Niasite and johanngeorgenstadtite, Ni2+4.5(AsO4)3 dimorphs from Johanngeorgenstadt, Germany

Niasite (IMA2019-105) and johanngeorgenstadtite (IMA2019-122) are Ni 4.5 2 + ( 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 ; density calc 5.222 g cm −3 ; optically uniaxial (–), ω 1.925(5) and ε 1.855(5) (white light), pleochroism O beige, E deep pink ( OY>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 ( Ni 3.69 2 + Co 0.66 2