Axinite

A “Big Ugly”

I’m sure there are some mineral collectors who have only beautiful, pristine, perfectly crystallized, valuable items. But I suspect more are like me, and that we have at least one “big ugly” in our collections. It should come as no surprise to anyone who knows me that I have far more than just one.

My reasons for picking up a “big ugly” range from it being something I have no idea what it is, to I know but it looks cool, to it has lots of crevices and cracks that might have micro-crystals, to I know/don’t know what it is, but it seems interesting in a broader context.

This big, ragged chunk of rock, 20 x 18 x 12 cm, is from a mine at Elkhorn, Montana (the one in the Elkhorn Mountains), and it fits all my criteria above for picking up a rock.

It turned out that the white stuff is calcite (known at the time), the dark, smooth chocolate-brown material is axinite, a calcium-iron-aluminum borosilicate (known in the area, but it doesn’t look like it ought to, and I didn’t know it when I picked it up), and the lighter slightly greenish brown fine-grained stuff is fibrous amphibole (unsuspected to the naked eye), probably actinolite, fibrous enough to be called the variety byssolite in places, and in my opinion also asbestiform because it is quite altered.

Axinite, Ca₂Fe²⁺Al₂BSi₄O₁₅OH, even from the same location, is more often a pretty lavender color, sometimes transparent to translucent, even gemmy. This dark brown stuff is practically opaque. It almost certainly is the iron-rich end member of the axinite group, axinite-(Fe), a group that also includes magnesium- and manganese-rich end members. The crystals here may also have physical inclusions of something to contribute to their opacity and general roughness.

The fibrous actinolite is also really rough and ratty, mostly (I think) because it is altering from original more solid crystalline material, but it may have developed this way by growing into cavities within the axinite and calcite. Everything in this piece is complexly intergrown.

The whole works is part of a skarn, a zone of alteration that is both metamorphosed (minerals reorganized by heat and pressure from a nearby igneous body, but essentially no overall chemical change) and metasomatized (changes including new minerals resulting from chemical exchanges in both directions between the igneous body and the surrounding country rock). Skarns are often pretty interesting, mineralogically.

The igneous bodies providing heat and chemicals to this skarn are probably both the Boulder Batholith and the small Black Butte Stock, and the country rock is probably Cambrian limestones and dolomites, which would be the source of the calcite (calcium carbonate) and the calcium in the axinite. The boron to make the axinite is almost certainly from the Boulder Batholith, where excellent axinite crystals have been found, both within the granite and along its margins.

Among the borosilicates, axinite may form at the expense of a common tourmaline like schorl, NaFe²⁺₃Al₆(Si₆O₁₈)(BO₃)₃(OH)₃(OH), in more calcium-rich situations like skarns related to limestones and dolomites.

So, despite its “ugliness,” for me this rock is intriguing, informative, and interesting, and therefore beautiful. Cat. No. 1866-A; collected September 2021.

A more gemmy, lavender axinite crystal from the southeastern margin of the Boulder Batholith, near Pipestone, Montana.