Albite

Albite is the sodium-rich end-member of the plagioclase feldspar series, with the formula NaAlSi3O8, forming a continuous solid-solution with calcium-rich anorthite. Crystallizing in the triclinic system, it typically shows repeated albite law and pericline law twinning, producing the fine striations visible on cleavage faces that are a key diagnostic feature for collectors and mineralogists alike. Pure albite is white to colorless with a vitreous to pearly luster and a hardness of 6 to 6.5, though it can appear pale green, blue, or show a floating sheen in the peristerite variety, prized as a moonstone-like gem material.

Typical albite occurs as a common rock-forming mineral in granites, alpine clefts, and especially in granitic pegmatites, where it often develops as the elegant blade-like habit known as cleavelandite, frequently intergrown with muscovite, lepidolite, tourmaline, spessartine, and apatite. The classic pegmatite source is Minas Gerais, Brazil, particularly the Doce and Jequitinhonha valleys and localities such as Divino das Laranjeiras and Galiléia, producing enormous, gemmy cleavelandite plates. In the United States, superb clear crystals come from Amelia Court House, Virginia, and from the San Diego County pegmatites of Pala and Mesa Grande, including the Himalaya Mine. Exceptional, large, well-formed crystals are also mined from the pegmatites of Gilgit and the Shigar Valley in Pakistan and from Nuristan, Afghanistan.

Particular, highly collectible albite comes from Alpine-cleft environments, where slow hydrothermal growth yields elongated, gemmy pericline-twinned crystals up to decimeter size. Famous examples originate from the St. Gotthard massif in Switzerland, the Val di Vizze and Valle Aurina deposits in Italy, and the French alpine fissures of St-Christophe-en-Oisans and St-Véran. Fine crystals also occur in the Italian pegmatites of Baveno, Piedmont, and in Canada at Mont Saint-Hilaire and the Francon quarry near Montreal. Albite additionally forms the feldspar component of maw-sit-sit, the ornamental albite-jadeite rock from Myanmar. Collectors seek albite for its crisp twin striations, association with rare pegmatite minerals, the translucent beauty of cleavelandite plates, and the scientific value of well-formed specimens in demonstrating triclinic crystallography and feldspar twin laws.