Sinkankasite

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Formula: Mn²⁺Al(PO₃OH)₂(OH).6H₂O
Hydrated phosphate of manganese and aluminium
Crystal system: Triclinic
Specific gravity: 2.27 measured, 2.25 calculated
Hardness: 4
Colour: Colourless
Luminescence: Not fluorescent under UV
Solubility: Readily soluble in dilute 1:1 hydrochloric acid
Environments

Pegmatites
Hydrothermal envronments

Sinkankasite is a late-stage hydrothermal alteration product of triphylite in complex zoned granite pegmatites (HOM).

Localities

At the Palermo No. 1 Mine, Groton, Grafton County, New Hampshire, USA, sinkankasite has been found on only one specimen. It occurs as flattened, circular, 2 to 3 mm sprays of white acicular crystals associated with vivianite in a fracture in massive triphylite (AM 69.380-382).

At the type locality, the Barker-Ferguson Mine, Keystone, Keystone Mining District, Pennington County, South Dakota, USA, sinkankasite occurs as complete pseudomorphs after triphylite and as cavity fillings formed from the partial or complete dissolution of triphylite. These pseudomorphs and cavity replacements range in size from less than 1 up to 10 cm³. Sinkankasite is also found along fractures that cut quartz, microcline, albite and muscovite. Secondary phosphate minerals associated with sinkankasite include vivianite, hureaulite, carbonate-rich apatite, strengite, barbosalite and fluellite.
The secondary phosphate mineralisation commenced with very late-stage hydrothermal alteration of triphylite, the primary phosphate. This alteration proceeded from the outside inward, especially when in association with microcline, albite and muscovite which apparently supplied the Al for sinkankasite. The early-formed secondary phosphates are vivianite and hureaulite, with vivianite dominant. They commonly occur as a network of veinlets transecting triphylite and as crystals in small vugs. Vivianite is found as transparent, blue, tabular to bladed 1 mm crystals and hureaulite is found as water-clear, pale pink to brown transluscent 3 mm crystals. Carbonate-rich apatite is younger than vivianite and hureaulite; in the vugs it forms white crusts and delicate boxworks and is composed of colourless, prismatic, bipyramidal crystals up to 0.5 mm in length. Spherulitic aggregates and 1 mm crystals of pink strengite are scattered randomly on the carbonate-rich apatite followed by, if not contemporaneous with, barbosalite in very dark greenish-blue 0.5 mm spherules. Fluellite, which is rather uncommon, appears to have formed with barbosalite and is found as clusters of colourless, dipyramidal crystals 1 mm across. Sinkankasite, which is rather common, is the last mineral in this sequence and is found as colourless, bladed, prismatic crystals up to 4 mm in length. It often occurs as divergent, radial clusters and as spheroidal aggregates. Several specimens, which contain only hureaulite and sinkankasite, have spherules of sinkankasite within hureaulite, indicating that in these specimens, sinkankasite formed just after the onset of hureaulite crystallisation, and was contemporaneous with it. Other secondary phosphates found at the Barker pegmatite, but not associated with sinkankasite, are laueite, torbernite, autunite and phosphuranylite (AM 69.380-382).
Sinkankasite from the Barker-Ferguson Mine - Image

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