On the presence of a hydrous component in a gemstone variety of intermediate olivine-type triphylite-lithiophilite, Li(Fe,Mn)PO4

Abstract

A single-crystal gemstone of intermediate olivine-type triphylite-lithiophilite with the composition Li(Fe0.56Mn0.42)P1.02O4 was investigated by vibrational spectroscopy and electron microscopy techniques. Though gem-quality, the crystal reveals lamellar, platy inclusions parallel to (100) with a length of up to > 10 μm and a thickness of up to 1–2 μm. These were identified as sarcopside, (Fe,Mn)3(PO4)2, yielding a higher Fe:Mn ratio than the host triphylite. Polarised infrared spectra of oriented triphylite sections show a weak, pleochroic band in the O-H stretching region that is centred at ~ 3444 cm-1. Its integrated total absorbance corresponds to a hydrogen content of only 3.7 wt.ppm H2O. Using peak-fitting techniques the band is decomposed into three component bands at ~ 3419, 3444, and 3474 cm-1 which are least intense in the [010] spectra and which show stronger absorptions in the [001] and [100] directions. Thus, O-H vectors almost parallel to the (010) plane, pointing closer to the a or to the c axes are assumed. Three ways of hydrogen incorporation are in agreement with this observation: (a) hydrous species at the oriented interfaces between triphylite host and sarcopside lamellae, (b) hydrogen from submicroscopic inclusions of hydrous phosphate minerals such as phosphoferrite-reddingite, (Fe,Mn)3(PO4)2 · 3H2O, (c) hydrous point defects in the structure of triphylite and/or sarcopside, which are possible under the assumption of vacancies at the Li (M1) and/or P sites.