MINERAL UBAHAN

Allanite in a metaluminous granite. This allanite has concentric zoning resulting from changing mineral composition during successive stages of growth. Allanite contains substantial amounts of Th and U, and sustains considerable radiation damage over time.The allanite grain has low birefringence, a result of radiation damage that essentially turns the crystal lattice into a glass

Chlorite replacing biotite in a muscovite granite. Small residual brownish patches of biotite still occur, as do dark bits of rutile or titanite The low first order, anomalous Berlin blue interference color indicates that this is an Fe-rich chlorite, as are the high birefringence patches of residual biotite.

Chlorite replacing biotite in a metaluminous granite. The green chlorite has only replaced the biotite on the top left of side of the biotite grain. The chlorite mostly has an low first order interference color, indicative of Mg-rich chlorite. The purple regions are of intermediate composition. The adjacent biotite has third order birefringence.

Sericite replacing plagioclase in a metaluminous granite. Sericite is grungy-looking fine-grained stuff that commonly replaces feldspars Sericite generally made up of very small crystals, its birefringence is irregular and generally low. Some of the larger crystals can be seen here to have first order birefringence. Albite twinning in the plagioclase is clearly visible.

Sericite replacing plagioclase in a metaluminous granite. At higher magnification, some of the grungy-looking clearly resolves into little, colorless, platy crystals. The crystals now clearly have up to middle first order birefringence. They have a positive sign of elongation and are probably small white micas that grew during subsolidus hydrothermal alteration

Serpentine in an altered harzbergite nodule in a kimberlite. Much of this sample is made up of calcite (colorless) and talc (darker browns). The serpentine is the yellowish material that occupies the thin veins. The calcite has very high birefringence, and the talc has irregular second and third order interference colors that are modified by the brown staining. The serpentine is most easily seen in the veins, and veins have lower first order birefringence and fibers perpendicular to the vein walls.