Sunday, December 1, 2013

Sunday Image: Cavity Minerals In Basalts

In cracks, fractures, geodes, gas vesicles of the Deccan Basalt lava flows occur secondary minerals of the zeolite family along with silica, calcite, apophyllite and iron hydroxides. They are often breathtaking in their fully faceted form and gorgeous color combinations.

Green apophyllite is one of the most common and appreciated minerals. I say mineral but really apophyllite is a general term used for three different minerals, fluor-apophyllite, hydroxy-apophyllite and natro-apophyllite. Most of the Deccan Basalt apophyllite are fluor- and hydroxy apophyllites. These specimens have a peculiar form. Their name is derived from the Greek apophylliso meaning "it flakes off". They have a basal cleavage like the micas which lends it to being easily split along one plane of weakness but an overall crystalline structure conforming to a tetragonal symmetry.

Here it is in the picture below growing like a creeper of other worldly kryptonite from a lustrous crystalline bed. 

The large off white crystals at the base of the geode are also apophyllite.  Zoom in and you will see large fleshy pink colored sheafs of stilbite clinging to the wall of the geode. The other minerals not easily recognizable are calcite, quartz and heulandite.

Source: The Gargoti Museum

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