What’s on the van? – Granite

This week’s What’s on the van? comes from Dr Dave Waters, Curator of Mineralogical Collections and Monica Price, Head of Earth Collections.

The rock that goes home for holidays
Every year, a group of Oxford undergraduates travel to north-west Scotland to unravel the fascinating geological history of the area by studying the local rocks and how they relate to each other. They stop at various places to explore the geology, including the ‘multicoloured  rock stop’, an exposure of rocks in a road-cut north of Loch Laxford. This is where our sample of granite was collected in 1998.

It is very old, part of a suite of ancient Precambrian rocks known as the ’Lewisian complex’, named after the Hebridean island of Lewis. It formed about 1.7 billion years ago when magma – hot molten rock – was intruded into an even older metamorphic rock called a gneiss. The magma cooled down, forming crystals of different minerals: pink potassium feldspar, white sodium feldspar, transparent grey quartz, and black grains of biotite mica and magnetite.

The students learn to identify the minerals by studying thin sections of the rock, just 0.03mm thick, under a petrological microscope. This allows them to examine the distinctive optical properties of each mineral under polarised light. The Oxford Earth Sciences Image Store shows you what they can see.

Each year, this piece of granite travels ‘home ‘ with the students on their fieldtrip, and helps to teach them about the different kinds of rock they will see in the field. It is part of the large collections of rocks, fossils and minerals in the University’s Earth Sciences Department and the Museum of Natural History that are available for students to study.