Section M 5.1.3.
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Olivine is a major consituent of the lunar crust. Olivine varies between the two end-member compositions of Fayalite (Fe2SiO4) and Forsterite (Mg2SiO4). The abbreviations Fa and Fo are often used. %mol are often added. For example Fa20 refers to an olivine which is 20%mol fayalite and 80%mol forsterite composition. Because olivine is such a major mafic mineral, it can be found in almost every rock sample - including such things an anorthosites which are virtually all plagioclase feldspar.

Mare basalts contain olivines which range in composition from about Fa20 to Fa70. Fe-rich compositions above Fa70 are rare, although Fa90-Fa100 samples have been found. These are from late stage mare basalts which have become rich in iron. Significant contaminant elements include Ca, Mn, Cr, and Al. These varies directly with Fe content, and possibly cooling rate. Cr is particularly notable as it is found at much greater concentrations (up to 0.6wt%) than in terrestrial olivines (approx. 0.1wt%). This is probably because the Cr is in the form of Cr2+ - i.e., a low oxidation state. This is due to the low oxygen partial pressures during mare basalt crystallization. Similarly Cr2+ is much more common in lunar pyroxenes than in terrestrial pyroxenes.

                      Forsterite              Fayalite

Composition           Mg2SiO4                 Fe2SiO4
Density               3.19-3.21               4.39         g/cm3
Molar volume          43.79 ± 0.03            46.39 ± 0.08 cm3
Formula weight        140.694                 203.79       amu
Melting point         2163 ± 20               1478 ± 2     K

Further thermodynamic data (enthalpy changes, etc.) can be found in NASA's Handbook of Lunar Materials.

General olivine parameters at 4 kBar:

Seismic Velocity (P)    8.13-8.32 km/s  
Seismic Velocity (S)    4.57-4.86 km/s  
Poisson's Ratio         0.24-0.27
Lab experiments on terrestrial olivines show the P and S wave velocities decreasing to 8.6km/s and 4.37 km/s for 70 bars.

Dielectric Constant    7.45-8.59   (fayalite)
Thermal conductivity   8.7 cal/(cm sec K)

Olivine gelatinizes in HCl. It is also very susceptible to hydrothermal alteration, to weathering, and to low-grade metamorphism. Only the latter might be relevant to the moon.

Colors range between yellow and green, and, strangely enough, olive!


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