Tin does not occur naturally by itself, and must be extracted from a base compound, usually cassiterite, stannite, cylindrite, franckeite
, canfieldite, and teallite.
The place in question is an entirely landlocked, mountainous, somewhat mysterious-seeming country which boasts a long mining history and has a secure reputation for turning out world-class specimens of--oh, let's see--metallic species (andorite, pyrarygrite, stannite, franckeite
, cylindrite), other primary ores (cassiterite, ferberite), and glamorous phosphates (phosphophyl-lite, paravauxite, vivianite, ludlamite) such as we have long admired and lusted after.
On some specimens, the blue vauxite forms casts after vanished spherical aggregates of wavellite to 8 mm diameter, and one specimen even shows metallic black microcrystals of franckeite
. Yes, of course, the source of these specimens is the famous Siglo XX mine, Llallagua, Bolivia.
andorite), the silver ore minerals may have been franckeite
and stannite (both of which commonly bear a few tenths of a percent silver), and sphalerite with micro-inclusions of tetrahedrite.
The same discovery produced about 30 specimens showing the very rare potosiite (a Pb-Sb-Fe-Sn sulfide) as lathlike, metallic gray crystals to 1 cm, some twinned to make fanlike forms, on drusy franckeite
, with zinkenite and arsenopyrite; Kosnar and Lavinsky had potosiite-bearing specimens of this description to 9 cm across.
The San Jose mine officially closed in late 1991, but recent searches for more of its world-class andorite specimens have produced, instead, a few very fine examples of the rare lead-tin-iron sulfosalt franckeite
, these found early in 2003 on the 380 level.
The exhibit also held rare franckeite
, andorite, helvite and canfieldite specimens, all from South America.
[Contributions to the Knowledge of Bolivian Mineralogy, V, Franckeite
The sulfides and Sulfosalts are very well represented, most notably arsenopyrite, bismuthinite, bournonite, cylindrite, franckeite
, pyrargyrite, pyrite, stannite, stibnite, teallite, and wurtzite Among the oxides, cassiterite and quartz are most important and siderite is the most important carbonate.
Associated minerals are: franckeite
, cylindrite, hocartite, rhodostannite and sphalerite.
Next, consider Brian's excellent specimens of the familiar sulfosalt species from the San Jose mine, Oruro Department: unusually lustrous black franckeite
in intergrown shaggy spherical aggregates to 7 cm; four fine thumbnails of andorite in thin, metallic gray dull-lustered loose aggregate s; and perhaps 50 specimens of stannite crystals in those tight little spheres we have seen before, but these overlaid heavily by unusually brilliant zinkenite needles.
Cylindrite and franckeite
have the same Q-H-Q-H sequence of stratification, but in franckeite
the width of the Q layer is twice that of the Q layer of cylindrite.