Sulphides within ultramafic rocks in ophiolites
Heazlewoodite, pentlandite, magnetite an native copper. Unst, Shetland Islands, Britain
Heazlewoodite (white, top) is intergrown with pentlandite (brown, with cleavage, centre) and rounded magnetite (light brown-grey, left) crystals, in a dunite. Minor amounts of magnetite also occur along fractures within pentlandite (bottom left, bottom centre) as a very characteristic intergrowth. Trace amounts of native copper (centre left) within magnetite are present but have tarnished to an orange colour. Dark grey areas are secondary silicates.
Polished thin section, plane polarized light, x80, air
Chromite, pentlandite, heazlewoodite, millerite and ferritchromit. Unst, Shetland Islands, Britain
Chromite (dark grey, top left) has a thin spongy ferritchromit rim (lighter grey). Pentlandite (brown, centre) shows good cleavage along (111) and has altered, partially along this cleavage, to heazlewoodite (white, higher reflectance, centre bottom) which in turn has altered to millerite (yellow-white, lower reflectance, bottom centre left) with a trellis-like parting/cleavage. Black areas are silicates.
Polished block, plane polarized light, x 160. oil
Chromite (dark grey, left) has thin spongy ferritchromit rims (lighter grey, top centre). Pentlandite (brown-yellow, centre bottom) shows its characteristic cleavage and surrounds a grain of pink nickel arsenide (centre right). Pentlandite is altering along fractures to heazlewoodite (white, higher reflectance, centre top) Silicates are black.
Polished block, plane polarized light, x 160. oil
Heazlewoodite, millerite and nickel arsenide. Unst. Shetland Islands, Britain
Small crystals of nickel arsenide (pink-brown, top right, centre left) are included within heazlewoodite (cream-white, centre) which is the main phase. Heazlewoodite shows multiple twinning along (0221) (bottom left) and alteration to lower reflectance, yellow millerite (bottom right) along a north-south oriented fracture. Black areas are silicates.
Polished thin section, plane polarized light. x 160, oil
Heazlewoodite, millerite, nickel arsenide. Unst, Shetland Islands, Britain
Heazlewoodite (white, centre), the main phase, encloses a small rounded crystal of nickel arsenide (pink, lower reflectance, centre left) and is altered along fractures and grain boundaries to millerite (light yellow-brown, lower reflectance, centre right). Black areas are serpentine.
Polished thin section, plane polarized light, x 160. oil
Heazlewoodite, millerite and nickel arsenide. Unst, Shetland Islands, Britain
This is the same field of view as previous section but with partially crossed polars. The strong anisotropy of heazlewoodite (blush-pink and blue-grey) and extreme anisotropy of millerite (golden yellow to deep blue) are visible. Millerite forms along the fractures in heazlewoodite but is associated with a phase with less intense anisotropy colours (orange-brown, left centre to blue-grey, centre right) which has the optical properties of godlevskite. Blue-green internal reflections of serpentine arc striking.
Polished thin section. partially crossed polars, x 160. oil