Hidden hemimorphite
From Montana USA
“Best” and “finest” are subjective judgements for most things, but it’s generally agreed that the finest hemimorphite crystals in the United States come from the Summit Mine (formerly called the Montana Silver Star Mine, not to be confused with the mines at Silver Star, Montana) near Radersburg, Montana.
Hemimorphite is a hydrated zinc silicate, Zn4Si2O7(OH)2 · H2O, which often forms beautiful colorless bladed crystals. Its name means “half form” because its crystallography does not have a mirror plane of symmetry across its long (c) axis, so it cannot form symmetrical doubly terminated crystals like quartz and calcite often do. Hemimorphite crystals are like the slats of a picket fence, pointy on one end and flat on the other, and they often grow in radiating sheafs of crystals.
My specimens in these photos from the Summit Mine aren’t really especially world-class examples of hemimorphite. In fact, unless you look closely, you don’t see any hemimorphite at all: it’s quartz. But the quartz has the form of hemimorphite blades because it encases hemimorphite crystals, making this a pseudomorph (“false form”) of quartz after hemimorphite.
Pseudomorphs are often seen where the second mineral replaces the first, but that’s not the case here. Most of the hemimorphite crystals are still there, but almost totally enclosed in the quartz crust; that special case is called a perimorph. In a few places, the hemimorphite crystal has dissolved away (or been removed some other way) from inside the encasing quartz leaving a hollow mold, and that special case is called an epimorph. But pseudomorph works well enough for me, and these are excellent examples of this from the Summit Mine. This is not a unique occurrence for pseudomorphs like this, but they are unusual worldwide.
The quartz encasing the hemimorphite isn’t a shell of fine-grained microcrystalline quartz (chalcedony) but is tiny quartz crystals (“drusy quartz”) which give the specimens a sparkly appearance. The light brown tint on a couple of the specimens is a dusting of tiny descloizite crystals, lead-zinc vanadate.
These specimens were collected by Leland Stanford, a well-known Helena mineral collector who passed away May 20, 2022, and I got them from Paul Senn. Each specimen is about 4 to 4.5 cm wide and one of them has encrusted hemimorphite blades about 2.5 cm long. Hemimorphite crystals as long as 12 cm are known from this mine.
The Summit Mine began about 1897, but apparently it was only mined for lead and zinc briefly in 1948 and had little production (Reed, 1951, Mines and mineral deposits (except fuels), Broadwater County, Montana: US Bureau of Mines Info. Circ. 7592). According to Chris Tucker who has explored the mine, the shaft reached 600 feet and exploited lead-zinc ores in a replacement body in the Mississippian Madison limestone (Mm, gray-blue on the map above). The location is on the eastern flank of the Devils Fence Anticline about 8 miles (13 km) southwest of Radersburg. The anticline is a fold probably created by a ramp in the Lombard Thrust, part of the Sevier Orogenic Belt that represents collisions and subduction on the western margin of North America and that extends from Alaska to Mexico.
The mineralizing fluids might be related to nearby andesite or diorite sills (pink Kd on the map) and other intrusions, all of Late Cretaceous age (80 to 70 million years old). For more information see Reynolds and Brandt (2006, Preliminary geologic map of the Townsend Quad: USGS OFR 2006-1138) and Mahoney and others (2008, Geologic map of the Devils Fence Quad: MBMG Open-File Report 565).
Excellent.
Own some these. They are a delight.