Mineral Monday

“He just smiled and gave me a wulfenite sandwich… “

This is one of the first mineral specimens I paid for, bought at the Geological Society of America convention in Milwaukee in 1970 for $6. Cat. No. 106.

Wulfenite, lead molybdate, is often an orange mineral that makes square, flat tablets. Many minerals are often elongate along their “c” crystallographic axes, and wulfenite also is, sometimes. But probably more often these flat crystals form, perpendicular to the “c” axis. The square shapes reflect the four-fold (tetragonal) symmetry of wulfenite.

They look like little sandwiches (or if you prefer, square ritz crackers with some darker central filling) because the initial deposit, now at the core between the brighter orange parts, may be either a different mineral, or maybe just wulfenite with some impurities. In the latter case we’d call these zoned crystals, in which the initial composition changed to something else (the brighter orange deposited on both sides of the original browner crystal).

Some wulfenite crystals like this are known in which the central band is actually mimetite, lead arsenate, indicating that the crystallizing solutions began more arsenic rich and changed to more molybdenum rich, and sometimes the central part is really wulfenite with impurities, which could be arsenic, or maybe vanadium, both of which often substitute for molybdenum.

But there is an alternative interpretation that says the “sandwich” isn’t really a central layer surrounded by two outer layers of orange “bread,” but is rather just a band on the surface around the center.

My specimen at top is from a classic wulfenite locality, the Erupción Mine in the Los Lamentos Mountains, Chihuahua, Mexico, but almost identical crystals from Ojuela in Durango, Mexico were studied by Kloprogge and Valera (2021, Characterization of “sandwich” wulfenite from the Ojuela Mine, Mapimí, Mapimí Municipality Durango, Mexico: Evidence of preferred secondary nucleation on selected wulfenite faces: Philippine Journal of Science 150(2):527-533) who showed that the central band is not really a central “wafer,” but is a surficial deposit on one particular crystal face (a tetragonal prism) on the sides of the wulfenite, where traces of many elements, including tungsten, copper, zinc, arsenic, vanadium, calcium and others substitute for both the lead and molybdenum in the wulfenite.

Unfortunately (he said ironically), my specimen’s crystals are almost completely undamaged, but there is one tiny corner of one that’s broken off, suggesting indeed that the central band does not extend into the interior of the crystal.

The pattern may have multiple origins, but in any case, it makes for a pretty sandwich-like crystal. The reddish-brown, dirty looking material on my specimen is tiny micro-crystals of descloizite, lead-zinc vanadate. The white substrate is calcite.

In 1772, this mineral originally had the cumbersome name “plumbum spatosum flavo-rubrum, ex Annaberg, Austria" (“yellow-red lead spear from Annaberg, Austria”), and later was called “Kärntherischer bleispath” (“Carinthian [i.e., the Austrian state of Carinthia] lead blades”) but happily for mineralogists it was renamed in 1845 for the Jesuit mineralogist Franz Xavier von Wulfen (1728-1805), who studied the lead ores of Bleiberg, Austria, now considered to be the type locality of wulfenite.

According to MinDat, the Los Lamentos Mountains were named for “the eerie shrieking and moaning sounds (lamentations) the wind makes passing through a series of vertical cracks at the crest of the range” above the Erupción mine.

Here’s an example from Ojuela. Obviously, the orange wulfenite crystals are prismatic rather than tabular, but you can see the central zoning on the large crystal at left center, which is 10 mm long. Associated with green mimetite. #1244.