New data on the mercury oxide-chloride mineral poyarkovite: The second find, and crystal-structure determination

Poyarkovite was first found in the Khaydarkan (Kyrgyzstan) antimony-mercury deposit in 1981. A second find is recorded in the oxidized cinnabar ores o f the Arzak mercury deposit, situated about 12 km northeast of the well-kno wn Terlig-Khaya mercury mine, Pii-Khem district, Tuva Republic, Russia. Her e, poyarkovite is closely associated with supergene eglestonite, calomel, t erlinguaite, montroydite, and native mercury. All these minerals occur in c avities with partly leached primary cinnabar. Poyarkovite is a rare constit uent in ores. It forms small, mostly irregular, grains as inclusions ih egl estonite and calomel aggregates, and sporadically as minute crystals growin g on surfaces of altered cinnabar. The Arzak poyarkovite has the same physi cal properties as the Khaydarkan specimen, i.e., vitreous to adamantine lus ter, purple or red-purple color, red or red-crimson streak, translucent (th in fragments and edges of grains), brittle, uneven or subconchoidal fractur e, no cleavage, and hardness 2-21/2. The measured density (9.56 g/cm(3)) is in good agreement with the calculated Value (9.64 g/cm(3)), refined accord ing to an X-ray structural study. The strongest seven lines in the X-ray po wder-diffraction pattern [d in Angstrom(I)(hkl)] are: 3.09(4-5)((5) over ba r 14), 2.96(4)(600), 2.83(10)((1) over bar 32), 2.74(8)(512), 2.61(6)((7) o ver bar 12), 1.884(4)((1) over bar (0) over bar 002), et 1.799(6)(640). The electron-microprobe analyses gave, on average, Hg = 91.70, Cl = 5.38, O = 2.45 (inferred value), total = 99.53 wt.%, corresponding to Hg3.000O1.005Cl 0.995. based on a total of five atoms. In the X-ray structural study, we ha ve used poyarkovite from Khaydarkan. The structure of poyarkovite [Hg3ClO, a 19.009(3), b 9.018(4), c 16.848(9) Angstrom, beta 110.82(3)degrees V 2700 (2) Angstrom(3), space group C2/c, Z = 24] was solved by direct methods, an d refined to R = 6.0% on the basis of 1614 unique reflections measured with MoK alpha radiation on a CAD-4 automated diffractometer. The unit cell con tains nine independent mercury atoms forming the six (Hg-Hg)(2+) dimers, wi th the Hg-Hg distances equal to 2.502(2)-2.565(3) Angstrom. Of six Hg-2 pai rs, three are formed by the 1 or 2 symmetry-related atoms. The mercury dime rs are located in three mutually orthogonal directions. Each O atom forms f our Hg-O bonds in a tetrahedral arrangement [1.95(2)-2.60(2) Angstrom, Hg-O -Hg 88.1(6)-121.4(9)degrees]. The shortest Hg-Cl distances are 2.816(10) An gstrom. In terms of only the Hg-Hg and Hg-O bonds, the poyarkovite structur e can be represented as two frameworks enclosed one inside the other. The f rameworks are bound by the translation of the C unit cell (0.5, 0.5, 0). Th ese interpenetrating frameworks of Hg and O atoms bind the Cl atoms togethe r; the latter complement the Hg coordination.