"Quartz is one of the most common minerals in the Earth's continental
crust. It has a hexagonal crystal structure made of trigonal
crystallized silica (silicon dioxide, SiO2), with a hardness of 7 on
the Mohs scale. Density is 2.65 g/cm³. The typical shape is a
six-sided prism that ends in six-sided pyramids"
Quartz occurs in hydrothermal veins and pegmatites. Well-formed
crystals may reach several metres in length and weigh hundreds of
kilograms. These veins may bear precious metals such as gold or
silver, and form the quartz ores sought in mining. Erosion of
pegmatites may reveal expansive pockets of crystals, known as
Quartz is a common constituent of granite, sandstone, limestone, and
many other igneous, sedimentary, and metamorphic rocks.
Tridymite and cristobalite are high temperature polymorphs of SiO2
which occur in high silica volcanic rocks. Lechatelierite is an
amorphous silica glass SiO2 which is formed by lightning strikes in
Quartz and glass are form of silica which does not dissolve in water at normal
condition, but does so at high temperature and pressure:
" study the solubility of amorphous silica at high temperatures and
high pressures ......The experiments were carried out in a bomb made
of Inconel X with an internal capacity of 89 cm3.."
Crystal size depends on the process rate. When grwon from melt, on the
rate of cooling, when from solution on the rate of solvent removal: