StilbiteZeolites are a common group of minerals, somewhat neglected by collectors, though stunning e
StilbiteZeolites are a common group of minerals, somewhat neglected by collectors, though stunning examples such as this rosy spray are very beautiful in their own right. Stilbite is one of the group’s more common minerals, though it was subdivided in 1997 into calcium (by far the most common) and sodium dominated versions which can only be told apart by geochemical analysis. The name (given by Abbe Hauy in 1801) comes from the Greek for to glitter, as it sparkles with pearly iridescence across its cleavage surfaces (planes of fragility within a crystal structure that reflect areas with few bonds holding the structure together).Zeolites are minerals born in hydrothermal systems, those vast circulations of hot briny water that move through and alter volcanic or metamorphic rocks. Most of the major deposits occur in cavities or as fracture filling veins in altered basalts in large igneous provinces (vast outpourings of lava that emerge during continental rifting or as a result of mantle plumes) like the Deccan Trapps in India or the North Atlantic Igneous province (covering Iceland, the British Isles (eg the Giant’s Causeway and Fingal’s Cave), Canada and others). Unsurprisingly they are usually hydrated, in this case sodium or calcium aluminium silicates resulting from the alteration of magmatic minerals such as feldspars. More rarely they occur in lightly metamorphosed sedimentary rocks formed of volcanic material, altered metamorphic gneisses and granite pegmatites.Stilbite is found in a wide variety of pastel colours, including orange, yellow and white. It is only rarely faceted for collectors, as its softness (3.5 on Mohs scale) and cleavage make the task difficult. Most of it forms as microcrystalline aggregates of interfelted crystals such as the 13 cm specimen in the photo, originally mined in 1968 in New Jersey and thought to be the best specimen ever mined the USA.LozImage credit: Joe Budd/Rob Lavinsky/iRocks.comhttp://www.mindat.org/min-3785.htmlhttp://www.minerals.net/mineral/stilbite.aspxhttp://www.iza-online.org/natural/Datasheets/Stilbite/Stilbite.html -- source link
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