Muzo habit

From Utah

If you know crystals, you might think these are steep pyramids of quartz, but you’d be wrong. I thought this tall, tapered, triangular-face habit was the one called “Tessin” for a canton in Switzerland where such quartz crystals are found. Tessin habit “faces” are actually sets of alternating prisms and steep positive rhombohedrons (narrow enough that they are barely visible even microscopically), similar to the alternation that makes “striations” on quartz crystals, but in the Tessin habit they alternate in such a way that the overall crystal tapers almost to a point.

But this is the Muzo habit, in which the faces that alternate are prisms and both positive and negative rhombohedrons (less steep than the rhombohedrons of the Tessin habit). It’s challenging to visualize this, and I apologize for the crystallographic jargon, but it has the effect of making one set of three “faces” more uniform in width while the other three faces taper almost to a point, or even disappear. You can see that in both crystals in my photo, where the surfaces that face upward (toward the camera) are triangular, and the two faces next to them are more rectangular and support the three triangular faces at the end of the crystal.

I’ve highlighted the edges in the version below to try to make the relationship clearer.

Muzo is a village in Colombia more famous as the source region for the majority of the world’s gem-quality emeralds.

Some of the faces also show diamond-shaped indentations from some other mineral that grew on the quartz (or that the quartz grew around) and that has since been eroded away. (See photo B in inset.) I don’t have a good guess for what those minerals might have been, so I’m open to suggestions.

These crystals are from the Deer Trail Mine, Marysvale, Utah. There are several different Deer Trail mines in Utah, but these are probably from the well-known Deer Trail in the Tushar Mountains in Piute County.

The Deer Trail Mine has been a major lead-zinc-silver-copper producer, with the metal ores replacing carbonate in the Permian Toroweap Formation. The mineralizing fluids were probably associated with intrusive and extrusive igneous rocks of the Marysvale Volcanic Field of southwestern Utah, mostly about 30 million years old and probably related to weak zones and crustal thickness changes along the northwestern margin of the Colorado Plateau.