Page 133 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS
SILICA GROUP
DISTINGUISHING Serpentine minerals have lower birefringence and lower refractive 1470 oc at atmospheric pressure, and the temperature of this inversion
FEATURES
indices than chlorite and fibrous amphiboles. Most chlorites exhibit does not change with increasing pressure. Finally, at 1713 °C cristoba-
either stronger birefringence or anomalous interference colours. lite melts and the liquidus boundary is reached. This diagram can, of
Brucite can show anomalous colours similar to chlorite, but brucite is course, be interpreted in the other direction, with liquid Si0 2 crystallis-
uniaxial. ing; it can then be determined which polymorph (minerals with same
*OCCURRENCE Serpentine minerals are formed during the alteration of ultrabasic chemistry but different structure) will be encountered at which tempera-
igneous rocks- dunites, pyroxenites and peridotites - at temperatures ture as crystallisation proceeds.
below 400 oc. Chrysotile probably forms first and antigorite then is Other forms of silica, not included in the diagram are coesite, a high
derived from it. pressure phase formed at high temperature ( 400--800 oq with stisho-
Chrysotile is the major variety of commercial asbestos and occurs as vite, a high density collapsed structural form found at very high pres-
economic deposits in Canada, South Africa and Russia. sures and temperatures. It is found in meteorite impact craters, and
minerals possessing stishovite structures may exist in the Earth's upper
Silica group Tektosilicates mantle. Quartz is an essential constituent in acid igneous rocks and
arenaceous sedimentary rocks, and is a common constituent in
The various forms of silica (Si0 2 is the formula for all silica minerals) can metamorphic rocks. The three main quartz minerals are described.
be represented on a simple P- T diagram (Fig. 2.29). This shows that the
lowest temperature form of quartz, called a-quartz (or low quartz), Quartz Si0 2 trigonal, cia 1.09997
inverts to ~-quartz (or high quartz) at 573 oc at atmospheric pressure;
the temperature of this inversion increases with increasing pressure n 0 1.544
(- 670:°C at 3 kb ). At 867 oc, ~-quartz inverts to tridymite; the tem- n. 1.553
a
o.oo9
perature of this inversion also increases considerably with increasing
Uniaxial +ve (length slow)
pressure (to - 1450 °C at 3 kb ). Tridymite inverts to cristobalite at
D = 2.65 H = 7
I~ *coLOuR Colourless.
I I HABIT Euhedral quartz crystals are prisms with hexagonal cross sections, and
I I may appear as phenocrysts in acid extrusive rocks, but quartz usually
I I
I I occurs as shapeless interstitial grains in igneous and metamorphic rocks
I I or as rounded grains in sedimentary clastic rocks.
I I
I I *cLEAVAGE None.
HIGH QUARTZ I I RELIEF Low, just greater than 1.54.
;f I
/ I I LIQUID *ALTERATION None.
/I I *BIREFRINGENCE Low, maximum interference colours are first order white or pale yellow.
/ I I
EXTINCTION Straight on prism edge.
/ I CRISTO- I
/ I BALITE I TWINNING Numerous types of twins occur, in particular Brazil (twin plane 1120)
/ I I
/ I I and Dauphine (twin axis is c axis), but twinning is not detectable under
/ I I the microscope because optic orientation in both twin parts is identical
/ I I in both types of twin.
TRIDYMITE I I
I .---.---1 OTHERS In some porphyritic acid extrusive and hypabyssal igneous rocks where
I __..--- I quartz occurs as phenocrysts, the crystals may show corroded margins
l--.--- I
----1 I because of a reaction between the quartz and the magmatic liquid.
--- I I occuRRENCE Quartz is an essential mineral in acid igneous plutonic rocks such as
I I
granites and granodiorites, but may be present in diorites and some
O 5!XJ ltxlO gabbros. In these, quartz occurs as shapeless grains. In rapidly cooled
0
Temperature ( C)
extrusive and hypabyssal rocks, for example rhyolites, dacites, pitch-
Figure 2.29 Pressure-temperature diagram for Si0 2 (after Tuttle & Bowen stones and quartz porphyries, quartz may occur as euhedral pheno-
1958). crysts. It is also found as large late formed crystals in pegmatites, and is a
120
121