Apophyllite While some minerals are singletons, many occur in groups with different varieties, one notable example being the garnet family. Many of these are what geochemists call solid solution series, were one ion can substitute for another in the crystal structure such as the pyrope (chromium)-almandine (iron)-spessartine (maganese) series in garnets. Apophyllite is one of these groups with three main members, named for the Greek words meaning it flakes off (leaf away from), since when heated they dehydrate and flake apart. Potassium interchanges with sodium and the hydroxyl radical (OH) with fluorine. They commonly form in the ex gas bubbles in basaltic lavas after cooling due to precipitation from mineralised ground/sea waters into the vesicles, in much the same way that amethyst, agate and quartz will form geodes. This example is what is known as a floater, forming unattached to the vesicle wall. Other geological settings include the baked zone around granites, particularly in limestones where apophyllite precipitates in fractures and cavities. They are little known and too soft for jewellery use, but fairly common worldwide, found associated with the zeolite family, another common vesicle filler in basalt. The most famous locality is the Deccan Trapp flood basalts that cover a fair portion of India, having erupted from the depths of the Earth as the age of the dinosaurs waned towards its final explosive punctuation. Other noteworthy ones include Brazil, Germany, Canada, the Kalahari manganese fields in South Afica (including the famous Wessels and NChwaning mines) and the USA though they occur in basaltic provinces worldwide. They are popular with collectors since they are plentiful, have a wide variety of colours and pretty shaped crystals. Since they are cheap, plentiful and attractive they make a nice addition to any cabinet. Hues include brown (sodium), green and colourless (the OH version, cubes are its typical crystal habit). The most common crystal shape is a double pyramid. This specimen comes from around Poona in India, on the Deccan Plateau. Loz Dear Readers, Most of our posts are not reaching your news feed due to fbs filtering system. If you wish to enjoy our posts more often, use the following for information on how to go about it: tmblr.co/Zyv2Js1VWMUJ0. Image credit: Photolitherland mindat.org/min-283.html webmineral/data/Apophyllite.shtml minerals.net/mineral/apophyllite.aspx galleries/Apophyllite
Posted on: Tue, 25 Nov 2014 09:51:57 +0000
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