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[GabiG]

Hi all :-) Could you please help me identify this cube shaped mineral? I found this rock with a few small cavities. They are filled with either quartz or calcite. Not sure which exactly :-/ Inside the largest cavity among the crystals sits a creamy colored cube. It also appears slightly translucent. It has perfectly straight sides. Could it be dolomite? Thank you for your help :-)

[Philippe Durand]

hello, it does not look exactly cubic;
it may be calcite

[GabiG]

OK :-) That's interesting. I didn't know calcite can grow in cubes like this. I thought it's similar to quartz (sort of pointy angular). That's why I always get confused about distinguishing small quartz and calcite crystals. What about the color? It's sort of creamy or darker even. Could it be iron staining? Thanks you! :-)

[Pete Richards]

I think the common colorless mineral is quartz, not calcite. You can easily test a tiny piece for hardness and solubility in dilute acid; the results should distinguish unambiguously between calcite and quartz.

The large cream-colored "cube" is, as has already been pointed out, not a cube at all, but a rhombohedron - the corners of the faces deviate systematically from 90°. The cracking shown in the sixth figure is almost certainly the cleavage of one of the rhombohedral carbonates - calcite, dolomite, siderite, magnesite, rhodochrosite, and a couple of others. Rhodochrosite can be ruled out because of color, and magnesite is unlikely because it is fairly uncommon. But the other three are all possible, as are intermediates like ferroan dolomite. On the basis of its appearance alone, I would guess it is dolomite with some iron content. There is no good way to tell between these three without a piece of this mineral that you could sacrifice to testing.

Keep an eye out for some other examples!

[GabiG]

Thank you so much again for all your information! Really appreciated :-) Yes, I need to look out for more but I don't hold out much hope. The island here is quite small and there are only limited places to find rocks with crystals. I don't want to break this one because it looks nice, although there might be more crystals inside. I will break a little of the other crystals though from a different spot to test if it they are quartz or calcite either with vinegar or oxalic acid. I don't have hydrochloric acid. It's too strong. What causes calcite to take on a rhombohedron shape ?

[Pete Richards]

[GabiG]

That is brilliant! Thank you so very much!! :-) It really puzzled me because from the few rocks I have been able to collect here, the calcite crystals I've seen have always had this triangular or pyramid appearance. That's why I keep confusing it with quartz. Even the websites I looked at showed the calcite with this sort of shape. Unfortunately, trying to clean iron stains, I've damaged a few pretty rocks with acid because I mistakenly thought they were quartz. I know there are other tests I can do, but I need practice! I'm glad that I'm learning! :-) Thank you again! :-)