Sphalerite and galena. Shullsburg, Wisconsin, USA

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Botryoidal sphalerite (light grey) comprises growth bands of different grain sizes; the coarser crystals (left) are well polished and overgrow earlier finer crystalline bands (centre left) with many polishing pits. Galena (white) is both crystalline and euhedral (centre), or botryoidal and poorly crystalline (centre left). Black areas are polishing pits, dark grey areas are carbonate (centre left) grains.

Polished block, plane polarized light, x 80, air

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