Niccolite, sphalerite and carborundum grit. Settlingstones Mine, North Pennines, Britain

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Coarse crystals of niccolite (light brown-yellow, top) have partial rims of sphalerite (light grey, top). Fine-grained angular carborundum grinding grit (light grey, similar reflectance to sphalerite, centre left and right) occurs within the resin (dark grey, centre). Black areas are polishing pits.The restricted grain-size and angular nature of the grit are characteristic. It is often accompanied by angular sulphide fragments which have been plucked from the section.

Polished block, plane polarized light, x 80, air

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