Quartz

Quartz Crystals from Le Collet d’Allevard, Isère, France. Dimension (mm): 45 x 44 x 33 mm. Modified from Source: www.abijoux.com

Composed of SiO2, quartz is one of the silica group minerals most common in Earth’s crust, but never found in meteorites as inclusions visible to the naked eye. Quartz in meteorites has been found in very small quantities in eucrites, other calcium-rich achondrites, and in the highly reduced E chondrites1.

As reported in the 2018 paper, “First evidence for silica condensation within the solar protoplanetary disk“, the research team found an AOA in the CR2 meteorite Yamato 793261 containing typical AOA minerals and ultrarefractory minerals along with grains of quartz (which formed at comparatively lower temperature). These quartz grains measured ∼5 µm in diameter.

 

 

 

 

 

 

 

High and Low Pressure SiO2 Polymorphs. Modified from Source: www.science.smith.edu

Low-pressure SiO2 polymorphs include quartz (α and β), tridymite and cristobalite. High-pressure polymorphs include coesite, stishovite

Quartz occurs in two forms (α and β) that are related by a displacive transformation. The high-temperature β form is not observed in nature because it converts into the low-temperature α form at 573 °C at 1 kilobar (0.1 GPa) of pressure.

 

 

 

 

 

Image Source: www.uwgb.edu

The structure of quartz can be most easily visualized as corkscrewing chains (helices) of silicon tetrahedra aligned along the c axis. The corkscrews take four tetrahedrons to repeat (or three turns in which each tetrahedron essentially rotates 120°). Each helix is connected to two adjacent ones at each tetrahedron.

There is only minor substitution of other elements for Si in quartz. Two trace substitutions postulated for quartz are: (Ti4+)tetrahedral ↔ (Si4+)tetrahedral and (Al3+, Fe3+)tetrahedral + (Fe3+, Na+, Li+, K+)interstitial ↔ (Si4+)tetrahedral. However, these substitutions are responsible for many of quartz’s variable colors. For example, amethyst gets its purple-violet color from Fe3+, rose quartz its pink from Ti4+ (and inclusions?), smoky quartz its gray-black color from Al3+, and citrine its yellow from Fe (and irradiation).


Some or all content above used with permission from J. H. Wittke.