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Spinels
Section M 5.2.2.
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Spinels

Spinels

The spinels, when combined, occur with minor abundances (typically below 2 %, but sometimes approaching 10 %). The general formula for the spinels, is AB2O4. "A" refers to the 4-cornered (tetrahedral) sites, and "B" refers to the 6-cornered (oxtahedral) sites. The basic structure is that of a cubic array of oxygen atoms, with "A" and "B" cations accommodating the appropriate sites. The Normal Spinel structure is where the divalent cation (eg. Fe2+) only occupies the "A" sites. If half of the divalent cations occur on the "B" sites, it is known as an Inverse Spinel.

The various compositions of spinels can be represented by a Johnston Compositional Prism (Lunar Sourcebook: Fig. 5.24; DHZ: Fig 201). The entire spinel group can be split into three main series: the Spinel, Magnetite, and Chromite Series; depending on whether the trivalent ion is Al, Fe, or Cr respectively.

The spinels are insoluble or soluble with difficulty in acids. Spinels are decomposed by fusion with KHSO4.

Density varies from 3.55g/cm3 (Spinel senso stricto: MgAl2O4) to 5.34g/cm3 (Franklinite: ZnFe2(3+)O4).

Most lunar spinels are represented by the three component system of FeCr2O4 - FeFeTiO4 - FeAl2O4. Mg often substitutes for Fe2+, although the Mg content is never greater than that of Fe. Most lunar spinels fall between the compositions of chromite (FeCr2O4) and ulvöspinel (Fe2TiO4). The typical cation substitution is Fe2+ + Ti4+ = 2(Cr,Al)3+. Other cations that often occur include V, Mn, Zr.

Spinels are ubiquitous in lunar mare basalts, and are invariably compositionally zoned. Chromite is typically the first mineral to crystallise from the parent melt. As the crystals grow, their TiO2 and FeO contents increase, whilst their Al2O3, MgO, and Cr2O3 contents decrease. Hence, the overall composition evolves towards that of ulvöspinel. Where titanian chromites and chromian ulvöspinels are found together, the latter phase is found in the form of overgrowths and rims surrounding the original chromite crystals.

Spinels also occur in highland rocks (eg. anorthosites, gabbros, troctolites and impact mixtures of these types). Spinels in anorthositic rocks tend to be chromite. Some highland rocks (especially the olivine-feldspar types [troctolites]) contain pleonaste spinel (midway between MgAl2O4 and FeAl2O4, but with some Fe and Cr).

Due to the low oxygen partial pressure, subsolidus reduction is often found in lunar spinels. Terrestrially, the presence of oxygen leads to oxidation rather than reduction. Lunar ulvospinels are often reduced to ilmenite + native Fe. In turn, ilmenite is (rarely) reduced to rutile + native Fe or to chromite + rutile + native Fe. The causes for this reduction are currently speculative.

Spinels

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