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Was gifted this . Just want to know what it is called ? enter image description hereenter image description hereenter image description here

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  • $\begingroup$ Please add more information; see this guide. $\endgroup$ – gerrit Mar 30 '17 at 11:55
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Can you scratch it with your fingernail? If yes, then this is a mineral called gypsum, a hydrated calcium sulfate (CaSO4·2H2O). It occurs in many forms, and this one is called satin spar.

It's a rather common mineral, and it is very fragile. Keep it away from sharp or hard objects, and keep it away from water.

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  • $\begingroup$ This is almost certainly correct $\endgroup$ – haresfur Mar 21 '17 at 23:52
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Regardless of the mineral composition, you have a great example of "antitaxial crystal growth". You can typically find fiber-like crystals where expansion of opposing surface walls moves in tandem with the crystal "boundary growth layer". I've seen antitaxial quartz and calcite crystals in shales and limestones on the hand-sample scale, like yours, as well as microscopic fibrous wings on pyrite botryoids in heavily deformed slates.

To simplify: when a crack in a hot, wet rock opens up, the surface walls of the crack will provide a fresh surface on which tiny crystals will precipitate and grow into the empty space beyond. If there is plenty of space and growth solution, the slowest-growing, most stable crystal faces will dominate and the void space will be filled with just a few, large, blocky crystals. However, if the void space opens very slowly, remaining no more than a thin, solute-rich seam, the side faces of the crystals will never grow, thereby limiting growth in an upward direction.

Antitaxial crystals should not be confused with minerals whose crystal morphology is genuinely fibrous, like, say, asbestos group minerals, or needle-like (acicular), like prehnite minerals. They're also pretty cool because structural geologists use the wavy growth directions to calculate stress directions as the host rocks are mashed and stretched over time.

Cool rock!

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