Exsolution of metallic copper from Lake County labradorite.

Some gem-quality labradorite phenocrysts in Miocene basaltic lava from Lake County, Oregon, have a schiller due to metallic copper; some have a transparent red or green colour. The CuO content of the crystals varies systematically with colour: 0-40 ppm for pale yellow, through approx 100 ppm for green to 150-200 ppm for red, and 80-200 ppm for schiller-bearing laths. The variation of Cu among different crystals is primary and reflects a variation in magma chemistry during plagioclase fractionation. The occurrence of colour or schiller is determined by the size of the Cu particles which, in turn, depend on the T at which exsolution is initiated. Schiller develops for the highest Cu content at highest T, whereas Cu colloids form at low T when Cu content drops.-L.C.H.