Granite
Granites are coarse-grained intrusive igneous rocks made of two different kinds of feldspar (potassium- and sodium-rich), together with quartz and a small…
Granite
Looking at granite between crossed polarisers makes it easier to distinguish the individual crystals. The rock is made up of interlocking rectangular feldspars…
Granite 2
This granite has an interesting texture, which is made visible by viewing it with crossed polarisers. Firstly, it has some very large feldspar crystals, set…
Granodiorite
Granodiorite is very similar to granite. It has less of the potassium variety of feldspar, more of the sodium feldspar (plagioclase). The dark minerals in this…
Granodiorite
Feldspars and quartz show up in shades of grey. Granite and granodiorite are acid rocks, i.e. they are mostly made of felsic minerals (quartz and feldspars)…
Gabbro (olivine gabbro)
Gabbro is a basic igneous rock, the intrusive equivalent of basalt. So, there is no quartz, and about half the rock is made of minerals (pyroxene and olivine)…
Feldspar (in anorthosite)
Some varieties of gabbro have few dark minerals, and are made almost entirely of plagioclase feldspar. This rock shows the interlocking texture of the feldspars…
Dolerite
Dolerite (or, in the USA especially, diabase) is the name given to basic igneous rocks found in small intrusions that are intermediate in grain size between…
Peridotite
Much of the Earth's mantle is made of peridotite like this, though it is less common to find such rocks at the Earth's surface. This peridotite is made of…
Basalt (olivine basalt)
Like many volcanic rocks, it has larger crystals in a very fine-grained matrix. The large crystals (mostly olivine) grew while the magma was held in a chamber…
