H is for Hopper

Not Edward.

Hopper, public domain photo.

A hopper is a container, shaped like an inverted pyramid or cone, with a wide upper opening to capture a lot of material, sloping sides, and a smaller lower opening to dispense the material in a more focused way. Ore bins, ore cars, and railway cars in mining often use hopper shapes.

In minerals, hopper crystals have similar shapes resulting in crystals that look hollow. The unusual form develops because particular crystallographic positions grow faster than others due to greater electrical (or electrochemical) attraction to those crystallographic positions. It has the effect of making edges grow faster than the centers of faces, and variability in the process can result in a step-like geometry down into the hopper, where the center of the face did not grow as fast.

For a point-by-point description of the process, please visit Steve Sorrell’s Mineral Matters post.

Here I just want to share two oddball examples of hopper crystals.

Dolomite after halite hopper, Fairview, Major County, Oklahoma. 6 cm wide. #333.

The rock above might look at first glance like some strange, crude carving of a low pyramid, or maybe you might see it as a squarish fossil of some sort. But it is the mineral dolomite, calcium-magnesium carbonate, which has replaced a hopper crystal of halite, common salt, sodium chloride. Such pseudomorphs (“false form,” in which one mineral takes the crystal form of another) occur in many places, but I think these from Major County, Oklahoma USA may be the most famous. At some localities there, the original halite is replaced by calcite or gypsum rather than dolomite. Here, dolomite appears to perhaps have filled in the old halite hopper (with the photo view from the bottom looking toward the top of the hopper), but in other examples, it seems that the dolomite replaces the halite as well.

The original hopper halite crystals probably grew in a drying, salty mud (Leitner and others, 2013, Origin of deformed halite hopper crystals, pseudomorphic anhydrite cubes and polyhalite in Alpine evaporites (Austria, Germany): Int. J. Earth Sci. (Geol. Rundsch.) 102, 813–829). The mud was laid down in shallow, hot flats in what is now Oklahoma in which the water was both mineral-rich and evaporating, during Permian time, about 265 million years ago (Guadalupian epoch). The environment must have been much like the modern coasts of the Persian Gulf, where similar mud flats are called sabkhas.

The resulting Blaine Formation contains beds of gypsum, halite, red mud, anhydrite, and dolomite, all indicative of evaporative conditions. Thus it isn’t surprising that once some halite crystals formed, when the chemistry of the water changed a bit, they were gently dissolved and replaced (or infilled without dissolving, but halite is so soluble that’s challenging to imagine) by dolomite or gypsum.

Wulfenite, Level 6, San Juan Poniente vein, Ojuela, Mexico. Field of view 3 x 5 cm. #1244.

My second example, above, is wulfenite, lead molybdate, from Ojuela, Durango, Mexico. The crystals are elongate tetragonal prisms, with the hopper forms extending down into the crystals parallel to the side prisms. As far as I can tell this is an unusual form for wulfenite in general and at Ojuela in particular, and I have not found any reports explaining its development. I’m confident that these are true hopper crystal growths, and not wulfenite growing around some other mineral (now gone) nor some kind of later etching out of the centers of the crystals.

For those who came here looking for Edward Hopper, here’s his painting Lime Rock Quarry II, 1926, Rockland, Maine. Via Bowdoin College Museum of Art.