Hemimorphite

from Idaho

I make no secret of the fact that I’m a hemimorphitophile. I like hemimorphite; it’s fairly common, and often makes sharp, vitreous crystals in attractive clusters.

Hemimorphite is a zinc silicate, named for its crystallography. “Hemimorph” means “half form,” because the crystals are not symmetrical along their length.

Imagine something like a doubly terminated quartz crystal, with similar pointy pyramids on both ends. Hemimorphic crystals (there are some other than hemimorphite itself) have different, non-symmetrical crystal forms on each end. In hemimorphite, usually one end is flat (called the pedion face) and the crystals grow from there to flattish laths on the other end often with the corners of otherwise pointy or rectangular shapes knocked off, sort of like a designer picket fence or tongue depressor, seen in several of the photos here.

This hemimorphite is associated with galena (lead sulfide, the main ore of lead) at the Democrat mine near Leadore, Idaho USA, where the deposit is hosted in Devonian Jefferson dolomite mineralized by fluids from a nearby granodioritic intrusion of late Cretaceous or early Tertiary age, around 70 to 50 million years ago. In 2017, the galena was mined mostly for its silver content, and I collected most of the specimens shown in this post in 2017 too.

Descloizite, which gives the light orange coating to some of the crystals, is a lead-zinc vanadate. It also forms the substrate to the sprays of hemimorphite in the photo above.

Geologist Bruce Cox explaining the setting, inside the Democrat Mine.

Reference: Cox & Antonioli, 2017, Ore controls of the Leadville (Junction) and Gilmore Mining Districts, Lemhi County, Idaho: A field trip guide: in Geology of the Leadore Area and other papers, 42nd Annual Field Conference, The Tobacco Root Geological Society, Cox, Hargrave, and McDonald, editors: Northwest Geology v. 46, p. 93.

The photo above shows a little 3-mm cluster of hemimorphite crystals situated in a ‘grotto.’ The white crystals are tipped by orange descloizite, which also coats the smaller downward-pointing cluster at right.

Usually orange-brown coatings like this would be limonite or some other iron oxide or hydroxide, but here it’s descloizite, lead-zinc vanadate. The orangish crust defining the cavity at left, and the red-orange misshapen crystals in it, are also descloizite.

In places the hemimorphite crystals are encrusted by a fine druse of later quartz.

More “tongue depressor” laths of hemimorphite coated with descloizite. Orange-red at left is also descloizite. Central lath is about 0.8 mm long.

If you are interested in the paper cited above, you can download for free the entire guidebook to the 2017 Tobacco Root Geological Society field conference at Leadore here.