Silica Minerals

The three silica minerals--quartz, tridymite, and cristobalite--are found on the moon. Unlike on Earth, where such minerals (especially quartz) are very common in the crust, they are rare on the moon. This is for two main reasons. Firstly, the lunar crust is not as "evolved" (i.e., processed via repeated melting epsiodes)--this tends to concentrate silica. Secondly, the moon lacks hydrous systems--many of which crystallize silica in a variety of forms (including cherts).

Quartz is stable at room temperature, whilst tridymite is stable between 870 and 1470^oC. Cristobalite is stable between 1470 and 1713^oC (silica melting point). All are metastable over a wider temperature range. All are metastable at room temperature and pressure. All can exist in two forms "alpha" (low-T) and "beta" (high-T) forms. The transformation is instantaneous--i.e., metastability is not seen. The differences are in the symmetry. The beta-form has a higher order of symmetry. E.g., alpha-quartz is trigonal, beta-quartz is hexagonal.

High pressure polymorphs coesite (formed at 450-800^oC and 38 kbar), and stishovite (130 kbar, > 1200^oC) have been found on Earth in impact structures. Coesites have also been reported from kimberlite xenoliths (source = deep mantle) whilst stishovite has only been reported from Meteor Crater, Arizona. Interestingly, neither have been found on the moon. This could be due to the rarity of silica in the meteorite "target" rock, or the rapid volatization of silica in the high vacuum present on the moon.

Lunar silica minerals concentrate with the KREEP elements. Quartz has been found in felsite clasts as needle-shaped crystals. These may represent the structural transformation of earlier tridymite. Quartz is also found in rare granite clasts (these are plutonic masses, lacking the hydrous minerals found in terrestrial granites).

The most common silica in mare basalt is cristobalite. This can make up to 5% (vol.) of some basalts. Some inversion does occur. Cristobalite and tridymite are often found together in mare basalts. Fine grained basalts tend to have cristobalite and quartz.

Typical contaminants include Al₂O₃, TiO₂, CaO, FeO, Na₂O.

                                     Physical Properties
                                 Formula weight SiO2 = 60.09 g

                                  Density          Symmetry

                a-quartz          2.648 g/cm3      Trigonal
                b-quartz          2.533 g/cm3      Hexagonal
                a-cristobalite    2.334 g/cm3      Tetragonal
                b-cristobalite    2.194 g/cm3      Cubic
                a-tridymite       2.265 g/cm3      Orthorhombic
                b-tridymite       2.192 g/cm3      Hexagonal
                coesite           2.911 g/cm3      Monoclinic
                stishovite        4.287 g/cm3      Tetragonal

Footnote: Symmetries quoted in NASA's Handbook of Lunar Materials is incorrect. The above are correct and verified from Deer, Howie and Zussman's Rock Forming Minerals ("DHZ") and The Lunar Sourcebook. See DHZ for space groups and further elaboration.

See the Lunar Handbook for tables of elastic constants vs. pressure,etc.

Cleavage is absent, except for tridymite which may exhibit a poor prismatic cleavage. Colors are clear or white. Cristobalite may show a yellowish color. Quartz is often colored on Earth, due to various impurities (e.g., amethyst). All are colorless in thin section.

Chemistry

All three are insoluble in acids, except HF. Coesite is almost insoluble in HF, but is dissolved in (NH₄)HF₂. Stishovite is even more insoluble in HF than coesite.

The silicas are soluble in molten Na₂CO₃.

Quartz can be, and is, produced artificially. This depends on the fact that quartz is much more soluble above 300^oC in sodium metasilicate than below. Solution and crystallization rates of quartz in water can be accelerated several orders of magnitude by NaOH and NaCl solutions. On Earth, quartz abundance and resistance to both chemical and mechanical attack leads to it being concentrated by erosional and sedimentary processes. Hence it forms the bulk of many sedimentary rocks (e.g., sandstones).

Tridymite has been crystallized from silica gel in the presence of fluorides and carbonates of K and Na.

Miscellaneous

Quartz exhibits piezoelectricity. I.e., an electrical field is produced when a crystal is subjected to mechanical stress. Similarly, an applied electric field will lead to deformation of the crystal.