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GeneralNegative crystals and their formation

9th Feb 2017 16:03 UTCDonn Cuson

Not sure if this is the right place for this question but here it is. I ran across a negative crystal in one of the quartz crystals that I collected. I had never seen one before and it took me a while to find out that it was called a negative crystal. I am trying to find out more about why during the formation of the quartz crystal itself that it would form an internal void in the shape of a crystal, with all the facets, faces and terminations of a crystal. In this case the negative crystal also does not have the normal pointed termination of a quartz crystal. Can anyone explain the why/how of a negative crystal formation?

9th Feb 2017 16:28 UTCEd Clopton 🌟 Expert

Neat specimen--where is it from, and how big is it?

Think of a crystal face not as part of a pre-ordained box-like form enclosing a crystal but merely as the contact between the crystal and the medium in which it forms. The surface between a crystal and the growth medium follows the same rules regarding orientation to the crystal lattice and the angles the faces form with one another, whether it is helping to enclose a crystal within the medium or to form a pocket of the medium within the crystal. Either way it is simply forming an interface between the crystal and the medium, and it doesn't know or care which function it is performing.

For crystal-chemistry reasons that are way over my head, molecules arrange themselves into flat crystal faces oriented at particular angles to optimize the energies in the crystal lattice. Under favorable conditions, a whole sheet of negative crystals--all oriented in parallel to the host crystal--may form along a thin crack in a crystal as the molecules heal the crack by rearranging themselves into "more comfortable" voids bounded by locally ideal crystal faces rather than a continuous crack bounded by random fracture surfaces.

Why a single void formed in that particular location in your crystal is difficult to say--maybe a particle of an impurity disrupted growth locally while crystallization proceeded normally on all sides until the interruption was fully enclosed, trapping a sample of the growth medium in the process.

Note that the c-axis of a negative crystal--yours as well--aligns with the c-axis of the host crystal, and all faces and edges of the negative crystal are parallel with their counterparts on the host crystal. In this case the negative crystal is distorted--it is stretched horizontally--so that instead of having a single point at the tip it has a line like the ridge of a roof along the top, right above the bubbles (more apparent in the second photo).

9th Feb 2017 17:48 UTCDonn Cuson

-- moved topic --

9th Feb 2017 18:06 UTCDonn Cuson

It’s a scepter head from Arizona, about 10mm across and the negative crystal is about 2mm

9th Feb 2017 18:17 UTCDonn Cuson

I have several other non-scepter crystals with the negative crystal orientated vertically and a good normal termination at the top. I can see now that the bubbles on the scepter negative crystal did interrupt the formation of the termination.

9th Feb 2017 19:37 UTCEd Clopton 🌟 Expert

It's probably the other way around--the attic-like termination created a convenient place for the bubbles to rest. At the time the negative crystal formed the gas forming the bubbles probably was still dissolved in the solution; otherwise they would have floated to the top of the cavity in which the host crystal was forming.

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