"Rock of the Month # 33, posted for March 2004" --- GCW sample 1493, from the Trinacria mine, Oruro Department. Obtained from David New, March 1993. A very rare mineral, described only from a few deposits in the Andes, within Bolivia and adjacent Argentina. The field-of-view is roughly 2 cm.

Note the spectacularly unusual crystal habit in the sulphosalt cylindrite, ideal formula FePb₃Sn₄Sb₂S₁₄, variously reported as triclinic or tetragonal. It was first described from Mina Santa Cruz at Poopo in the late 19th century. The small crystals of this soft, dense, steel-grey to black metallic phase occur as cylindrical prisms with a pronounced concentric, coaxial internal structure, hosted in a massive matrix. The Fe-Pb-Sn sulphosalt is also reported from the Llallagua Sn deposit in Bolivia (Bandy, 1944) and from Huanuni. It is illustrated by Bauer and Tvrz (1974) and Picot and Johan (1977). The latter document the peculiar internal structure of the mineral, with concentric layers each 2-5 microns thick. This structure generates black crosses in reflected cross-polarized light.

Cylindrite has more recently been reported from the Pirquitas Ag Sn deposit in Jujuy province, northwest Argentina, a complex multi-stage deposit notable for very fine intergrowths of ore minerals. The complex mineralogy includes many minerals of Sn, Ag, Pb, Bi, As, Sb, Tl, In and other elements (Paar et al., 1998). This locality lies about 60 km south of the Bolivian border, and the cylindrite occurs in a recently-discovered vein with other unusual Pb-Sn-Sb ore minerals such as potosiite and franckeite.

References

Bandy,MC (1944) Mineralogy of Llallagua, Bolivia. Tucson Gem and Mineral Society Spec.Pap. 1, 67+16pp., reprinted 1976.

Bauer,J and Tvrz,F (1974) A Field Guide in Colour to Minerals, Rocks and Precious Stones. Octopus Books Ltd, London, Engl. translation, 208pp.

Paar,WH, de Brodtkorb,MK, Sureda,RJ and Topa,D (1998) A microprobe study of complex Ag-Sn ores from Pirquitas, Jujuy province, Argentina. Abs. International Mineralogical Association 17th General Meeting, 118, Toronto, 9-14 August.

Picot,P and Johan,Z (1977) Atlas des Minéraux Métalliques. BRGM Mem. 90-1977, 407pp., p.143 (in Fr.).