Abstract
Merelaniite is a new mineral from the tanzanite gem mines near Merelani, Lelatema Mountains, Simanjiro District, Manyara Region, Tanzania. It occurs sporadically as metallic dark gray cylindrical whiskers that are typically tens of micrometers in diameter and up to a millimeter long, although a few whiskers up to 12 mm long have been observed. The most commonly associated minerals include zoisite (variety tanzanite), prehnite, stilbite, chabazite, tremolite, diopside, quartz, calcite, graphite, alabandite, and wurtzite. In reflected polarized light, polished sections of merelaniite are gray to white in color, show strong bireflectance and strong anisotropism with pale blue and orange-brown rotation tints. Electron microprobe analysis (n = 13), based on 15 anions per formula unit, gives the formula Mo4.33Pb4.00As0.10V0.86Sb0.43Bi0.33Mn0.05 W0.05Cu0.03(S14.70Se0.30)Σ15, ideally Mo4Pb4VSbS15. An arsenic-rich variety has also been documented. X-ray diffraction, electron diffraction, and high-resolution transmission electron microscopy show that merelaniite is a member of the cylindrite group, with alternating centered pseudo-tetragonal (Q) and pseudo-hexagonal (H) layers with respective PbS and MoS2 structure types. The Q and H layers are both triclinic with space group C1 or C 1 ¯ . The unit cell parameters for the Q layer are: a = 5.929(8) Å; b = 5.961(5) Å; c = 12.03(1) Å; α = 91.33(9); β = 90.88(5); γ = 91.79(4); V = 425(2) Å3; and Z = 4. For the H layer, a = 5.547(9) Å; b = 3.156(4) Å; c = 11.91(1) Å; α = 89.52(9); β = 92.13(5); γ = 90.18(4); V = 208(2) Å3; and Z = 2. Among naturally occurring minerals of the cylindrite homologous series, merelaniite represents the first Mo-essential member and the first case of triangular-prismatic coordination in the H layers. The strongest X-ray powder diffraction lines [d in Å (I/I0)] are 6.14 (30); 5.94 (60); 2.968 (25); 2.965 (100); 2.272 (40); 1.829 (30). The new mineral has been approved by the IMA CNMNC (2016-042) and is named after the locality of its discovery in honor of the local miners.
Highlights
Merelaniite, from Merelani, Tanzania, was first identified unknowingly in 2012 by Simonoff andWise as thin wires of “molybdenite” piercing chabazite crystals [1,2,3]
Initial Raman and chemical studies were performed on a specimen in the collection of the Smithsonian Institution
The new mineral merelaniite (Figures 1–3) occurs rarely and sporadically on specimens extracted from the gem mines in the Merelani hills near the town of Arusha, Lelatema Mountains, Manyara
Summary
Merelaniite, from Merelani, Tanzania, was first identified unknowingly in 2012 by Simonoff and. This review culminated in the publication of a collector-based overview in The Mineralogical Record [7], and the identification of several exotic and/or unusually well crystallized sulfide phases In amongst these phases the authors noted the presence of what they termed ‘incompletely characterized sulfide whiskers’. These “whiskers” were described as tightly coiled metallic cylinders with a chemical composition predominant in molybdenum, lead, and sulfur. They were found alongside many different mineral species from the assemblage, yet at the time, they defied a simple explanation. Initial Raman and chemical studies were performed on a specimen in the collection of the Smithsonian Institution (temporary research number 3323, and cataloged under NMNH 177015)
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