Plagioclase

Feldspars have a framework structure of linked SiO₄ and AlO₄ tetrahedra. The ratio Si:Al varies between 3:1 and 1:1. Ordering of Si and Al can lead to complexities in the feldspar structure. The vast majority of lunar feldspars are members of the plagioclase group (exceptions being the K and Ba enriched feldspars). Plagioclase feldspars consist of solid solutions between albite (NaAlSi₃O₈) and anorthite (CaAl₂Si₂O₈), due to substitution of NaSi and CaAl.

The precise structures of intermediate compositions are not that well known.

Lunar plagioclases are depleted in Na relative to terrestrial plagioclases.

Highland impact melts and KREEP (K, REE, P encriched) rocks have more Na-rich compositions (i.e., less anorthite), and are also enriched in the KREEP elements.

Coarse-crystalline plagioclases (e.g., from the anorthosites: almost 100% plagioclase) have a much more restricted range of compositions.

Plagioclase may undergo sub-solidus reactions. For example, the exsolution of certain elements (Ca, Fe, Mg, Si, etc.) to form exsolution lammellae. If the spacing of such lamellae is correct, they can form a diffractive grating. The resulting irridescence is known as labradorescence, and has been used as a decorative stone.

Density is in the range of 2.6-2.7g/cm³. Symmetry is triclinic. Under crossed-polars, lamellae twins provide a characteristic feature of plagioclase crystals. Measuring distinction angles of such twinning can provide the anorthite:albite composition of the feldspar. In hand specimen, plagioclases are usually tabular or lath shaped, and have a whitish color (although colored varieties do exist).

Anorthite is soluble in HCl, whilst albite is not.

Most lunar plagioclases have compositions greater than 80% anorthite.

Current known abundances:

Mare bassalts        15-35 %   Roughly the same for high-Ti and low-Ti
Anorthosites         40-98 %   (<70 % very rare)
Crystalline breccias 50-75 %   Lunar highlands only
Vitric breccias      15-50 %   Very fine grained
Light-mixed breccias 70-90 %   Most abundant at Apollo 16 site
Soil                 10-60 %   Resemble local bedrock

Physical Data

Molar volume:  100.73 +-0.15 cm3
Formular weight: 278.21g
See the Lunar Materials Handbook for elasticity, enthalpy, and electric data.

Seismic velocities: P   6.5 (10bars) - 7.1km/s (10000bars)
                    S   3.6 (10bars) - 3.8km/s (10000bars)
Melting point: 1823 K