Optical properties of lunar glass spherules from Apollo 14 fines

Abstract

Optical spectra of lunar glass spherules are similar to observed spectra of silicate glasses containing both Fe^{2 +} and Fe^{3 +}. Increased absorption in the visible region upon heat treatment in the laboratory is attributed to an increased fraction of Fe^{3 +}. From previous thermodynamic studies in sodium disilicate glass (Johnston, 1964) it is observed that below a fixed oxygen partial pressure the Fe^{2 +} /Fe³⁺ ratio, at a given temperature no longer depends on oxygen pressure but is controlled by the disproportionation reaction 3FeO = Fe₂O₃ + Fe. It is suggested that among the consequences of this disproportionation reaction are (a) that the Fe²⁺/Fe³⁺ ratio in lunar glass may be a measure of the thermal history of the glass-this is supported by the fact that glasses of quite similar chemical composition show significantly different absorption coefficients in the visible-and (b) that the ability for the disproportionation to occur in the liquid and supercooled liquid phases may explain why iron metal is frequently found to be associated with the glassy phase. Birefringence which has been observed in rapidly heat-treated and quenched lunar glass spherules is shown to be explicable only on the basis of a chemical alteration, such as disproportionation or oxidation. Index of refraction changes which result on heat treatment of lunar spherules are not reconcilable with usual glass transition phenomena and hence also suggest chemical change.