Microcline

Microcline has exactly the composition of orthoclase and is a different mineral. The difference is order: in microcline the aluminium and silicon atoms occupy specific sites in the framework, and in orthoclase they are distributed more randomly. Ordering happens slowly and only at low temperature, so microcline is what potassium feldspar becomes when it cools gently over a long period.

That distinction is invisible in a hand specimen and is exactly the case the identity resolver cannot settle from composition and crystal system alone — the two share a formula, and the ordering that separates them is not a field any general mineral database records.

Under a petrographic microscope with crossed polarisers, microcline shows a distinctive cross-hatched or tartan twinning that nothing else does. It is close to conclusive, and it is the reason the distinction from orthoclase is routine in thin section and near-impossible without one.

In hand specimen, colour is the only practical hint. Microcline is the feldspar most likely to be green: amazonite's blue-green comes from trace lead in the structure, and no orthoclase shows it.

Microcline forms in slowly cooled granites and, spectacularly, in pegmatites — the coarse-grained late-stage bodies where individual crystals can reach metres. It also forms by the slow inversion of orthoclase already in a rock, given enough geological time at low temperature.

Because ordering takes time, the presence of microcline rather than sanidine or orthoclase says something about a rock's thermal history: it cooled slowly, or it has been cool for a very long while.

Mined with the other potassium feldspars as a flux for ceramics and glass, where it is not distinguished commercially from orthoclase — the two behave identically in a kiln.

Amazonite is the exception. The green variety is cut as a gemstone and carved decoratively, and has been since antiquity: amazonite beads appear in Egyptian and Mesopotamian jewellery.