Garnet mica
schist; Austrian Alps
This view is of the flat cleavage surface of a medium-grade
schist. The rock has a purplish sheen because both dark mica
(biotite) and light mica (muscovite) are present. It is just
coarse enough to distinguish individual mica flakes, and
some of them are reflecting the light from the illuminating
lamps and appear as white specks. Also visible are large
dull red-brown garnet crystals up to 1 cm across. Some of
the garnets show a regular hexagonal outline. This texture,
with large crystals growing in a finer matrix, is quite
common in metamorphic rocks, and is called porphyroblastic
texture. We have to make a distinction between
porphyroblastic texture in metamorphic rocks and porphyritic
texture in igneous rocks, since although they look similar
they have very different origins. In porphyritic texture,
big crystals grew first from molten material, and the fine
matrix crystallized later. In porphyroblastic texture, the
fine solid matrix of crystals was there first, and big new
crystals grew in it.