The Silicates Minerals

The 90% of the Earth’s crust is composed of silicate minerals, comprising of oxygen and silicon. The silicate minerals are the ones that form rocks. The classification of this mineral is based on the silicate group structure, which consists of oxygen and silicon in varied proportions. Among the rock-forming minerals, this forms the majority.


The mineral that is found in abundance in the Earth are the silicate minerals, like Quartz, Mica, Feldspar, Pyroxene, Olivine and Amphibole, all belong to the silicate mineral group.
Silicate tetrahedral, composed of oxygen and silicon bind with other Cations (atoms that cannot be positively charged because they have lost an electron to become positively charged) in order to arrive at other silicate minerals.


Classification of silicate minerals:
Silicates are by and large the biggest group of minerals, comprising of mostly oxygen and silicon. Ions such as calcium, magnesium, aluminum and quartz are also added to it. Certain essential silicate minerals that form rock are quartz, pyroxenes, feldspars, garnets, olivine, mica and amphiboles.


Olivine
Olivine resembles green crystals; they are particularly seen in certain metamorphic and igneous rocks. The majority of crystals are so tiny that a magnifying glass or a hand lens becomes essential to view them.

Shape: Orthorhombic  (usually a many-sided prism that has an overall sphere shape)

Fracture: Conchoidal, brittle

Hardness: 6.5-7 on ⦁ Mohs Hardness Scale

Streak: White

Color: Green (but sometimes yellow or brown)

Luster: Greasy

Cleavage: Yes, but the crystals do not break easily along it

The Pyroxene Minerals
The Pyroxene minerals are usually found in extrusive igneous rocks named basalt and in meteorites. The various kinds of Pyroxene minerals are Diopside, Augite, Enstatite, Wollastonitea and Hypersthene. Si2O6 is included in all these varieties. The difference is that in certain varieties magnesium (mg) and iron (Fe) are present, while in others sodium (Na) is present or all these three elements are combined with pyroxene minerals. The general characteristics of the ordinary Pyroxene minerals like Augile are as follows:

Fracture: Most have an uneven and brittle fracture

Shape: Orthorhombic or monoclinic

Cleavage: Two planes that meet at nearly a 90-degree angle

Luster: Glassy or metallic

Hardness: 5-6.5 on ⦁ Mohs Hardness Scale

Streak: White, light green or light brown

Color: Black

Amphibole
Among the amphibole group there are varieties of minerals; however, Hornblende is the one which is generally found. In the majority of igneous rocks we find tiny crystals of Hornblende. Generally they appear as dark spots. In order to see them, either use a magnifying glass or a hand lens.

Hornblende

Fracture: Uneven brittle fracture

Shape: Monoclinic  (crystals look like short, six-sided columns)

Cleavage: Two planes that meet at a 124-degree angle

Luster: Glassy or milky

Hardness: 5-6 on Mohs Hardness Scale

Color: Black or dark green, translucent to opaque

Streak: Gray-green or gray-brown


Mica Minerals
Certain rocks sparkle due to the presence of mica minerals! Normally we find them in igneous, metamorphic rocks like, schist and in granites. Reflection of light on their plane surface causes them to sparkle. It is here where the minerals break through their cleavage planes. The breaking takes place with ease all along their cleavage planes, so much so that certain crystals shear into several thin layers that appear as pages of a tiny book. The Colonial Americans made use of these big sized mica pages to substitute for their glass windows.

The properties of the two major varieties of mica which are generally found, are as follows. Look at the similarity in their other properties than their color. This is the way to discriminate them.

Feldspar
Feldspar is the most common mineral in the Earth’s crust, so you are very likely to find it in the rocks you collect. In all three rock types, but it is most common in intrusive igneous rocks like granite where the crystals look white or pink.

There are several types of feldspar. The characteristics of the two most common types are listed below. These two common types of feldspar are difficult to tell apart besides their color. Color can be helpful, but beware, because the same mineral can often have different colors. The sure way to tell these two apart is by looking at the crystal surfaces for thin parallel groves called striations. Plagioclase feldspar has striations but orthoclase feldspar does not.

Quartz
The mineral that we find most in the crust of the earth is Quartz.
In pure quartz, we find only the elements oxygen (O) and Silica (Si). In case, after magna is cooled and silica is the residue, after the formation of feldspars, there is a chance for quartz to form.

Hardness: 7 on Mohs Hardness Scale
Color: Quartz is either white or colorless. Certain types are smoky or pink.
Luster: Greasy or Glassy.
Cleavage: None
Streak: White
Shape: Trigonal (crystal is a prism of 6 sides and the end is in the shape of a pyramid).

Structural Classification
The silicates can be divided into many subclasses depending on their structures. The following subclasses are:

Nesosilicates
This is the simplest subclass of the silicate group having single tetrahedron, which means the tetrahedrons are not bonded to one another. It is also referred to as Orthosilicates. The structure of this type of silicate imparts a close packing and bonding of ions to create a stronger and harder structure. Gemstones are more abundant in this class than any other group. The common members of this subclass are Andalusite, Euclase, Forsterite, Datolite, Fayalite and many more.

Sorosilicates
This subclass member has double tetrahedrons bonded by one ion of oxygen. The structure of sorosilicate is very unique with the shape of an hourglass. This type of silicates is the smallest in appearance when compared to the other subclasses. Some of the common members are Epidote, Hemimorphite, Zoisite, Allanite, Ilvaite and Idocrase.

Inosilicate
This subclass has two groups of members, one is the single chain and the other is the double chain members. The silicate and oxygen ratio is 1:3 respectively in a single chain, and in double chain it is 4: 11. Single chain examples are Lorenzenite, Okenite, Augite, Neptunite and many more. The double chain examples are Actinolite, Edenite, Anthophyllite, Cummingtonite etc.

Cyclosilicate
This type of silicates also forms chains, but in the shape of a ring. The ratio of silicon to oxygen is 1:3. The examples are Axinite, Benitoite, Cordierite, Baratovite etc.

Phyllosilicate
This subclass member has tetrahedron rings, which share oxygen to other tetrahedron rings. The sharing is in a specific way, which develops sheetlike forms. The ratio of silicon to oxygen is 1:2.5. The examples are Apophyllite, Chrysocolla, Cavansite etc.

Tectosilicate
The other name of this subclass is “Framework Silicates” because the structure is uniquely designed with the tetrahedron projecting outside from all the directions. In this structure, the other tetrahedrons get all the shared oxygen and the ratio of silicon to oxygen is 1:2. Some of the examples are Andesine, Bytownite, Microcline, Oligoclase and the like.

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