In these images a pale pink garnet (with an olivine inclusion) in a mantle-derived peridotite xenolith is surrounded by fine-grained brown material referred to as a “kelyphitic rim”.  The kelyphite consists of intergrown brown picotite spinel and aluminous pyroxenes.  In plane light the spinel is brown to opaque and the pyroxenes appear as colourless blades (A) and under crossed nicols (B) the pyroxenes have first order interference colours.  This section is only about 10 µm thick (note that the maximum interference colour of the olivine is first order red), and more detail can be seen in this kelyphitic rim than in many examples in sections of the standard thickness.  Note that there are two zones to the kelyphite, an outer zone in which individual grains are visible and an inner zone that is too fine-grained to allow individual grains to be identified.  Kelyphite forms as the result of partial re-equilibration of the garnet during rise to the surface in a hot kimberlite magma.  The effects of heating and lowering of pressure caused transformation of the garnet to a lower pressure assemblage of spinel + pyroxenes.  (Some kelyphitic rims contain glass indicative of melting of the garnet.)  These images are of a garnet peridotite xenolith from the Hamilton Branch kimberlite in Kentucky.  Views are 2.2 mm across.  A ppl, B x-nicols.