The Earth Story

A tale of two fluids Once upon a time there was a granite that rose from the bowels of the Earth towards the surface. At some point, it stalled in the crust and the 800 degree (C) magma started cooling amongst the country rocks in which it found itself. As the principal minerals, quartz and feldspar formed, the remaining liquid became enriched in silica, and as the end of crystallisation approached cooled very slowly and started to form large crystals of iron hued amethyst. After the granite had finished cooling, and approached the surface due to the erosion of the overlying rock groundwater saturated in calcium carbonate started to move through the rocks at the crustal level at which the amethyst found itself. As a result of a change in conditions the calcite began to exsolve from the solution and precipitated as a layer of dogtooth calcite crystals atop the amethystine substrate. Later still, iron enriched fluids draped a trace of red haematite staining over the assemblage, producing the final result you see before you. The specimen comes from the Hubei province of China, and measures 11.0 x 10.8 x 8.6 cm.  Loz Image credit: Joe Budd/Rob Lavinsky/iRocks.com