Apatite with actinolite

A detective story

This little 9-mm specimen is colorless apatite with green needles of either actinolite or epidote or both. But I only figured that out a few years ago; this is another in the category of “had it 50 years” – although in this case, I thought I knew what it was, but my thinking was in error.

The specimen was part of my Indiana University mineralogy professor Dr. Beck’s collection that I received on his death in 1971. He kept little well-formed crystals like this in glass tubes with cotton in the bottom and a cork in the top, labeled with a code number, in this case 13-B. That correlated with a hand-written chart, A to E across the top and 1 to 20 down the side, so you had a total of 100 spaces for information. Which for 13-B said: “Rock crystal, rutilated, Birkenstock, Switzerland, F. Krantz.” Rock crystal is quartz, rutile needles are pretty common inclusions in quartz, and Krantz was a prominent German mineral dealership, started in 1833 by Adam August Krantz, and still in business.

A Facebook group I follow called Mineral of the Week chose “inclusions” for one week’s theme about four years ago. I dug this little guy out to photograph it for that group; it was still held into a plastic box with sticky goo where I mounted it in 1972, but the goo was embrittled and yellow.

Microscopic examination of the specimen immediately suggested “not rutile” – rutile is rarely green. Then there were the crystal faces, which also immediately screamed “not quartz.” I spent a LOT of time clicking around and finally decided quite confidently that the clear mineral is apatite, calcium phosphate, whose name means “deceiver” and this time it lived up to it. Hardness validated the crystallography and the conclusion that it is apatite. And the green needles are likely actinolite or some similar amphibole.

If both mineral names were wrong, the locality might be too. MinDat, the online mineral database, has no location similar to “Birkenstock, Switzerland.” There are two important locations for actinolite and related minerals included in colorless apatite: The Knappenwand area near Salzburg, Austria, and various places in the Gilgit-Baltistan area of Pakistan. It’s pretty unlikely that the Pakistan locations were producing much for the world mineral collecting market in the 1960s or earlier when Dr. Beck probably obtained this, so I attributed it to the Austria locality. At least that’s closer to Switzerland! Whether this confusion came from a misidentification by the dealer (unlikely), a mix-up in labels by Dr. Beck or me (possible), or whether it is a correct but undocumented locality for incorrectly labeled minerals, I can’t know.

I think anything included in apatite is a lot more interesting than inclusions in quartz, which happen all the time, so I’m pleased with this new identity for this old specimen, and of course I love the chase for information. Things like this were favorites of Dr. Beck – he threw such crystals at me and my friends in Advanced Mineralogy class, knowing we’d see the vitreous, colorless crystal as probably quartz and the strands as rutile, given our levels of inexperience. We would have to measure the angles of the faces and make careful crystal drawings that were useful in identification (or at least, elimination of something like quartz).

This crystal is complex enough that its faces are a challenge to identify (at least for me 54 years after that class), but after a lot of looking I think the diamond-shaped face at center is a common second-order hexagonal dipyramid, s (112¯1), the faces angling away from it are mostly various prisms, and the big one at top center/top right is probably a first-order pyramid, but to be honest I really don’t know how to orient the crystal to show its hexagonal symmetry.

After I shared it in the Mineral of the Week group a knowledgeable expert confirmed that the piece is most likely Alpine and likely from Knappenwand, not from Pakistan (and I guessed correctly that the Pakistan material began coming out only 20 or so years ago).

Just to be clear: don’t think of this as actinolite needles spearing though the apatite crystal. I’m sure the actinolite was there when the apatite grew around it; conceivably (but I think very unlikely) they grew simultaneously.

I re-mounted the specimen in its little box with fresh mineral tac (sticky goo) that will serve, I hope, for another 53 years. Cat. No. 13-B. The two photos are of the same specimen with somewhat different lighting and magnification.