Iron-Manganese Phosphates with the Olivine - and Alluaudite-Type Structures: Crystal Chemistry and Applications

摘要

Iron-manganese phosphates are common accessory minerals occurring in granitic pegmatites, in metamorphic rocks, and in meteorites. In rare-elements pegmatites, primary phosphates of the triphylite-lithiophilite series [Li(Fe~(2+),Mn~(2+))(PO_4)-Li (Mn~(2+),Fe~(2+))(PO_4)] form masses that can reach several meters in diameter, enclosed in silicates. During the oxidation processes affecting the pegmatites, these olivine-type phosphates progressively transform to ferrisicklerite-sicklerite [Li_(1-x)(Fe~(3+), Mn~(2+))(PO_4)-Li_(1-x)(Mn~(2+),Fe~(3+))(PO_4)] and to heterosite-purpurite [(Fe~(3+),Mn~(3+)) (PO_4)-(Mn~(3+),Fe~(3+))(PO_4)], according to the substitution mechanism Li~+ + Fe~(2+)?□+ Fe~(3+). This oxidation sequence was first observed by Quensel (1937), and then confirmed by Mason (1941).