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Other Silicates
Section M 5.1.5.
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Other Silicates

All of these silicates have been described as of trace concentrations in the regolith, by NASA.

Zircon

Zircon has the formula of ZrSiO4, but tends to concentrate REEs. It is also noteworthy due to its usefulness in dating rocks. This is because it is a refractory mineral which resists remelting and often incorporates Hf, Th, U in its crystal structure. Hence it is well suited to U-Pb dating (and is the basis of terrestrial "oldest crust" rock dates). It has also been shown that their U-Pb systematics are unaffected by meteor bombardment! Although rare, small, and difficult to work with, lunar zircons have proved to be extremely important in dating lunar samples - especially the very old highland material.

Zircons also tend to keep fission tracks (linear damage caused by spontaneous fission of U atoms), and is hence also suited to fission track dating methods.

The main source of lunar zircons are the rare high-silica "granitic" rocks. Sample 15405 (breccia of quartz monzodiorite composition) contained 0.6 vol% zircon. However, most lunar zircons are found as isolated grains in soils and breccias. This is due to the rarity of lunar granitic rocks, and the durability of zircons. They have also been found in metamorphosed basalt clasts - possibly due to the breakdown of original tranquillityite.

Density is in the range of 4.6-4.7 g/cm3 for fresh samples. Colors are red/brown/yellow/grey/green. It is slowly attacked by hot concentrated H2SO4.

The crystal group is tetragonal.

Pyroxferroite

This is the lunar iron analogue of pyroxmangite. Composition is (Mn, Fe) SiO3, although terrestrial pyroxmangite never has more than 25 % Fe : Mn. Pyroxmangite is pinkish-lilac in color. Pyroxferroite is found in mare basalts and Fe-rich basalts. Although produced artificially in the lab, pyroxferroite (rather than pyroxmangite) has never been found on Earth.

Amphibole

The amphiboles are a complex group of minerals. On Earth they are typically formed during metamorphism, or in some igneous rocks. They can also survive the sedimentary cycle. General composition is (Ca, Mg, Fe) (Si, Al)8 O22F although many different substitutions are possible.

Garnet

NASA list garnets as being possible contamination products and may not be of indigenous lunar origin. On Earth, garnets are surprisingly common, and are produced by moderately high pressure metamorphism. Before everyone gets excited that these might be mined for semi-precious gems to be shipped back to Earth, it should be borne in mind that the majority of garnets on Earth are worthless due to inclusions, fractures, or non-gem compositions. I, myself, have seen hundreds if not thousands of garnets in thin section - all have had inclusions in them. I have even picked moderate sized samples (8mm or so) from cliffs in Scotland!

Tranquillityite

Composition Fe8(Y,Zr)2Ti3Si3O24, this mineral is named after the Apollo 11 landing site. Density is 4.7 g/cm3. This is found in lunar mare basalts where it forms small laths less than 100µm long. It is often associated in small pockets with apatite and pyroxferroite. These are the last minerals to crystallize. It is semi-opaque and non-pleochroic. In transmitted light, it is deep red, but is grey in reflected light. The deep red is due to the high Ti content.

Other Silicates

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