Amgaite, Tl3+2Te6+O6, a New Mineral from the Khokhoyskoe Gold Deposit, Eastern Siberia, Russia

Abstract

The new mineral amgaite was discovered at the Khokhoyskoe gold deposit, 120 km W of Aldan town, Aldanskiy District, Sakha Republic (Yakutia), Eastern Siberia, Russia. Amgaite forms fine-grained colloform aggregates up to 0.05 mm across, and is often intimately intergrown with avicennite, unidentified carbonates and antimonates of Tl. Other associated minerals include gold, silver, acanthite, arsenopyrite, pyrite, berthierite, chalcocite, weissbergite, chlorargyrite, calcite, quartz, goethite etc. Amgaite is dark reddish brown to black. It has submetallic luster, black streak, brittle tenacity and conchoidal fracture. Its density calculated from the empirical formula and powder XRD data is 8.358 g/cm3. Its Mohs’ hardness is ca. 1.5–2. Optically, amgaite is uniaxial. In reflected light, it is gray with a bluish shade, very weakly anisotropic with rare brownish red internal reflections. Reflectance values for the four COM wavelengths [Rmin, Rmax (%)(λ in nm)] are: 13.5, 14.2 (470); 12.7, 13.2 (546); 12.3, 12.7 (589); and 11.7, 12.3 (650). The Raman spectrum shows bands of Te–O and Tl–O bonds and confirms the absence in amgaite of H2O, OH–, CO32– groups and B–O bonds. The chemical composition is (electron microprobe, wt.%): MgO 0.43, CaO 1.62, Fe2O3 0.36, Tl2O3 66.27, Sb2O5 3.48, TeO3 27.31, total 99.47. The empirical formula based on 6 O apfu is Tl3+1.74Ca0.17Mg0.06Fe3+0.03Te6+0.93Sb5+0.13O6. Amgaite is trigonal, space group P321; unit-cell parameters are as follows: a = 9.0600(9), c = 4.9913(11) Å, V = 354.82(8) Å3, Z = 3. The strongest lines of the powder X-ray diffraction pattern [dobs, Å (I, %) (hkl)] are as follows: 3.352 (100) (111), 3.063 (15) (201), 2.619 (49) (300), 2.065 (18) (221), 1.804 (28) (302), 1.697 (8) (321), 1.625 (9) (411). The crystal structure of amgaite is the same as of synthetic Tl3+2Te6+O6. The new mineral is named after the Amga River, the basin of which hosts the type locality, Khokhoyskoe occurrence. The type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, with the registration number 5773/1.

Tellurium mineralization of the Priozernoe gold-quartz deposit (Northern Urals, Russia)

“…Из недавних находок отметим новый теллурат Tl амгаит Tl 3+ 2 Te 6+ O 6 , обнаруженный среди продуктов окисления пирит-адуляр-кварцевых метасоматитов Хохойского рудопроявления золота в Якутии (Kasatkin et al, 2022).…”

Kalithallite, K3Tl3+Cl6⋅2H2O, a new mineral with trivalent thallium from the Tolbachik volcano, Kamchatka, Russia

“…Recently the fourth mineral with Tl 3+ was discovered, amgaite Tl 3+ 2Te 6+ O6 (Kasatkin et al, 2022). All these four minerals are extremely rare in Nature due to the particular and unusual formation conditions, under very high oxidizing potential necessary for the appearance of Tl 3+ ; all other known minerals with species-defining Tl contain this element in univalent form.…”

“…Kalithallite became the third mineral with species-defining trivalent thallium, after avicennite Tl 2 O 3 (Karpova et al , 1958) and chrysothallite K 6 Cu 6 Tl 3+ Cl 17 (OH) 4 ⋅H 2 O (Pekov et al , 2015c). Recently a fourth mineral with Tl 3+ was discovered, amgaite Tl 3+ 2 Te 6+ O 6 (Kasatkin et al , 2022). All these four minerals are extremely rare in Nature due to the particular and unusual formation conditions, under very high oxidising potential necessary for the appearance of Tl 3+ ; all other known minerals with species-defining Tl contain this element in univalent form.…”