Analcime

Analcime forms primarily through low-temperature hydrothermal processes in volcanic rocks. When basaltic lava solidifies, gas bubbles become trapped as vesicles. Over thousands to millions of years, groundwater heated to 50 to 200 degrees Celsius by residual volcanic heat circulates through these cavities. The water dissolves silica, aluminum, and sodium from the surrounding basalt, and as conditions stabilize, analcime crystallizes on the vesicle walls. It typically appears as one of the later-forming zeolites, following minerals like natrolite, thomsonite, and heulandite in the paragenetic sequence.

Analcime also forms diagenetically in sedimentary environments, particularly in saline, alkaline lake deposits. Volcanic ash falling into alkaline lakes reacts with the sodium-rich water at temperatures as low as 25 to 50 degrees Celsius. The volcanic glass dissolves and reprecipitates as analcime over geological time. This process is well-documented in the Green River Formation of Wyoming and Colorado, where extensive analcime beds formed in Eocene-age lake sediments approximately 50 million years ago.

The distinctive trapezohedral crystal habit of analcime, which makes it look like rounded cubes, results from its isometric crystal structure. Despite being classified as isometric, analcime is actually pseudocubic, with subtle departures from true cubic symmetry. The trapezohedron is a 24-faced form where each face is an irregular four-sided polygon. This habit is so characteristic that experienced collectors can identify analcime on sight from across a room. The water molecule in the structure can be driven off by heating to approximately 400 degrees Celsius, causing the crystal to become opaque and crack.

Analcime is most easily recognized by its distinctive trapezohedral crystal habit, which produces rounded, pseudo-cubic crystals with 24 faces. No other common mineral consistently forms trapezohedra, making this habit practically diagnostic. Crystals are typically white to colorless with vitreous luster and range from a few millimeters to several centimeters across. Hardness of 5 to 5.5 means analcime just barely scratches glass.

Distinguish analcime from leucite, which shares the trapezohedral habit but occurs only in potassium-rich volcanic rocks (never in basalt vesicles) and has a duller, greasy luster. Leucite also has a slightly higher specific gravity (2.47 versus analcime's 2.24 to 2.29). Distinguish from garnet, which also forms trapezohedra but is much harder (6.5 to 7.5) and denser (3.5 to 4.3). If in doubt, test hardness with a steel knife. Analcime is scratched easily by steel, while garnet is not. When heated in a closed tube, analcime releases water and becomes cloudy, confirming its identity as a hydrous zeolite. In dilute hydrochloric acid, analcime dissolves slowly and forms a silica gel, a behavior shared with other zeolites.