Granite porphyry

Granitic magma usually cools and solidifies deep below the Earth's surface in the form of large bodies – plutons and batholiths. Effusive rocks – rhyolites – form when a smaller batch of molten rock erupts onto the surface. Granitic magma that ascended from depth, but does not reach the surface and solidifies at depth in the form of magmatic dikes, produces a rock called granite porphyry. Different cooling conditions result in variable crystal-size of individual minerals. Whereas all grains can be identified with the naked eye in the granites, the rhyolites are very fine-grained. Granite porphyries represent a compromise between both end-members.

Granite porphyry contains larger crystals (quartz in the displayed sample) surrounded by fine-grained groundmass. The individual groundmass grains of cannot be identified with the naked eye. This texture is classified as porphyritic and is the basis of the rocks name. Next to the quartz crystals, other principal minerals of this rock are potassium feldspars. Mafic minerals are represented by biotite crystals. Granite porphyries may have various colours including light- or dark-grey, reddish or yellowish. The black veinlets in the displayed sample are greisen veins, i.e. metasomatised rock containing ore minerals, for example – cassiterite (tin oxide).

The occurrences of granite porphyry are scattered over Krušné hory Mts. and Slavkovský les Highland as an entourage of larger bodies – granitic batholiths.

Sample donated by Mr. Marek Plachý.