David Virgo

OXIDATION STATE OF IRON IN PLAGIOCLASE FROM LUNAR BASALTS.

Abstract Mossbauer spectra of 57 Fe at 77 and 295°K in the plagioclase from basalts 10044 and 12021 show that iron is in the high-spin ferrous state and located at at least two distinct positions with different coordination numbers. Some excess resonant absorption is probably due to Fe 3+ although the Fe 3+ doublet could not be positively resolved. The ratio Fe 3+ /(Fe 2+ + Fe 3+ ) is estimated to be between 0.02 and 0.1.

Re-evaluation of the cation distribution in orthopyroxenes by the Mössbauer effect

Abstract The resolution of Mossbauer spectra of orthopyroxenes is improved by measurement of the absorbers at temperatures close to liquid nitrogen. Some previously published data on the ferrous ion distribution betweenM1andM2 have been reevaluated.

Cooling History of Orthopyroxenes

Order-disorder transitions between ferrous iron and magnesium in orthopyroxenes [minerals close to the composition (Fe, Mg) SiO3] occur rapidly between approximately 480� and 1000�C. Disordering and ordering have been studied experimentally. The determination of the metastable ferrous iron site occupancy in orthopyroxenes from rapidly cooled volcanic rocks provides information on the cooling rates, especially from 600� to 480�C.

Mössbauer Effect and High-Voltage Electron Microscopy of Pyroxenes in Type B Samples

Site occupancy numbers for ferrous iron, magnesium, and calcium at the Ml and M2 sites in lunar clinopyroxenes are estimated from nuclear gamma-ray resonant absorption spectra of 57Fe. The cation distribution is ordered; calcium and magnesium prefer M2 and Ml, respectively. The distribution corresponds to an equilibrium at a temperature lower than 680�C. Crystals cleaved and sectioned by diamond-knife ultramicrotomy were examined by high-voltage (200 kv) electron microscopy and diffraction. Uniform 300-to 600-A-wide bands that correspond to single crystal domains were found. Correlation of the bands with magnetic ordering at low temperatures is considered.

State and Location of Iron in Apollo 11 Samples

The oxidation state of iron has been investigated in the lunar soil, in clinopyroxene and glass separated from the soil, and in clinopyroxenes separated from two igneous rocks (microgabbros). In the soil, iron is exclusively in the ferrous state at octahedrally coordinated positions or in the metallic state. A small amount of metallic iron is probably paramagnetic. The ferric to ferrous iron ratios in the clinopyroxenes are zero with an error limit of ±0.01. The site occupancy in the clinopyroxenes is an ordered one, calcium and magnesium occurring at the M2 and M1 positions, respectively. Ferrous iron fills the remaining M1, M2 sites.