ES2 Metamorphic Rocks - Metamorphic Microstructures #13
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Poikiloblasts.
Commonly a porphyroblast will grow by addition of material to its outer surface
more rapidly than unwanted material (such as matrix quartz grains) can be removed from the
interface. The porphyroblast traps the material as inclusions, forming a poikiloblast. In the
example, the left-hand side shows a poikiloblast of colourless, moderate-relief andalusite,
with inclusions mainly of quartz and biotite, while on the right a high relief
staurolite poikiloblast is packed with tiny quartz inclusions.
Field of view is 3 mm across.
Andalusite-staurolite schist, Banff coast, NE Scotland. Sample BB1.
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