Igneous Rocks

Granites and Gabbros

Igneous rocks are rocks which have melted their way from deep within the crust of the Earth through cracks and vents in the older rocks. In some cases they force their way through to the surface and appear as molten lava. More often they slow down before they reach the surface and cool into solid rock.. As they cool, the different minerals which they contain form themselves into crystals of different and often beautiful shapes. There are two things to remember about this cooling. First, the slower the melt cools, the larger (lie crystals, because large crystals take a very long time to form. Second, the minerals do not all crystallize out at the samc time. The ones which solidify first are free to assume their perfect crystal shapes, while still surrounded by other minerals in pasty form. When these last materials crystallize at lower temperatures, they have to make do with the spaces that are left. Often there is not room for these ones to assume their perfect forms.

Prismatic crystals of white quartz (silicon dioxide) SiO2 and iridescent Chalcopyrite (supplied of iron and copper (Cu Fe SZ). These are crystals of quartz which have been free to develop perfect shapes.

Rosa Pohrino Granite: with normal granite texture and large feldspar crystals

The first example is of pink granite called Rosy Pohrino, from Spain, with normal granite texture and large feldspar crystals

Red foliated Granite in which the minerals have been streaked out by pressure. They have been partly melted and pressed into layers, like leaves.

The Red foliated Granite is very interesting because It cooled first as ordinary granite, but later it was reheated and compressed. This compression crushed the crystals and begin to draw the stone out, so that it has become 'banded'. In other words, this igneous rock was metamorphosed, or changed into another state and then deformed into layers, like leaves.


Granite and Gabbro in the Edgware Road

Swedish Red Granite from the Green Man public house, at the corner of Bell Street and Edgware Road, London N1.

Black Gabbro from Marks & Spencer's doorways, Edgware Road, N1

The oval panel is of black 'granite'. It is actually a Gabbro which contains no quartz. Quartz is white and its absence accounts for the black colour of the Gabbro.


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