Niasite and johanngeorgenstadtite, Ni 2+4.5(AsO 4) 3 dimorphs from Johanngeorgenstadt, Germany

Kampf, Anthony R.; Nash, Barbara P.; Plášil, Jakub; Smith, Jason B.; Feinglos, Mark N.

Niasite (IMA2019-105) and johanngeorgenstadtite (IMA2019-122) are inline-formula M7inlinescrollmathml chem normal Ni normal 4.5 normal 2 + 24pt17ptsvg-formulamathimg67e7bf650b4293cac05d6c77463b844f ejm-32-373-2020-ie00004.svg24pt17ptejm-32-373-2020-ie00004.png (inline-formulaAsO4)inline-formula3 dimorphs from Johanngeorgenstadt, Saxony, Germany. The two new minerals occur in association with one another and with aerugite, bunsenite, quartz, rooseveltite and xanthiosite. This mineral assemblage is apparently secondary in origin and most likely formed from the breakdown of primary nickeline under dry (low relative humidity) and oxidizing (high oxygen fugacity) conditions. Both minerals are found in sugary aggregates of irregular, rounded grains or short prisms. Niasite properties are as follows: colour violet-red to red-orange; streak pale pink; transparent; resinous to subadamantine lustre; brittle tenacity; no cleavage; conchoidal fracture; Mohs hardness inline-formula∼4; densityinline-formulacalc 5.222 g cminline-formula−3; optically uniaxial (–), inline-formulaω 1.925(5) and inline-formulaε 1.855(5) (white light), pleochroism inline-formulaO beige, inline-formulaE deep pink (inline-formulaO<E). Johanngeorgenstadtite properties are as follows: colour pink-orange; streak pale pink; transparent; resinous to subadamantine lustre; brittle tenacity; inline-formula{010}, inline-formula{110} and inline-formula{1−10} cleavage; curved and stepped fracture; Mohs hardness inline-formula∼5; densityinline-formulacalc 4.801 g cminline-formula−3; optically biaxial (–), inline-formulaα 1.83(1), inline-formulaβ 1.86(1), inline-formulaγ 1.88(1) (white light), inline-formula2Vmeas 78(1)inline-formula, pleochroism inline-formulaX violet, inline-formulaY light olive, inline-formulaZ yellow (inline-formula M32inlinescrollmathml X > Y > Z 51pt10ptsvg-formulamathimg6c90d70906091ab313bd2c8d800287a8 ejm-32-373-2020-ie00005.svg51pt10ptejm-32-373-2020-ie00005.png ). Raman spectra of both minerals are dominated by the stretching vibrations of inline-formulaAsO4 tetrahedra and confirm that both minerals are anhydrous. Electron microprobe analyses give the empirical formulas (inline-formula M34inlinescrollmathml chem normal Ni normal 3.69 normal 2 + 28pt17ptsvg-formulamathimg147b05fb89cd76d095431f876e9cc813 ejm-32-373-2020-ie00006.svg28pt17ptejm-32-373-2020-ie00006.png inline-formula M35inlinescrollmathml chem normal Co normal 0.66 normal 2 + 32pt17ptsvg-formulamathimg764468023aa917dba17dc1838d0de979 ejm-32-373-2020-ie00007.svg32pt17ptejm-32-373-2020-ie00007.png inline-formula M36inlinescrollmathml chem normal Fe normal 0.03 normal 2 + 30pt17ptsvg-formulamathimg61ca96612ba2e7f3e6d59aa6549de4d2 ejm-32-373-2020-ie00008.svg30pt17ptejm-32-373-2020-ie00008.png inline-formulaAl0.02inline-formulaNa0.02inline-formula M39inlinescrollmathml chem normal Cu normal 0.01 normal 2 + 32pt17ptsvg-formulamathimg8dc555472b0482461bb7fa2da5412628 ejm-32-373-2020-ie00009.svg32pt17ptejm-32-373-2020-ie00009.png )inline-formulaΣ4.43inline-formulaAs3.03O12 and (inline-formula M42inlinescrollmathml chem normal Ni normal 3.56 normal 2 + normal Co normal 0.75 normal 2 + normal Cu normal 0.13 normal 2 + 88pt17ptsvg-formulamathimg80ddbbf784c5988aa231ef7dff220fa1 ejm-32-373-2020-ie00010.svg88pt17ptejm-32-373-2020-ie00010.png inline-formula)Σ4.44inline-formulaAs3.02O12 for niasite and johanngeorgenstadtite, respectively. Niasite is tetragonal, inline-formula M45inlinescrollmathml I true normal 4 normal ¯ normal 2 d 25pt12ptsvg-formulamathimg5fac0e34a7a818e2a111a26d9f5f9850 ejm-32-373-2020-ie00011.svg25pt12ptejm-32-373-2020-ie00011.png , with inline-formulaa=6.8046(8), inline-formulac=18.6190(13) Å, inline-formulaV=862.1(2) Åinline-formula3 and inline-formulaZ=4. Johanngeorgenstadtite is monoclinic, inline-formulaC2∕c, with inline-formulaa=11.933(3), inline-formulab=12.753(3), inline-formulac=6.6956(17) Å, inline-formulaβ=113.302(8)inline-formula, inline-formulaV=935.9(4) Åinline-formula3 and inline-formulaZ=4. The structure of niasite (inline-formulaR1=0.0226 for 471 inline-formulaIo>2σI reflections) is the same as that of jeffbenite, as well as those of several garnet-like synthetic phases. Johanngeorgenstadtite (inline-formulaR1=0.0375 for 355 inline-formulaIo>2σI reflections) has an unprotonated alluaudite structure.

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Kampf, Anthony R. / Nash, Barbara P. / Plášil, Jakub / et al: Niasite and johanngeorgenstadtite, Ni2+4.5(AsO4)3 dimorphs from Johanngeorgenstadt, Germany. 2020. Copernicus Publications.

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