Morphology and Optical Properties of Precipitated Vivianite, Fe3(PO4)2·8H2O

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Crystals of vivianite, Fe3(PO4)2∙8H2O, belonging to the monoclinic prismatic class 2/m , are grown by precipitation from aqueous solution at temperatures from 25 to 56 °C. At 25 °C aggregates are dominating, but at 35 °C and above single crystals are more common, being tabular along {010} and most often with parallelogram outline; in a few cases twins are observed. The outline is distinct and consistent with published lattice parameters, but at low temperature the {010} faces are very rough as judged from interference colors seen in the polarizing microscope. Lateral faces belonging to the {110} form are visible on crystals grown at 45 and 56 °C, and the widths of their projections on {010} as measured with a micrometer eyepiece served for estimating crystal thickness, making use of the face angle from lattice parameters. Optical parameters are determined by a combination of three methods: 1) measurement of birefringence with an Ehringhaus 6λ compensator, 2) Kamb's method for the optic axis angle determined by the angle‐of‐flight method in conoscopic illumination, and 3) visual interferometry using a polarizing microscope incorporating a Mach–Zehnder interference device. Reasonably precise values of principal refractive indices in the expected range are found.
Original languageEnglish
Article number1900226
JournalJournal of Crystal Growth
Issue number6
Number of pages5
Publication statusPublished - 1 Jun 2020

ID: 243428194