Page 135 - A Practical Introduction to Optical Mineralogy

SiliCATE MINERAlS SiliCA GROUP

common constituent in hydrothermal veins accompanying various ore CLEAVAGE A poor prismatic cleavage occurs.
minerals. Quartz is one of the most common detrital minerals owing to RELIEF Moderate, RI considerably less than 1.54.
its lack of cleavage, hardness and stability. Because of this quartz is a ALTERATION None.
common and often essential mineral in coarse terrigeneous rocks such as BIREFRINGENCE Low: very low first order colours.
conglomerates and sandstones, and also occurs in siltstones and mud- INTERFERENCE Very difficult to obtain because of very small crystal size.
stones where its fine grain size is such that detection may not be possible FIGURE
with a microscope. In many sedimentary rocks, including some lime- *TwiNNING Common on { 110}; seen as wedge shaped (sector type) twinning on
stones, authigenic quartz will form. The secondary quartz actually grows basal plane.
around pre-existing grains or forms well-developed crystals, and is occuRRENCE Rare in rocks although tridymite may be found in quickly cooled igneous
formed during diagenesis after deposition of the sediments. Quartz rocks such as rhyolites, pitchstones, dacites and so on. It may be found in
occurs in many metamorphic rocks, usually remaining in the rocks until association with sanidine (see under 'Feldspar group') and sometimes
high grades of metamorphism are reached. At these highest grades a augite or fayalitic olivine. Tridymite has been recorded from high tem-
reaction perature thermally metamorphosed impure limestones.
muscovite + quartz-+ K-feldspar + sillimanite
Cristobalite Si0 2 tetragonal, cia 1.395
may occur, and the grain size of these high grade gneisses will be about no 1.478
the same as granites (greater than 3 mm). ne 1.484
a o.oo3
orthorhombic Uniaxial - ve
Tridymite Si0 2
D = 2.38 H = 6-7
c =-y
coLOuR Colourless.
*HABIT Minute square sectioned crystals are common, but often occurs as
skeletal fibrous crystals in cavities.
CLEAVAGE None.
RELIEF Moderate, considerably less than 1.54.
ALTERATION None.
BIREFRINGENCE Very weak.
INTERFERENCE Basal sections give a uniaxial negative figure but minute crystal size
FIGURE makes this difficult to obtain.
TWINNING Twins on { 111} but not seen in thin section.
occuRRENCE Usually found in cavities in volcanic rocks, ant! has been discovered in
/ some thermally metamorphosed sandstones. Since both tridymite and
/
/
/ cristobalite occur as metastable forms outside their stability field, no
/
/ conclusions can be drawn about the P-T conditions of formation of
/
a = (3 rocks containing these minerals.
n. 1.469-1.479
np 1.470-1.480
n, = 1.473-1.483
a = o.oo4
2V, = 40°-90° +ve
OAP is parallel to (100)
D = 2.26 H = 7
coLouR Colourless.
*HABIT Six sided tabular crystals common, but usually very tiny.
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