Struvite-(K), KMgPO46H2O, the potassium equivalent of struvite a new mineral

Abstract

The new mineral struvite-(K) is the natural potassium equivalent to struvite NH4MgPO4·6H2O. It was discovered independently at two different localities: (1) at the famous sulphosalt locality Lengenbach in Binntal, Switzerland, where it occurs in a dolomitic rock of Triassic age, in close association with various Pb-As-sulphosalts (mainly rathite). It forms extremely fine needles reaching up to a maximum length of about 0.3 mm. The needles are elongated along the crystallographic a -axis, they are completely colourless and transparent. The acicular crystals are generally well developed, and several forms such as {0 0 1}, {0 1 0}, {1 0 1}, {0 1 2}, {1 1 0}, {1 1 1}, {0 1 2} could be identified by optical goniometry. Single-crystal study showed the mineral to be orthorhombic, with space group Pmn 21 (from analogy to struvite), a = 6.903(3), b = 6.174(2), c = 11.146(3) A, V = 475.0(2) A3 (refined from powder data). Qualitative chemical data derived from EDS analyses on an SEM gave major K, Mg, P, and traces of Sb, Fe, and Cu. Attempts to prepare samples for quantitative EMP analyses failed, therefore a structure determination (using a Bruker AXS three-circle diffractometer) and a refinement of K versus NH4 were carried out which prove that the mineral represents the pure K end-member. Optically, the mineral is biaxial positive, with 2 V Z = large, α = 1.490(2), β = 1.493(2) (for 589 nm), γ could not be measured, the optic axes plane (OAP) is perpendicular to the needle axis, therefore β parallels the crystallographic a -axis. At the Lengenbach occurrence, struvite-(K) is a product of the latest stage of hydrothermal activity to supergene alteration. A second occurrence (2) is Rossblei, Schladminger Tauern, Styria, Austria, an abandoned galena mine. The host rock of the Pb-mineralisation is a sericite-schist belonging to the polymetamorphic basis of the Schladminger Tauern. The mineral occurs as pseudomorphosed aggregates of dirty white colour reaching up to several millimetres . The aggregates represent close intergrowths of fine-grained struvite-(K) and newberyite Mg(PO3OH)·7H2O. Cell parameters refined from the powder data, after deduction of newberyite lines, are a =6.878(1), b =6.161(1), c =11.100(1) A, V =470.41(9) A3. No additional physical, optical, morphological data could be derived due to the close intergrowth of the two minerals. Struvite-(K) from Schladming obviously represents a recent alteration product.