Forsterite marble, metamorphosed Durness Limestone

Ledmore North quarry

Outcrop

Complex banded marble, metamorphosed Durness Limestone, Ledmore
The yellow-green bands are rich in serpentine.

Hand specimen

Forsterite marble, metamorphosed Durness Limestone, Ledmore
Forsterite is magnesium silicate, the pure variety of the mineral olivine. It is colourless, so it cannot be distinguished from the white calcite of the marble. However, if water gets in as the rock cools, it breaks down to serpentine, which has a characteristic yellow-green colour seen here in certain bands and fractures.

Thin section

Forsterite marble, metamorphosed Durness Limestone, Ledmore
The forsterite is in cracked round grains set in dusty-looking calcite. The Durness limestone was originally made of dolomite, calcium magnesium carbonate. Where it also contains silica, a chemical reaction can occur during metamorphism to make magnesium silicate (forsterite) plus calcium carbonate (calcite).

Plane polarized light. Field of view 6 mm across

Forsterite marble, metamorphosed Durness Limestone, Ledmore
The rounded grains of forsterite show up in bright colours agains a background of pearly speckled calcite.

Crossed polars. Field of view 6 mm across

D.J. Waters, Department of Earth Sciences, May 2003