Mineralogy and crystallography of Si-bearing Ca phosphate in angrite meteorites

Abstract

We performed mineralogical and crystallographic study on silico-phosphates in angrite meteorites. The silico-phosphate is present as up to 30 micrometer euhedral grains and contains 10-15 wt% SiO2. The electron diffraction images by FIB-TEM reveal that it has an apatite structure, which is consistent with earlier works by Raman spectroscopy and SEM-EBSD analyses, and so it is called "silico-apatite". Approximately half of P5+ is replaced with Si4+. Fe3+ and Ti4+ partly replaces Ca2+, and Cl- may be similarly replaced with vacancy in order to maintain charge balance in the apatite structure, and thus the composition is written as Ca9(Fe3+,Ti4+)(PO4)4(SiO4)2(Cl,  )2.