a-quartz | b-quartz

Quartz [SiO2]

Mineral Chemistry
Quartz is the most commonly found of all minerals at the earth's surface, and occurs in igneous, metamorphic and sedimentary rocks. Quartz, an essential constituent of acid igneous rocks, is also a principal gangue mineral of veins radiating from intermediate and acid igneous rocks. Because of its resistance to weathering under ambient pH conditions, quartz accumulates as a principal constituent of sediments. Quartz solubility does however increase dramatically in alkaline fluids.

The colour of quartz shows great variety. Names such as rock crystal, milky quartz, blue quartz, amethyst, rose quartz, citrine, smoky quartz, and cairngorm arise from the visible appearance of quartz, with colour attributable to impurities. Cryptocrystalline silica, containing minute crystals of quartz with submicroscopic pores, has the group name of chalcedony. This group includes materials of uniform colour such as jasper, flint or chert, and banded materials such as agate.

Structure
Quartz has a framework structure of SiO4 tetrahedra linked by shared 'corner' oxygens. At atmospheric pressure, a-quartz transforms to b-quartz at 573oC by small displacements of the linked SiO4 tetrahedra - a displacive phase transition. This transition occurs at higher temperatures with increasing pressure. The low temperature form, a-quartz, has lower symmetry than high temperature b-quartz, due to distortions in the structural framework. Thus, six-fold screw axes in b-quartz become three-fold screw axes in a-quartz.

Both a- and b- quartz are enantiomorphic - a relationship similar to that between right and left-handed gloves. In right-handed quartz, spirals of tetrahedra twist one way, while in left-handed quartz spirals twist in the other direction. These relationships are described by the enantiomorphic space groups, P3121 (No. 152) and P3221 (No. 154) for a-quartz and P6222 (No. 180) and P6422 (No. 181) for b-quartz.

A number of polymorphs of silica, SiO2, are found in nature. Coesite and stishovite are high pressure and temperature polymorphs of SiO2. High temperature crystalline forms of SiO2 are tridymite and cristobalite and, like quartz, have low- (a-) and high- (b-) structural modifications.

Listed below are structural parameters for a- and b- quartz.

(i) a-quartz

Structure Trigonal
Space Group: P3221 (No. 154)
a=4.9134 Å c=5.4052 Å
a=b=90.0, g=120.0
Z=3

Atomic Positional Parameters
Si 3a 0.4699 0.0000 0.0000
O1 6c 0.4141 0.2681 0.1188


(ii) b-quartz

Structure Hexagonal
Space Group: P6222 (No. 180)
a=4.9965 Å c=5.4546 Å
a=b=90.0, g=120.0
Z=3

Atomic Positional Parameters
Si 6g 0.5000 0.0000 0.0000
O 12k 0.4147 0.2078 0.1666

References
Y le Page and G Donnay (1976) Acta Crystallographica, B, 32, 2456.

AF Wright and MS Lehmann (1981) Journal of Solid State Chemistry, 36, 371.