Piemontite metaquartzite
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Fact sheet
Description: 

This sample was found to the NE of Ben Hope in the far north west of Scotland. It comes from the "famous" diamond locality described by John Faithfull in the Scottish Journal of Geology (2007, volume 43, pages 33-40). It is a pink mica-rich quartzite, with small quantities of tiny dark-red, black and colourless crystals. It was the colourless garnets that led Matthew Forster Heddle (in the late 1890's) to suggest diamonds may be present in the rock. The recent investigation has confirmed that the "diamonds" are instead garnets. The sample is from an erratic boulder, but is almost certainly derived from local Moine rocks.

The sample contains an oxidized manganese-rich silicate and oxide assemblage: piemontite, muscovite, spessartine (Mn-rich garnet), K-feldspar, hematite, braunite, and rare zircon. Rotations 1 and 3 focus on the piemontite which is remarkably pleochroic, whereas rotation 2 shows the Mn-rich muscovite, more piemontite, iron/manganese oxides and tiny colourless garnets. Look closely as the garnets are difficult to see.

In Place
In Time
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Accuracy:
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Moderate
Timescale:
Timescale: 
Proterozoic
Ma = Millions of years ago
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Type: 
igneous
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