| General Formula: SiO2 |
Sample: M-20 | System: Hexagonal (trigonal) |
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| 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. |
