QUARTZ

General Formula:
SiO2
Sample: M-20 System:
Hexagonal (trigonal)
Orthoquartzite
Subrounded quartz grains cemented in a matrix of silica. The grain boundaries are clearly marked by the fine grained inclusions.
Field of View 2.7 mm, plane light
Orthoquartzite
Note the variety of orientations present in the randomly oriented quartz grains.
Field of View 2.7 mm, crossed polars
Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Optical Properties

Colour
Pleochroism
colourless
non-pleochroic
Form typically anhedral to highly irregular in igneous and metamorphic rocks. Detritial grains are are more or less equant.
Relief/
RI
low positive relief
nw = 1.544
ne = 1.553
Cleavage not observed
Birefringence
Interference Colours
0.009
Maximum interference colour range up to first order white, with a tinge of yellow
Twinning non observed
Interference Figure/
Optic Sign
2V
unixial positive, but strained displaying undulatory extinction may show a biaxial interference figure with a small separation of the isogyres. Optic Orientation optic axis is the c axis. Elongate crystals cut from end to end are length slow.
Composition essentially pure SiO2, although trace amounts of Ti, Fe, Mn, Al may be present. Alteration not readily altered and is very stable in weathering environments.
Occurrence one of the most widely abundant minerals, present in a wide variety of environments. Distinguishing Features low relief, low birefringence, lack of cleavage, uniaxial positive.


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