Classifying igneous rocks• Igneous rocks come in a range of different textures and chemical compositions.• We can easily classify hand-specimens by colour and by grainsize.
Simple classification• As a rule of thumb, rocks rich in ferromagnesian minerals tend to have a darker colour and higher density than rocks rich in quartz and alkali feldspar. Obsidian is the obvious exception.• Volcanic rocks tend to have very fine grains. Grainsize generally increases with the time taken for crystallisation.
Classifying igneous rocks• Igneous rocks come in a range of different textures and chemical compositions Ultramafic Mafic Intermediate AcidVolcanic Basalt Andesite Rhyolite< 1mmHypabyssal Dolerite Micro-diorite Micro-granite1-5mmPlutonic Peridotite Gabbro Diorite Granite>5mm