Surface textures
More Black Pine Mine crystals
Sometimes the nature of crystal faces is inherent in the crystal itself and how it grew, with irregularities or complex crystal faces interacting. But this post is about minerals deposited on the surface of other minerals, where they probably inhibit growth and result in patterns that look like they were etched into the original mineral’s surface.
The specimen at top is a quartz crystal, a Japan-law twin from the Black Pine Mine near Philipsburg, Montana USA. The twin (shown at left above) is interesting enough, but one face has abundant patterns “etched” into the surface of the quartz crystal. Actually, they are not etched at all, but represent places where some mineral was on the surface while the quartz grew, with the fan-like sprays of the other mineral preventing quartz from growing in those areas.
The most common mineral at Black Pine that has such a fan-shaped habit is pyromorphite, lead-phosphate-chloride, or at least something in the series between pyromorphite and mimetite (lead-arsenate-chloride). I’m confident that that is what was once deposited on this crystal surface, and that it was later completely removed, probably during the overall mineralization process rather than by later weathering.
The second example, also from Black Pine, is the quartz and tetrahedrite above. It’s really nothing very exciting, but two of the quartz crystal faces show those fans that I think were once pyromorphite or pyromorphite-mimetite series.
Even when they are still present, and they are quite abundant at Black Pine, without analysis it’s difficult to impossible to tell pyromorphite from mimetite there. Brown to lavender and yellow hexagonal prisms from Black Pine have traditionally been labeled pyromorphite, but Jensen and Nikischer (2012, Some notes on the mineralogy of the Black Pine Mine, Granite County, Montana: Mineral News, v. 28, no. 6, p. 1–10) note that much of the well-known brownish prismatic material from Black Pine often labeled pyromorphite is actually seen to be mimetite when it is analyzed.
It’s possible (but I know of no systematic analyses) that white, acicular (needle-like) material may be pyromorphite vs. the stubby well-formed hexagonal prisms that Jensen and Nikischer identified as mimetite. Either way, the photo above shows some of those white acicular crystals in isolated needles and some sheafs and fans that are probably similar to the material that produced the textures on the quartz crystal faces above.
I’ve had the twin at top for decades (it’s my catalog no. 3). The others are recent acquisitions from the collection of Bill Stanford via Butte mineral dealer Paul Senn.
If you happen to be interested, I’ve recently uploaded about 40 new photos of minerals from Black Pine to the MinDat Black Pine page, which was surprisingly lacking in many photos. You can also see them in order on my MinDat home page.