Galena and bournonite. Navan, Ireland

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Galena (blue-white) has a characteristic triangular pit (black, top left) due to the plucking along its perfect (100) cleavage. It carries abundant sulphosalt inclusions (blue-green, centre) which have the optical properties of bournonite. The inclusions range in size from submicroscopic to approximately 10 lam in diameter and many lie along grain boundaries of the galena. The presence of small sulphosalt inclusions is very common in galena. They are largely responsible for the trace and minor element chemistry often mistakenly reported to be in solid solution. Their similarity in optical properties, and small grain size (which makes chemical analysis difficult to impossible), means that they are often unreported or unidentified.

Polished block, plane polarized light, x 160, oil

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