Mn-Mg OLIVINES OF LANGBAN-TYPE AND THEIR SYNTHETIC EQUIVALENTS

Abstract

Tephroites and Mn-Mg olivines are the major olivines in the
Langban ore region, while forsterite is rare. Forsterite solid solution in tephroite
varies over a wide range, from Fo60Te40 to pure tephroite. Compositions in the
range of Fo90 TelO to Fo60 Te40 have not been encountered in the studied population of 30 samples. Most of the Mn-Mg oiivines, especially those from the Umgban mine, have a nearly ideal "picrotephroite" composition which is half-way between forsterite and tephroite. With occasional exceptions the olivines may be considered
as binary systems with only subordinate amounts of other cations which
makes it possible to determine cation ordering from plotting a- against b-axis and to estimate the crystallisation temperature, The Umgban "picrotephroitesll are approximately 62-94 % ordered, with a mean ordering of 75 %, Such extensive ordering implies that the crystallisation temperature of the olivines could not have been very high. Synthesis experiments have been performed in the forsteritetephroite system at temperatures below 7000 C and at a pressure of 1 kbar. The results are not very conclusive because of the sluggishness of the reactions but
they may suggest a temperature range of 500-600° C and moderate to low pressures for the formation of the Ungban "picrotephroites" ,