Niasite and johanngeorgenstadtite, Ni²⁺_4.5(AsO₄)₃ dimorphs from Johanngeorgenstadt, Germany

Abstract

Niasite (IMA2019-105) and johanngeorgenstadtite (IMA2019-122) are Ni4.52+(AsO4)3 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 ∼4; densitycalc 5.222 g cm−3; optically uniaxial (–), ω 1.925(5) and ε 1.855(5) (white light), pleochroism O beige, E deep pink (O<E). Johanngeorgenstadtite properties are as follows: colour pink-orange; streak pale pink; transparent; resinous to subadamantine lustre; brittle tenacity; {010}, {110} and {1−10} cleavage; curved and stepped fracture; Mohs hardness ∼5; densitycalc 4.801 g cm−3; optically biaxial (–), α 1.83(1), β 1.86(1), γ 1.88(1) (white light), 2Vmeas 78(1)∘, pleochroism X violet, Y light olive, Z yellow (X>Y>Z). Raman spectra of both minerals are dominated by the stretching vibrations of AsO4 tetrahedra and confirm that both minerals are anhydrous. Electron microprobe analyses give the empirical formulas (Ni3.692+Co0.662+Fe0.032+Al0.02Na0.02Cu0.012+)Σ4.43As3.03O12 and (Ni3.562+Co0.752+Cu0.132+)Σ4.44As3.02O12 for niasite and johanngeorgenstadtite, respectively. Niasite is tetragonal, I4¯2d, with a=6.8046(8), c=18.6190(13) Å, V=862.1(2) Å3 and Z=4. Johanngeorgenstadtite is monoclinic, C2∕c, with a=11.933(3), b=12.753(3), c=6.6956(17) Å, β=113.302(8)∘, V=935.9(4) Å3 and Z=4. The structure of niasite (R1=0.0226 for 471 Io>2σI reflections) is the same as that of jeffbenite, as well as those of several garnet-like synthetic phases. Johanngeorgenstadtite (R1=0.0375 for 355 Io>2σI reflections) has an unprotonated alluaudite structure.