing of oxygens among SiO4 tetrahedra" is called a nesosilicate.

The second nesosilicate image down on the right is of the mineral sphene. Its molecular formula is CaTiSiO5. Each crystallographic unit cell of sphene contains at least one molecular unit. The actual space group P 21/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 Si2O76-) 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. 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 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 polyhedra, connected with  groups
 * columns with six-membered rings of, and , and polyhedra (REE - rare earth elements)
 * groups, octahedra, and disordered  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:, where 0.4 ≤ x ≤ 0.9, x+y+z=1 and y1+z1=1.

IMA Formula:.

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 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  octahedra, where the sheet units have the chemical composition, 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.

Kaolinite is a clay mineral, a layered silicate mineral, with one tetrahedral sheet of silica linked through oxygen atoms to one octahedral sheet of alumina  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, however, in ceramics applications the formula is typically written in terms of oxides, thus the formula for kaolinite is *2*2.

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 SiO4 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 (KAlSi3O8), Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8)."

"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

 * 1) Most minerals on Earth are oxides.