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Interesting cube shaped mineral surrounded by quartz or calcite inside a rock
  
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GabiG




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PostPosted: Dec 30, 2019 11:57    Post subject: Interesting cube shaped mineral surrounded by quartz or calcite inside a rock  

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 :-)


Cube mineral 1.JPG
 Mineral: Dolomite
 Locality:
North Laxey Mine area, Laxey, Isle of Man
 Dimensions: 9mm
 Description:
Cube shaped mineral inside rock cavity among quartz or calcite.
 Viewed:  694 Time(s)

Cube mineral  1.JPG



Cube mineral 2.JPG
 Mineral: Dolomite
 Locality:
North Laxey Mine area, Laxey, Isle of Man
 Dimensions: 9mm
 Description:
Cube shaped mineral inside rock cavity among quartz or calcite.
 Viewed:  691 Time(s)

Cube mineral  2.JPG



Cube mineral 3.JPG
 Mineral: Dolomite
 Locality:
North Laxey Mine area, Laxey, Isle of Man
 Dimensions: 9mm
 Description:
Cube shaped mineral inside rock cavity among quartz or calcite.
 Viewed:  693 Time(s)

Cube mineral  3.JPG



Cube mineral 4.JPG
 Mineral: Dolomite
 Locality:
North Laxey Mine area, Laxey, Isle of Man
 Dimensions: 9mm
 Description:
Cube shaped mineral inside rock cavity among quartz or calcite.
 Viewed:  693 Time(s)

Cube mineral  4.JPG



Cube mineral 5.JPG
 Mineral: Dolomite
 Locality:
North Laxey Mine area, Laxey, Isle of Man
 Dimensions: 9mm
 Description:
Cube shaped mineral inside rock cavity among quartz or calcite.
 Viewed:  695 Time(s)

Cube mineral  5.JPG



Close-up of Cube mineral 1.jpg
 Mineral: Dolomite
 Locality:
North Laxey Mine area, Laxey, Isle of Man
 Dimensions: 9mm
 Description:
Close-up of cube shaped mineral inside rock cavity among quartz or calcite.
 Viewed:  693 Time(s)

Close-up of Cube mineral 1.jpg



Close-up of Cube mineral 2.jpg
 Mineral: Dolomite
 Locality:
North Laxey Mine area, Laxey, Isle of Man
 Dimensions: 9mm
 Description:
Close-up of cube shaped mineral inside rock cavity among quartz or calcite
 Viewed:  692 Time(s)

Close-up of Cube mineral 2.jpg



Cavity holding Cube mineral .JPG
 Mineral: Dolomite
 Locality:
North Laxey Mine area, Laxey, Isle of Man
 Dimensions: 9mm
 Description:
Rock with several cavities, one of which contains a 9mm cream colored cube-shaped mineral.
 Viewed:  693 Time(s)

Cavity holding Cube mineral .JPG


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Philippe Durand




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PostPosted: Dec 30, 2019 13:37    Post subject: Re: Interesting cube shaped mineral surrounded by quartz or calcite inside a rock  

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

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GabiG




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PostPosted: Dec 30, 2019 14:21    Post subject: Re: Interesting cube shaped mineral surrounded by quartz or calcite inside a rock  

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! :-)
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Pete Richards
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PostPosted: Dec 30, 2019 14:24    Post subject: Re: Interesting cube shaped mineral surrounded by quartz or calcite inside a rock  

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!

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GabiG




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PostPosted: Dec 30, 2019 15:05    Post subject: Re: Interesting cube shaped mineral surrounded by quartz or calcite inside a rock  

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 ?
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Pete Richards
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PostPosted: Dec 30, 2019 15:32    Post subject: Re: Interesting cube shaped mineral surrounded by quartz or calcite inside a rock  

GabiG wrote:
(snip) What causes calcite to take on a rhombohedron shape ?


The complete answer to your question would be a book! A bare bones answer is that the symmetries of crystals are determined by the symmetries of the arrangements of the atoms inside them - their crystal structures - symmetric arrangements of atoms that vary widely from mineral to mineral but are always the same for a given mineral. Interactions between the crystal structure and details of the growth process produce the external shapes of the crystals. While all calcite crystals have three-fold symmetry, some are rhombohedra (some of them steep, some of them shallow) while others are scalenohedra and still others have combinations of these and other forms.

This particular rhombohedron has the same shape as the blocks into which calcite breaks because of weakness in the structure in particular directions (cleavages).

Calcite in fact is the most variable mineral we have, in terms of its crystal morphology. Literally thousands of different shapes have been documented. Your little rhombohedron is a nice simple form, very typical for calcite and its relatives.

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GabiG




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PostPosted: Dec 31, 2019 05:16    Post subject: Re: Interesting cube shaped mineral surrounded by quartz or calcite inside a rock  

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! :-)
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