Cuprite var. chalcotrichite

Exploring minerals from the Black Pine Mine, Montana USA

Reporting on explorations of the many new specimens I recently got from the Black Pine Mine near Philipsburg, Montana.

Cuprite, simple copper oxide, occurs in the oxidized zones of copper sulfide deposits. It’s unusual from Butte, where the oxidized zone is largely eroded away; four of the five photos (three by my friend Dan Evanich) of cuprite from Butte on MinDat are from the Bullwhacker Mine on the east side of the district, near the uplifted flank of the East Ridge, where the oxidized zone, high in the deposit, is still present.

Cuprite’s name is straightforward, named in 1845 by Wilhelm Karl von Haidinger from the Latin "cuprum" for copper.

Cuprite seems to be uncommon at the Black Pine Mine near Philipsburg as well. There’s only one photo on MinDat, but the Mineralogical Record article on Black Pine (by Dave Waisman, 1992) has a photo of a 9-mm grainy-botryoidal example that’s really similar to the 26 little specimens I got recently from Paul Senn. “Botryoidal” is from Greek words for “a bunch of grapes,” and refers to the rounded masses that you see in the left specimen here, which is 15 mm high.

Besides its botryoidal habit, cuprite often forms nice octahedrons, but there is a variety called chalcotrichite. That name is from Greek χαλκός khalkos, "copper" and τριχωτός trichotós, "hair" = "hairy copper," for its typical appearance as fine filaments. But that name is also applied to an interlocking framework of long, narrow cuprite crystals that make for amazing forms, ranging from tiny cityscapes to things like red circuit boards. The one example I have here from Black Pine is only about a half millimeter across.

The third photo is the result of an experiment. I left a grain of Black Pine cuprite in hydrochloric acid overnight. It dissolved, the water evaporated, and the precipitate was the feathery turquoise-blue material in the photo. It’s probably a hydrous copper chloride, of which there are many, but the closest mineral is probably eriochalcite, copper chloride with two water molecules. It’s water soluble as would be expected.

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