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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