Section M 5.1.1.
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The pyroxenes are a major group of silicate minerals found in the lunar crust.

The basic pyroxene structure is based on chains of SiO4 tetrahedra. Two different kinds of octahedral sites hold cations of varying sizes. Hence complex substitutions are possible. Such cations include Mn, Fe2+, Cr3+, Cr2+, Ti4+, Ti3+, and Al. K is too large to fit into either site. Pyroxene compositions are described in terms of MgSiO3 (enstatite), CaSiO3 (wollastonite), and FeSiO3 (ferrosilite), although 50 % wollastonite compositions never crystallise with the pyroxene structure.

There are two main groups of pyroxene, depending on the crystal system: Orthopyroxene (orthorhombic) and Clinopyroxene (monoclinic). Orthopyroxenes are low in Ca. Clinopyroxenes are also split into low Ca (pigeonite) and high Ca (augite) forms.

Lunar pyroxenes show reactions below their melting temperatures. Exsolution of clinopyroxene into augite (high-Ca clinopyroxene) and pigeonite (low-Ca clinopyroxene). Study of these exsolution lamellae has shown that igenous and metamorphic pyroxenes have cooled slowly. Estimated cooling rates range from 1.5-0.2 K/hr for lava flows 6m thick (Apollo 15 data).

Shock lamellae are also seen in pyroxenes. These are caused by meteoroid impact. These are rare though.

Pyroxenes in highland coarse crystalline rocks (eg. anorthosites) tend towards the Mg rather than Fe compositions.

Density:  2.8-3.7 g/cm3
Seismic Velocities:  6.8 km/s (10bar) to 9 km/s (10000bar)
                     P wave, Density=3.2g/cm3  (Sonoma Co.)

See the Handbook of Lunar Minerals for Elastic, Thermodynamic, and
Electric data.

Melting Point:  1830 +- 2 K
Enthalpy of Melting:  61.5 kJ

Orthopyroxene   (Mg,Fe) SiO2
Varies from Enstatite (MgSiO2) to Ferrosilite (FeSiO2)
Density varies from 3.2-4.0 g/cm3.
Colour: Enstatite (greens) to Ferrosilite (browns)
Pleochroism: red/browns/yellows/etc
Partially decomposed by HCl.

Pigeonite (Mg,Fe,Ca)(Mg,Fe) Si2O6
Density: 3.17-3.46 g/cm3
Colour: Brown/black/green (yellowish in thin section)
Pleochroism: Greens/browns
Insoluble in HCl

Augite (Ca,Mg,Fe2+,Fe3+,Ti,Al)2 (Si,Al)2O6
Density: 3.19-3.56g/cm3
Colour: Brown/green/black
Pleochroism: Green/yellow/bbrown/violet
Insoluble in HCl


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