This common mineral is a master of ice formation and massive crystal growth

Nature · Published 3 October 2026 Swipe more like this →

In short: Microcline is a potassium-rich mineral found in granite and pegmatites. While it shares a chemical formula with orthoclase, its unique triclinic structure creates a distinct cross-hatch pattern under a microscope. Beyond its geological significance, this mineral acts as a powerful ice-nucleating agent and can grow into gargantuan, record-breaking crystals.

Microcline, a potassium aluminium silicate, is a fundamental tectosilicate mineral. Its name derives from the Greek for small slope, reflecting the fact that its triclinic crystal system results in prism angles slightly smaller than 90 degrees. This distinguishes it from its polymorph, orthoclase, which is monoclinic. The two minerals are often physically identical, requiring x-ray or optical examination to tell them apart. When viewed under a polarizing microscope, microcline reveals a characteristic grating-like structure caused by minute multiple twinning, a feature that emerges as orthoclase transforms during slow cooling.

The scale of microcline can be staggering. The largest documented single crystals, discovered at the Devil's Hole Beryl Mine in Colorado, measured approximately 50 by 36 by 14 meters, potentially ranking them among the largest crystals of any material ever found. In other forms, microcline appears as amazonite, a green variety misnamed by Spanish explorers who mistakenly associated it with the Amazon Basin. It also frequently appears as perthite, where it contains thin lamellae of exsolved albite.

Beyond its structural quirks, microcline plays a surprising role in the atmosphere as an exceptionally active ice-nucleating agent. Scientists have recently begun to map how water molecules bind to its surface to trigger this process. In industrial and regulatory contexts, the mineral is known as E555. It has been the subject of scrutiny by the European Food Safety Authority, which issued a scientific opinion on it in 2008 and a formal call for technical and toxicological data in 2018 regarding its use as a food additive.

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