Labradorite, Uncut and Polished

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I wrote last time about my new collection here, promising more on labradorite, the small shimmering blue/green/black stone in the top tray.

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It’s found in mafic igneous rocks including gabbro, basalt, norite and anorthosite; and some metamorphic and detrital sedimentary deposits.

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1″ (25 mm)

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Labradorite sometimes shows the schiller effect, a sparkling shimmer of iridescent red, orange, blue and yellow colors. This phenomenon has been named after the mineral- ‘labradorescence.’

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Per Wikipedia: ((Ca, Na)(Al, Si)4O8), a feldspar mineral, is an intermediate to calcic member of the plagioclase series. It has an anorthite percentage (%An) of between 50 and 70. The specific gravity ranges from 2.68 to 2.72. The streak is white, like most silicates. The refractive index ranges from 1.559 to 1.573. Twinning is common. As with all plagioclase members, the crystal system is triclinic, and three directions of cleavage are present, two of which form nearly right angle prisms. It occurs as clear, white to gray, blocky to lath shaped grains in common mafic igneous rocks such as basalt and gabbro, as well as in anorthosites.

 

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Inclusions

I’m grappling with photographing these shiny, reflective objects and I’m still struggling. I managed to find my camera’s exposure override so I gave 2+ stops more exposure to compensate for the white background (all exposure meters want to average the whole picture to 18% grey; paradoxically, white backgrounds require MORE exposure [wider f-stop, longer shutter speed or lower ISO rating]).

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Reverse

I have switched to LED spotlights of 8 watts to fight the heat but that leaves my depth-of-field very shallow. I begin to see why the pros use black backgrounds.

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Finally, here is another shot of pyrite from last post taken with a hand lens. A specimen this large, examined close up, looks very little like gold, which seldom forms crystals-which in any case would be isometric and not cuboid like pyrite.

 

 

Unless noted,text/images by todgermanica, noncommercial use free with attribution. Ad clicks pay only WordPress-not me.

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