Caledonite with Linarite (#30)
|North America||Near Baker, San Bernardino County, California, USA||5 ¼" x 3" x 2"|
Here are two very uncommon minerals with similar chemical compositions and origins. The chemical formula of caledonite is Pb5Cu2(SO4)3(CO3)(OH)6. It was first described from a locality in Scotland, and Caledonia is the Roman (Latin) name for the area now called Scotland. Linarite, PbCu(SO4)(OH)2, was originally found in Linares, Andalusia, Spain. Both minerals contain lead (Pb), copper (Cu), and sulfate (SO4). They are intimately associated on the same rock, so they obviously share a common origin. These two minerals occur in lead-copper ore deposits as secondary minerals in the oxidized zone. What does this mean?
Secondary minerals are those that form from alteration processes affecting the primary (or original} minerals in a rock. The oxidized zone of an ore deposit is a region above the water table where surface water and/or ground water and dissolved chemicals percolate through the rocks, resulting in chemical reactions that alter the ore minerals. Such reactions generally transform the sulfide ore minerals into sulfates, which further react with minerals such as calcite to form carbonates.
There are numerous old lead-copper mines in San Bernardino County, California, but which one produced this specimen is not stated. Note that the colorful minerals are a flattened coating on a surface, which suggests that they formed in a crack or fissure in the host rock, which then split in two. Perhaps there is another specimen somewhere that corresponds to the other side of the fissure.