Northstarite, a new lead-tellurite-thiosulfate mineral from the North Star mine, Tintic, Utah, USA

Abstract

Northstarite, Pb₆ (Te⁴⁺O₃)₅(S₂O₃), is a new mineral from the North Star mine, Tintic district, Juab County, Utah, USA. It is an oxidation-zone mineral occuring in a vug in massive quartz-baryte-enargite-pyrite in association with anglesite, azurite, chrysocolla, fluorapatite, plumbogummite, tellurite, zincospiroffite, and the new mineral adanite. Crystals are beige short prisms with pyramidal terminations, up to about 1 mm in length. The mineral is transparent to translucent with adamantine luster, white streak, Mohs hardness 2, brittle tenacity, irregular fracture, and no cleavage. The calculated density is 6.888 g/cm³. Northstarite is uniaxial (–) and nonpleochroic. The Raman spectrum is consistent with the presence of tellurite and thiosulfate groups and the absence of OH and H₂O. Electron-microprobe analyses gave the empirical formula Pb_(5.80)Sb³⁺_(0.05)Te⁴⁺_(5.04)S⁶⁺_(1.02)S²⁻_(1.02)O₁₈. The mineral is hexagonal, space group P6₃, with a = 10.2495(5), c = 11.6677(8) Å, V = 1061.50(13) ų, and Z = 2. The five strongest X-ray powder diffraction lines are [d_(obs) Å(I)(hkl)]: 3.098(100)(113), 2.957(88)(300), 2.140(42)(223), 1.7335(41)(413), and 1.6256(31)(306). The structure (R₁ = 0.033 for 1476 I > 2σI reflections) is a framework constructed of short (strong) Pb–O and Te–O bonds with channels along the 6₃ axes. The thiosulfate groups at the centers of the channels are only weakly bonded to the framework.