Cholla Cat

All the pretty micro-mineral colors

This wulfenite (lead molybdate) crystal is 6 mm wide. Photo is a stack of 13 photos to enhance depth of field.

When Michael Shannon and Malcolm Alter explored a remote, old, unnamed adit in the Buckskin Mountains of western Arizona in 2021, they encountered thick piles of cholla cactus needles brought in by rodents, and a live “ring-tailed cat” (a coatimundi, related to racoons), leading to their name “Cholla Cat” for this little mine. It has become famous for some really colorful specimens. I acquired those shown here from Shannon Family Minerals.

The location in La Paz County, west-central Arizona, is within or marginal to a huge, complex, enigmatic zone called the Mojave-Sonora Megashear (green dashed lines on the USGS geologic map above), a feature postulated to be part of the southwestern boundary of North America at various times from Precambrian to at least Jurassic (Anderson and others, eds., 2005, The Mojave-Sonora Megashear Hypothesis: Geol. Soc. Amer. Special Paper 393). The zone contains Precambrian rocks (gray on the map above, including in the Buckskin Mountains) along the southwestern flank of the Colorado Plateau that have been dismembered by faulting related to the San Andreas and Walker Lane (red highlights on the map) as well as basin-and-range normal faulting. All of this could easily allow mineralizing fluids to enter the rocks.

I might tackle aspects of the Mojave-Sonora Megashear in a future tectonic post; this one is mostly just to share some of the pretty minerals from the Cholla Cat mine.

Some of the prettiest combination pieces are yellow to orange mimetite (the granular material above; lead arsenate-chloride) and bladed yellow-orange wulfenite (lead molybdate) on dark purple fluorite. Field of view about 6 mm.

Fluorite at Cholla Cat seems to typically form octahedrons, but they seem to rarely be nice smooth crystals. My two specimens above show the rough character, which is really mostly intergrown and parallel-grown cubes that have an overall octahedral shape, perhaps seen best in the crystals at far right in the right-hand photo. Each of these plates of fluorite is about 35 mm long, and the color differences are real. The one on the left is associated with tiny yellow mimetite and a couple larger broken tabular wulfenite crystals. Sometimes the crystals have enough complex forms, such as the tetrahexahedron, that they are almost little balls.

I believe the white to tan crystals above (each specimen about 38 mm) are willemite, zinc silicate, but I’m not confident of that. They fluoresce blue-white as willemite sometimes does, but these stubby crystals don’t show willemite’s trigonal crystallography very well. A decent alternative would be cerussite, lead carbonate (but these do not react in acid) or possibly anglesite (lead sulfate). Striated crystal faces occur in both of those and in these specimens. I’m confident from the crystallography that they are not hemimorphite (another zinc silicate). They really need analysis. The orange crystals nestled in the left specimen are wulfenite.

Calcite in these specimens tends to be almost flat blades, close to the habit sometimes called “poker chip,” but some clusters of scalenohedral crystals form crusts, and occasionally, absolutely flat hexagonal plates like those above occur (with yellow mimetite). The crystals near center in the photo above are each about one millimeter across.

The dark red-orange material in the photo above (FOV about 40 mm), with calcite, is possibly mimetite, but it could be toward vanadinite (lead vanadate chloride, which forms a series with mimetite) or even descloizite (lead-zinc vanadate). It’s more botryoidal than the tiny yellow mimetite prisms that are more common, at least in my specimens.

This last photo shows bright yellow mimetite prisms on purple fluorite. FOV about 3 mm.