This rock sample is known as the Roche Rock which is found near Roche SW991596 in Cornwall. This particular rock may represent the final stages in the consolidation of a parent magma, in this case a granite.
                 In the final stages, volatile elements are released through joints and fissures which result in significant changes in some of the parent rock. One of these modifications is tourmalinization.
                The Roche Rock illustrates a case where tourmalinization has resulted in a tourmaline-quartz rock in which none of the original minerals except quartz has survived.
                Sections A and C taken in plane polarised light show crystals of a variety of  tourmaline known as schorl with their characteristic pleochroic pale blue and yellow/brown colours. C is the same section as A but turned at 90deg. to illustrate pleochroism, a change of colour when the section is rotated under plane polarised light (ppl). The near middle centre crystal with a bubble is 1.5mm in length.
                B is the same section as A. B, taken with crossed polars, illustrates the bright interference colours shown by tourmaline in contrast to the greys and whites of quartz.
                D and E are photo's taken with a scanner, E in ppl, D in cross polarised light (xpl). The photo's illustrate the value of xpl, in this instance the ability to see the quartz mosaic in shades of white and grey interference colours and the bright interference colours of tourmaline.  

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