Analcime

Analcime is usually classified as a zeolite mineral, but structurally and chemically it is more similar to the feldspathoids. Analcime is not classified as an isometric crystal, as although the crystal structure appears to be isometric, it is usually off only by a fraction of an angle. However, there are truly isometric samples of the mineral, which makes its classification even more difficult. Due to the differences between the samples being too slight, there's no merit from having multiple species names, so as a result analcime is a common example for minerals occurring in multiple crystal systems and space groups.

It was first described by French geologist Déodat de Dolomieu, who called it zéolithe dure, meaning hard zeolite. It was found in lava in Cyclops, Italy. The mineral is IMA approved, and had been grandfathered, meaning the name analcime is believed to refer to a valid species til this day.

Properties

Analcime crystals always look pseudocubic. Its common crystal forms include trapezohedron, truncated trapezohedron with cubic faces, and more rarely either as a truncated trapezohedron, or the crystals can take the shape of a truncated cube that is typical to bixbyites. The crystals can occur either individually, as interconnected, form groups on plates or even in druzy form. Crystal masses can also form veins sometimes. Individual crystals are euhedral, meaning they have well defined faces. When on a matrix, the mineral takes a granular habit, meaning the crystals become anhedral. The color of the mineral varies due to trace impurities. The mineral is weakly piezoelectric and pyroelectric, meaning it produces a weak electric charge when it's rubbed or heated, hence the name analcime. Other characteristics include the fact that the mineral can have a blueish white fluorescence when inspected under short UV light, and a creamy white-yellow one inspected under long UV light. It does not show any pleochroic or radioactive properties. Analyses regarding the mineral's symmetry vary in results, however the most prevalent one being tetragonal. The mineral usually has polysynthetic twins, which are only visible in thin sections when the specimen is inspected under polarized analyzed light. Analcime mainly consists of oxygen (50.87%), silicon (25.51%), aluminum (12.26%), sodium (10.44%) but otherwise has a negligible amount of hydrogen (0.92%) as well. The highest quality specimens take the form of a trapezohedron, and can reach up to 25 cm. These specimens are associated with serandite, aegyrine and natrolite, and can be found at Mont Saint-Hilaire, Quebec. Of the pinkish-white specimens, the mineral takes the shape of well-formed cubes which can exceed 10 cm, originating from several sites in Val di Fassa, Trentino, Italy.