Vol 10, No 1 (2024)

The new mineral gajardoite-(NH₄), ideally (NH₄)As³⁺₄O₆Cl₂[(Ca_0.5□_0.5)(H₂O)₅], is found at the Khovu-Aksy Ni-Co deposit, Republic of Tyva, Russia. Gajardoite-(NH₄) occurs as tiny lamellar coarsely hexagonal or irregular curved and divergent crystals up to 0.01 mm in size. The crystals are combined in groups, rosette-like clusters or spherulitic aggregates up to 0.2 mm, which are intimately intergrown with annabergite, arsenolite, and pharmacolite on a matrix of skutterudite, safflorite, and other minerals. The new mineral is colorless, white in aggregates, transparent with a white streak and a vitreous lustre. It is brittle, with a perfect cleavage on {001}. The Mohs hardness is ~1½. The calculated density (D_calc) is 2.583 g/cm³.Gajardoite-(NH₄) is optically non-pleochroic, uniaxial (–), ω = 1.745(10), ε = 1.558(5) (589 nm). The chemical composition determined by electron microprobe (wt. %, H₂O content calculated by stoichiometry) is as follows: (NH₄)₂O 3.17, Na₂O 0.40, K₂O 1.07, CaO 5.28, As₂O₃ 67.25, Cl 12.21, H₂O 15.30, O=Cl –2.76, total 101.92. The empirical formula based on four As and 11 O atoms per formula unit is [(NH₄)_0.72K_0.13Na_0.08]_Σ0.93Ca_0.55As³⁺₄O₆Cl_2.03(H₂O)₅. Gajardoite-(NH₄) is hexagonal, space group P6/mmm; the unit-cell parameters are as follows: a = 5.263(3), c = 16.078(5) Å, V = 385.8(5) ų, Z = 1. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] include 16.08 (34) (001), 5.36 (34) (003), 4.565 (41) (100), 3.466 (23) (103), 2.637 (100) (110), 2.360 (25) (113). Gajardoite-(NH₄) is an ammonium analog of gajardoite KCa_0.5As³⁺₄O₆Cl₂ · 5H₂O. Their structural identity is confirmed by powder X-ray diffraction and infrared and Raman spectroscopy.

Crystallization conditions of olivine in dunites of the Guli massif (Siberian Platform) are established using petrological, geochemical, mineralogical and thermobarogeochemical methods. The formation of olivine and chromite from picrite-meimechite ultramafic magmas was followed by the fractionation of clinopyroxenes from basaltic systems with the formation of residual high-alkaline melts. The calculations in PETROLOG and COMAGMAT programs based on the compositions of inclusions and minerals, as well as using olivine-spinel geothermometer, showed that a magmatic system, which was responsible for the formation of dunites of the Guli massif, evolved during the crystallization of olivine in a wide temperature range of 1520–1250 °C in a magma chamber at a depth of about 17 km.

Native gold from seven placers of the Amyl-Sistighem ore placer region (the Republic of Tyva, Krasnoyarsk region) is classified on the basis of its structure, morphology and chemical composition, which are studied by optical and electronic microscopy. Three chemical types of native gold are identified: Ag-Au, Hg-Ag-Au, Cu-Ag-Au. There are rounded and angular particles with and without a high-fineness rim. Seventeen types of native gold are established on the basis of combination of various characteristics. These varieties characterize the mineralogical-geochemical types of primary ores, the distance of the transportation of clastic material, the degree of mechanogenic and crystallophysical deformations of gold particles, and the duration of their occurrence in a gold-bearing layer under relatively calm conditions. Our results can be used in planning thematic pre-project and prospecting-assessing work at this territory, whereas methodological issues can be applied in other gold placer areas.

The technology of preparation of an ancient binder mixture used in construction work at the Gonur Depe archeological site (Southeast Karakum desert) is interpreted on the basis of study of its mineral composition and textural-structural features. The material is composed of thin prismatic-angular and fibrous aggregates of reduced gypsum (60–70 %) and β-bassanite (3 %) enclosed in clay-siliceous matrix (20–30 %). The mixture also contains grains of quartz (5–10 %), feldspar (up to 1%), mica (up to 1 %) and other minerals. The mixture was produced from a crushed natural gypsum stone, which was calcinated in air. In this case, gypsum is converted to β-bassanite and partly to anhydride. The morphology and sizes of mineral particle confirm the presence of the calcination stage up to 200 °C. The further addition of water to a dry mixture caused rapid thickening and cementation of the material, which could be used for whitewashing and plastering of the walls.