Garnet sillimanite gneiss - Isle of Harris
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Fact sheet
Description: 

This coarse-grained, Lewisian metamorphosed granite comes from Rodel on South Harris in the Western Isles of Scotland. The rock probably originated as a siltstone sediment in the Archean, but was metamorphosed when it was subjected to temperatures in excess of 800°C during the Laxfordian orogeny in the Proterozoic period.

In thin section the coarse-grained texture is dominated by two types of layer. The first layer type is composed of garnet (slightly lower relief and isotropic) and coarse-grained sillimanite (slightly lower relief and medium birefringence colours). The remainder of the rock is a finer-grained matrix of quartz, biotite and feldspars. The garnet was highly cracked by deformation and pressure changes as the rock returned to the Earth’s surface.

In Place
In Time
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Location description: 
Rodel, South Harris, Western Isles, Scotland
Accuracy:
Location precision: 
Poor
Timescale:
Timescale: 
Archean
Ma = Millions of years ago
In Focus
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Type:
Type: 
metamorphic
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