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Very unusual calcite crystal
  
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Pete Richards
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PostPosted: Dec 21, 2016 21:10    Post subject: Very unusual calcite crystal  

In recognition of the current topic on faden quartz, I offer another strange and distantly related distorted crystal, this one calcite. This is very rare, to my knowledge a one-time occurrence involving only a handful of crystals.

The one of these crystals I have seen is apparently typical of this small batch. It appears to be monoclinic: a flattened rhomboid colorless crystal, complete and free of attachment. A thin white line leads perfectly straight from one acute corner to the opposite one. This linear structure appears (no thin sections allowed!) to lack the microstructure that characterizes classic faden crystals. For the purposes of this topic, I will avoid referring to it as a faden, and just call it The Line. The crystal has well formed, sharp faces.

The one I have seen was brought to me for examination by George Robinson in 1987; I think it resides in the Canadian Museum of Nature, but I am not sure. It is listed as coming from Black Rock, South Africa, which could refer to several mines in the Kalahari, depending on which were operating in the late 1980s.

I was able to study it on the optical goniometer and thereby work out its geometry. While it appears monoclinic, all the faces can be assigned to common forms known from calcite elsewhere in the region.

The crystal is elongated along The Line, which turns out to pass from one {101} (cleavage rhomb) face to the one at the opposite end through the crystal. The whole crystal morphology is distorted by the fact that the major crystal grew from this linear seed, not from a point of attachment, as is typical.

The first image shows this wonderful crystal. The second is a SHAPE drawing reproducing its morphology, and listing the forms involved. The third is a SHAPE drawing of an undistorted crystal using the same forms. Other undistorted crystals with somewhat different shapes could be drawn varying the relative sizes of the faces. But what is striking is the wonderful perfectly balanced modification of the form by the growth of the crystal around The Line.

By inference, The Line is in all probability a single calcite crystal, elongated as stated. I have no idea how it formed. Some cave formations form thin tubes that are single crystals, but The Line is so thin that, if hollow, water would probably move through it only by capillarity, not by gravity. In any case, there it is. The origin of The Line a mystery; the rest of the morphology is explainable because The Line occurred.

I mentioned that there were several of these crystals found at the same time. One has to wonder if they all formed at different places along The Line, a one and only line, like a string of beads!



Calcite N'Chwanning 1b.jpg
 Mineral: Calcite
 Locality:
Black Rock Mine, Black Rock, Kalahari manganese field, Northern Cape Province, South Africa
 Dimensions: About 2.5 cm
 Description:
 Viewed:  1497 Time(s)

Calcite N'Chwanning 1b.jpg



MonoCalcite 1.jpeg
 Mineral: Calcite
 Description:
 Viewed:  1496 Time(s)

MonoCalcite 1.jpeg



MonoCalcite 2.jpeg
 Mineral: Calcite
 Description:
 Viewed:  1523 Time(s)

MonoCalcite 2.jpeg



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Collecting and studying crystals with interesting habits, twinning, and epitaxy
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Tobi




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PostPosted: Dec 22, 2016 03:01    Post subject: Re: Very unusual calcite crystal  

I gave this crystal the 20,000th "Thumb up" of FMF :-)
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marco campos-venuti




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PostPosted: Dec 22, 2016 05:18    Post subject: Re: Very unusual calcite crystal  

It is possible that calcite and quartz fadens are formed in two different ways?


faden group.JPG
 Mineral: Faden Quartz Crystals
 Locality:
Pakistan
 Dimensions: 4-8 cm
 Description:
 Viewed:  1384 Time(s)

faden group.JPG


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Roger Warin




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PostPosted: Dec 22, 2016 05:42    Post subject: Re: Very unusual calcite crystal  

Yes if the cause of these formations of bubble ribbons is different and not induced by a spacing of the walls of the diaclase.
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marco campos-venuti




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PostPosted: Dec 22, 2016 06:24    Post subject: Re: Very unusual calcite crystal  

Pete Richards wrote:
The whole crystal morphology is distorted by the fact that the major crystal grew from this linear seed, not from a point of attachment, as is typical.


I guess also the shape of quartz faden crystals depends by a preexisting thread. They have the same shape!
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marco campos-venuti




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PostPosted: Dec 22, 2016 06:28    Post subject: Re: Very unusual calcite crystal  

This is a Faden Gypsum from China. Again the same generic shape by growing over a thread. The thread is curved and has no bubbles.


Gypsum faden crystals, Guilin, China.jpg
 Mineral: Gypsum (variety faden)
 Locality:
Guilin Prefecture, Guangxi Zhuang Autonomous Region, China
 Dimensions: 14 cm
 Description:
Gypsum faden crystals
 Viewed:  1388 Time(s)

Gypsum faden crystals, Guilin, China.jpg


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