FMF - Friends of Minerals Forum, discussion and message board

[John Nash]

This is a fun surprise. I found this in a small vug in basalt on the edge of a parking lot. The locality I've mentioned before, the Onion River, just south of Lutsen, Minnesota. The specimen is about 6 cm long and entirely clear calcite. It broke off a vein of massive calcite. When I took it out, I thought I'd lost the tip, in the pocket mud it looked flat. But look at the termination! Anyone who enjoys calcite and its strange crystal forms want to tell me what to call this?

[gemlover]

How about a "truncation?"

[Peter Megaw]

Or "George"

Seriously, this looks like a "c" face to me...

Perhaps Pete Richards can throw more light given his love for calcite crystallography

[Susan Robinson]

Hi John,
According to my husband, who is a mineralogist, he thinks the extra face in question is probably a basal pinacoid, and {0001}, commonly referred to as a c-face by collectors.
I hope this information helps you.
Susan Robinson

[Pete Richards]

Yes, this extra face is almost certainly the pinacoid, a face which is perpendicular to the three-fold symmetry axis. There should be one such face at each end of the crystal, but your crystal is typical in only having one end developed.

Other faces, most easily seen in the "top" view (your last image) occur in groups of three or six, the latter not uniformly distributed around the crystal, but sort of in groups of three pairs. The first of these belong to a rhombohedron, and the others belong to a scalenohedron. Calcite has many of each of these types of forms, which differ in the angles they make with the vertical axis and the three horizontal axes. It is not possible to make any reliable guesses about which rhombohedron and which scalenohedron this crystal displays. In the overdrawn copy of your third image, below, the face labeled 1 is the pinacoid, the face labeled 2 and the two which are of the same shape (but not size, because of difference in degree of growth) belong to a rhombohedron, and the face labeled 3 and its five equivalents belong to a scalenohedron. Your second image makes clear that there is an additional rhombohedron, more shallowly inclined to the three-fold c-axis, though we only see one face of this form.

I have a friend in Minnesota who occasionally sends me calcite crystals from the basalts along the west shore of Lake Superior, more or less the same general area this crystal came from in all probability. There are definitely some very interesting calcites to be seen, and many involve several generations of growth on the crystals.

The rhombohedral faces of this crystal show signs of overgrowth, and if this had continued long enough, these faces might have disappeared, replaced by the scalenohedron.

[John Nash]

Thanks Pete! I appreciate your drawing it out for me.

Most of the basalts along the western shore of Lake Superior (Minnesotan's call it the North Shore) aren't very vuggy. There are some very big (but very empty) pockets minutes north of Duluth and here and there there are pockets, sometimes with zeolites, up to around Lutsen, about 90 miles NE up the coast.

John