Purpurite from Spodumene Pegmatites of the Kolmozero Deposit, Kola Peninsula, Russia

The morphology and assemblage of purpurite Mn3+(PO4) from spodumene pegmatites of the Kolmozero deposit (Kola Peninsula), as well as its chemical composition and crystal structure, are characterized using a comprehensive study with X-ray powder diffraction, electron microscopy, single crystal X-ray diffraction, and Raman spectroscopy. Purpurite typically occurs as fine-grained aggregates (crusts) on spodumene and lithiophillite, as well as part of a complex mixtures of secondary minerals, which fill caverns in pegmatite. The Kolmozero purpurite is characterized by a high Fe content and its composition with a Mn/(Mn + Fe) ratio of ∼0.55 is close to the heterosite Fe3+(PO4). The Raman spectrum of purpurite contains no O–H vibration bands and is similar to those published for heterosite. The crystal structure of natural purpurite is solved for the first time in this work. Purpurite is orthorhombic, space group Pbnm, the unit cell parameters are: a = 4.7783(3), b = 9.7417(5), c = 5.8388(4) Å, V = 271.79(3) Å3, R1 = 2.42%. The crystal structure of purpurite is based upon layers of vertex-shared strongly distorted (Mn3+, Fe3+)O6 octahedra linked via PO4 tetrahedra.