MINERALOGY - GLY 252 Tectosilicates - Quartz

Department of Geology

“Leading Minds”

Framework silicates Each tetrahedron shares all four oxygens with neighbours.

Form a 3-D structure.

Feldspar and quartz

Silica Minerals The SiO4 tetrahedra share four O atoms with their

neighbours and join up to form a interlocking 3-D framework. The sharing of oxygens results in the Si:O ratio of 2:1, hence SiO2

Si-O links form strong covalent bonds and interlocking the structure results in a high hardness.

Several SiO2 polymorphs are stable at various P & T Most common mineral is quartz

Physical properties Hardness: 7 Habit: Hexagonal prisms with pyramidal terminations, massive Vitreous – glassy looking Colourless-white-grey when pure Purple – Amethyst Brownish-grey – Smokey quartz Pinkish – Rose quartz Yellowish – Citrine No cleavage Quartz is chemically inert- it is thus

extremely resistant to chemical and physical weathering and is consequently very abundant

The most abundant mineral in the crust

Stability of SiO2 -polymorphs

Tectosilicates Low Quartz

001 Projection Crystal Class 32

Tectosilicates High Quartz at 581oC

001 Projection Crystal Class 622

Tectosilicates Cristobalite

001 Projection Cubic Structure

Tectosilicates Stishovite

High pressure → SiVI

Tectosilicates Low Quartz Stishovite

SiIV SiVI

Komatiite

650oC

Coarse Fine

Grain size

Basic mineralogical/chemical classification

GRANITE RHYOLITE

DIORITE ANDESITE

GABBRO BASALT

Classification of igneous rocks

There are various mineralogy and geochemical classification schemes for igneous rocks

QAFP: CLASSIFICATION

QAFP: a classification system based on the proportions of the following minerals Q = QUARTZ A = ALKALI FELDSPAR F = FELDSPATHOID P = PLAGIOCLASE

QAFP – phaneritic rocks

QAFP – aphanitic rocks