Hydrothermal

Rocks formed by a variety of processes that involve fluids, transfer of material in fluid, or crystallisation from silicate-bearing fluid or fluid-rich melt.
  • vein1 A vein filled with feldspar and quartz cuts a metaconglomerate of Neoproterozoic age in Namaqualand, South Africa.
  • IMG 4653 Huge radiating crystals of white diopside form a hydrothermal calc-silicate occurrence several metres across, among medium grade schists and gneisses. This kind of occurrence may mark a channel for escaping metamorphic fluid. SE British Columbia, Canada.
  • chalcopyrite1 Massive sulphide ore deposit comprising bornite (purple), chalcopyrite (bright yellow) and pyrrhotite (pale yellow) photographed underground at Okiep, South Africa.
  • IMG 6107 Greisen vein cutting partly kaolinised granite at Cligga Head, Cornwall. Greisen consists of quartz and muscovite replacing feldspar of the granite adjacent to a narrow vein that contains black tourmaline and some cassiterite (tin oxide).
  • IMG 4622 A lenticular quartz segregation in an upper amphibolite-facies schist. These result from the redistribution of silica in solution in metamorphic fluids. Sample from Baffin Island, Canada.
  • IMG 4623 A coarse kyanite-quartz segregation in amphibolite-facies schist. Sometimes fluids can carry significant amounts of other chemical species, even aluminium, generally thought to be rather immobile. SE British Columbia, Canada.
  • IMG 6350 Granitic pegmatite with 'graphic' intergrowths of quartz in alkali feldspar, and large narrow biotite flakes. Width of view ca. 40 cm. From a slab in a stone merchant's yard.