Abstract
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.
Highlights
Johanngeorgenstadtite (R1 = 0.0375 for 355 Io > 2σ I reflections) has an unprotonated alluaudite structure. In their description of the new mineral paganoite, NiBi3+As5+O5, Roberts et al (2001) told the story of the enigmatic specimen on which it was discovered. This specimen was obtained by American mineral dealer David New in 1981 from a mineral shop in Germany but was without a label
Feinglos, who is a co-author of the present paper, recognized the specimen to contain an unusual assemblage of minerals, including bunsenite, NiO, aerugite, Ni17As6O32, and xanthiosite, Ni3(AsO4)2, which is unique to one mineral locality, Johanngeorgenstadt, Saxony, Germany
One of the authors of the present paper (JBS) obtained a sample thought to contain petewilliamsite; this mineral was determined to be niasite, one of the new minerals described. This sample was traced to the late James Ferriaolo, who is thought to have obtained it as surplus material from the American Museum of Natural History (AMNH)
Summary
In their description of the new mineral paganoite, NiBi3+As5+O5, Roberts et al (2001) told the story of the enigmatic specimen on which it was discovered. One of the authors of the present paper (JBS) obtained a sample thought to contain petewilliamsite; this mineral was determined to be niasite, one of the new minerals described . This sample was traced to the late James Ferriaolo, who is thought to have obtained it as surplus material from the American Museum of Natural History (AMNH). The AMNH collection card and the original label (Fig. 2) indicate that the specimen, noted as including 10 pieces containing aerugite and xanthiosite, was obtained in June of 1914 from the collection of Walter F. The niasite cotype in the American Museum of Natural History collection is the holotype for johanngeorgenstadtite
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