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Let's Play, "What is it?"


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First some background:  This is a little mineral that I picked up on a bluff overlooking the and on the AZ side of the Colorado River near the "Needles" area, just below where I-40 crosses into California.  It caught my eye because it was the only dark pebble sitting on the surface; it was surrounded by "blond" material, mostly white dirt with some marble fragments and marbleized fossils pieces.  The dark color seen on the surface is not "desert varnish" by the true color of the mineral. 

Specs:  It has a hardness of 7-7.5 Mohs, SG of 2.58 (measured), colorless streak and is gray, gray-blue all the way through.  I made the cut using a diamond cutting wheel to check the color inside and to provide a sample to perform some powder tests on.

Pics:  Taken with my iphone 5S using a lamp with a clear, 60 watt, incandescent bulb.

What is it?  Yes, I know what IT is. :4chsmu1:

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Edited by 4meter
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We have a winner!  Au Seeker has the winning answer of.... Cordierite. 

Cordierite is a ring, aluminum, silicate similar to Beryl.  The key to IDing Cordierite is the SG of the mineral; which is the lowest SG of almost any silicate mineral and the change in color as the mineral is rotated (pleochroism) as can be seen in the last two mineral pics I posted.  The mineral is also called a "Water Sapphire" and was used by the vikings as a navigational aid when the sun was below the horizon or behind clouds for extended periods.

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