Grain size and associated terminology

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Magmatic context

Grain size terminology:

• Very coarse-grained rock contains over 3 cm long crystals;

• Coarse-grained is 5 mm to 3 cm;

• Medium-grained is 1 to 5 mm;

• Fine-grained is less than 1 mm;

• Glassy, aphanitic - not crystallized.

Non-homogenous grain size:

• Phenocryst – mineral phase that is distinctly larger than the rest of the phases (groundmass).

  • Porphyritic rock – points to at least two crystallization stages occurring in distinct conditions. Porphyritic intrusive phases indicate that the last intrusive phase was emplaced in the upper crust. This proximity to surface is essential to give raise to porphyry-style of mineralization.

• Oikocryst mineral – mineral with slow nucleation and prolonged growth that encloses a large amount of more rapidly nucleated minerals.

  • Poikilitic texture – refers to an oikocryst-bearing rock;

  • Ophitic texture – special case for pyroxene oikocryst enclosing plagioclase in mafic intrusive rocks.

Magmatic or magmatic-hydrothermal context

• Pegmatite intrusion – pegmatites are (very) coarse-grained intrusive rocks, with constitutive minerals that are centimeter to several meters in size. To be a pegmatite, the minerals need to have crystallized from a magma, which is generally volatile-rich and evolved (pegmatite in gabbro is an exception, though it also developped from a residual melt).

• Myarolitic cavity – defined as a sharp grain size increase in part of a magma intrusion. The cavity is usually ovoid in shape, with a hollow space in its core (that may have been filled with clay before weathering). The cavity points to the local accumulation of magmatic volatiles in parts of on intrusion emplaced in the upper crust.

Metamorphic context

• Porphyroclast – crystal that is larger than other minerals in the rock. The porphyroclast is a relic of a mineral from the pre-deformation, coarser-grained, rock (it can display strain shadows and rotated tails).

• Porphyroblast – large crystal (larger than most minerals in the rocks) that is full of inclusions (poecilitic or poikilitic grain).

  • Snowball garnet – garnet porphyroblast grown during a deformation event that induced the rotation of the mineral and its inclusions baggage.

  • Porphyroblasts can be pretectonic, intertectonic, syntectonic, or post-tectonic.