Differences Between Ulexite and Selenite

Physical Properties

1. Ulexite (hardness 2.5) is slightly harder than selenite (hardness 2).
2. When a piece of Ulexite with two polished parallel surfaces is placed on a newspaper, the words seem to “float” on the top surface. This can be seen as much as 30 degrees away from the perpendicular. When a slab of selenite with two polished perpendicular faces is placed on a newspaper, the words can be read as long as your eye is directly over the ends of the fiber and no more than 10 degrees away from the perpendicular. The image does not appear to float.
3. Directing a laser beam through one polished side of Ulexite, as reported by G. Donald Garlick and W. Barclay Kamb in the 1991 issue of the Journal of Geological Education, projects rings of light. This does not happen with Selenite.
4. Ulexite usually is found with muddy looking impurities, whereas Selenite is found more often in the pure state.
5. Specific gravity of Ulexite is 1.6 to 1.9, whereas Selenite has a specific gravity of 2.32
6. They both give a white streak test

Chemical Properties

1. Ulexite is a hydrated borate, NaCaB₅O₉ · 8H₂O whereas Selenite is a form of gypsum CaSO₄ . 2H₂0.
2. Ulexite will not dissolve in water whereas Selenite degrades in water.

Other Information

Ulexite is named for its discoverer, a German chemist, G. L. Ulex. Consequently, the mineral Ulexite is capitalized. Selenite produces light patterns similar to the moon, so is named after Selene, the Greek Goddess of the Moon). Therefore, selenite is not capitalized.

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