Feldspar, Feldspathoid and Quartz group

By : Trishnangini Deka

Dpt. Of Applied Geology

Bangalore University.

FELDSPAR GROUP

Introduction

This is the most important group of rock forming silicates which occur in igneous, sedimentary and metamorphic rocks. They constitute about 2/3rd of the igneous rocks.

Atomic structure

The feldspars are tectosilicates with a structure of a framework silicate of (Si, Al) O4 are linked to one another by sharing oxygen ions in all directions. The resultant framework of tetrahedra has large interstices,occupied by the pottasium ions in the alkali feldspars.

Types

Feldspars are of two types :

1)Alkali feldspar(microcline,orthoclase and sanidine)

2)Plagioclasefeldspar(albite,oligoclase,andesine,labradorite,bytownite and anorthite)

Chemical composition They are alumino silicates of potassium,

sodium, calcium and barium, and may be regarded as isomorphous mixtures of the four end member given below, of which the first 3 are common while the fourth is rare.

1)Orthoclase and microcline. (KAlSi3O8)

2)Albite. (NaAlSi3O8)

3)Anorthite (CaAl2Si2O6)

4)Celsian (BaAl2Si2O6)

Physical properties 1)Colour-White,grey or flesh

2)Form-Tabular

3)Lustre-Vitreous

4)Streak-White

5)Cleavage-2 sets, but one is better developed.

6)Fracture-Even to uneven

7)Hardness-6

8)specific Gravity-2.57 to 2.7

Optical properties In Plane Polarised light

1)Colour-Colourless, turbid

2)Form-irregular

3)Relief-Low to moderate

4)Cleavage-Absent

In Crossed Nicols

1)Anisotropism-Anisotropic mineral

2)Twinning-Plagioclase feldspar typically shows lamellar twinning

3)Interference colour-The feldspars show grey or white interference colours of low

order.

Varieties 1)Sanidine-A high temperature

pottasium feldspar 2)Adularia-Low temperature

orthoclase 3)Moon stone-Opalescent adularia

or albite 4)Aventurine-Gem variety of albite 5)Amazon stone-Bright green

microcline

Occurrence1)Plagioclase are abundant in all types of igneous rocks except the ultrabasic rocks.

2)In basic and intermediate lavas, plagioclase phenocrysts, as well as small grains in the ground mass are abundant and exhibit zoning.

3)Alkali feldspars are restricted to alkali and acid igneous such as syenites, granites, metamorphic gneisses and less frequently in sedimentary rocks like arkose.

Feldspathoid Group

Introduction

The feldspathoids are chemically similar

to feldspars in that they are aluminosilicates of chiefly potassium, sodium and calcium but with minor amounts of other ions.

Atomic structure Their structures are aluminosilicates

framework in whose interstices the cations are bound and which occasionally play host to unusual anions as well.

CompositionAt high temperature there is complete

solid solution to kalsalite(K4Al4Si4O16) but at most magmatic conditions, intermediate compositions are not possible and nepheline typically has 3:1 Na:K ratio.

Physical properties 1)Colour-White or gray

2)Lustre-Sub vitreous to somewhat greasy

3)Streak-White

4)Cleavage-Poor cleavage

5)fracture-Sub conchoidal

6)Pleochroism-Non

Optical properties 1)Colour- Colourless

2)Relief- Low posituve most common, also low negative

3)Interference Figure -uniaxial negative

4)Alteration- Nephaline may alter to clay minerals,analcime,sodalite,calcite,and cancrinite

OccurenceNephaline is a common mineral in alkali

rich silicon poor plutonic and volcanic igneous rocks such as nepheline syenite, foidite and phonolite.

Nepheline also may be found in some contact metamorphosed rocks adjacent to alkali rich intrusives, presumably as a result of sodium metasomatism.

QUARTZ GROUP

Introduction

Quartz is the most abundant and an essential constituent of igneous, sedimentary and metamorphic rocks. Due to their piezoelectric effects,these are the most sought after mineral in the electronic industry. In nature,quartz occurs as crystalline, cryptocrystalline or amorphous variety.

Atomic StructureThe structure of quartz is

formed by Sio4 tetrahedra by sharing each of their corners with another tetrahedron. As a result,the Si:O ratio becomes 1:2 and it results into a stable 3-Dimensional frework (tectosilicates) with identical properties throughout the crystal.

Chemical Composition

The quartz usually occurs in nature in its pure form as Sio2.However ,some traces of other elements are also known to occur. The most common amongst them is of Al,Fe,Na,Li or K. Sometimes fluid inclusions may be added to the silica structure during the cavity filling processes. Hence, the substitution of Al for Si is also accompanied by the introduction of Na or Li within the atomic structure

Physical properties 1)Colour- Pure quartz is colourless but with addition of impurities, it turns smoky, pink or grey. The clear transparent variety is termed as rock crystal and violet (or purple) variety as amethyst. 2)Form- Crystallised, crystalline, cryptocrystalline or amorphous variety are common. Crystals occur as hexagonal prisms terminated by rhombohedral faces. 3)Streak- Colourless 4)Lustre- vitreous to sub vitreous 5)Fracture- Conchoidal 6)Cleavage- absent 7)Hardness- 7 8)Specific gravity- 2.65

Optical properties In Plane polarised light 1)Colour-Colourless 2)Form-Irregular(anhedral),rounded or as phenocrysts having euhedral form in some porphyritic igneous rocks. 3)Relief-Low 4)Cleavage-absent

Between crossed nicols1)Anisotropism-Mineral is isotropic2)Interference colour-It shows dull double refraction

Occurence1)Quartz is a very common mineral found in a

wide variety of geological environments. 2)It is abundant in felsic to intermediate intrusive

and extrusive igneous rocks such as granites, granitic pegmatite, granodiorite, quartz diorite, rhyolite and may be found in small amounts in diorite, gabbro, syenite and volcanic equivalents

3)In metamorphic rocks it is abundant in slate, phyllite, gneiss and quartzite of various types.

4)Hydrothermal vein and replacement deposits typically contain quartz as a gangue mineral.

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