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Colorless crystals with sturmanite
  
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Pete Richards
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PostPosted: Nov 10, 2016 14:25    Post subject: Colorless crystals with sturmanite  

Recently I was examining one of my specimens of sturmanite from N'Chwaning #2 Mine. This piece contains beautiful gemmy yellow sturmanite crystals and many tiny colorless crystals that have from the beginning been labeled as quartz. They have tapered, striated "prisms" with small pointed teminations, too small to see the component faces. They look like quartz of Tessin habit.

But I saw a reflection from a tiny face in the termination where there should not be a face. And a look at Mindat showed that quartz from N'Chwaning is rare and not like these crystals.

Off to the SEM (lucky me). The images clearly were not of quartz, and EDS quickly showed that they were calcite! Rather unusual calcite morphologically, but quite reasonable.

Attached are some SEM images and my SHAPE drawing with the Miller indices of the forms I think comprise these little gemmy crystals.

Perhaps you have some too....



quartz1.jpg
 Mineral: Calcite
 Locality:
N'Chwaning II Mine, N'Chwaning mining area, Kuruman, Kalahari manganese field (KMF), Northern Cape Province, South Africa
 Dimensions: Crystals are about 0.8 mm tall if complete
 Description:
Colorless, horizontally-striated calcite somewhat mimicking quartz
 Viewed:  11286 Time(s)

quartz1.jpg



Quartz2.jpg
 Mineral: Calcite
 Locality:
N'Chwaning II Mine, N'Chwaning mining area, Kuruman, Kalahari manganese field (KMF), Northern Cape Province, South Africa
 Description:
 Viewed:  11238 Time(s)

Quartz2.jpg



Quartz3.jpg
 Mineral: Calcite
 Locality:
N'Chwaning II Mine, N'Chwaning mining area, Kuruman, Kalahari manganese field (KMF), Northern Cape Province, South Africa
 Description:
 Viewed:  11282 Time(s)

Quartz3.jpg



calcite2.jpeg
 Description:
Interpretation of calcite morphology, using only well known, common crystal forms for calcite.
 Viewed:  11278 Time(s)

calcite2.jpeg



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Roger Warin




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PostPosted: Nov 11, 2016 08:40    Post subject: Re: Colorless crystals with sturmanite  

Hi Peter,
Like you I am a lover of calcite, certainly not as competent.
I observe that as in Rhisnes (Belgium), the rhombohedron {101} and the pinacoid {001} terminate the N'Chwaning crystal (and also the others {021} and {443}).
Do you know the geochemical relations which condition this relatively rare dipyramidal {883} form for calcite, at least when it is dominant?
In Rhisnes, the deposit is very close to an area that produced marble.
Roger.
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Pete Richards
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PostPosted: Nov 11, 2016 10:38    Post subject: Re: Colorless crystals with sturmanite  

Hi Roger,

I wish I had something wise to say! For calcite in general, the hexagonal dipyramid {883} is considered rare, especially as a dominant form. But it is typical for calcite from several localities in Ohio, found at a specific stratigraphic horizon over its outcrop band that stretches for about 100 miles north-south. These rocks have never been exposed to elevated temperatures - fluid inclusions indicate temperatures of mineralization not greater than 80°C. And any other metamorophic influences are hundreds of kilometers away.

In these crystals, one can sometimes see phantoms that demonstrate that an earlier form was dominated by the scalenohedron {211}.

(I should state that I am using indices here that are relative to the morphological unit cell with c:a=1:0.8543. The structural unit cell is four times as elongate along the c axis and consequently the "l" index is four times as large for a given form.)

I cannot imagine what aspects of crystallization these crystals, or yours for that matter, might have in common with those at N'Chwaning II, that might explain the common aspect of their forms!



calcite.jpg
 Description:
Calcite, Erie County, Ohio. Drawing of an unusual specimen that has {883} and {211} about equally developed.
 Viewed:  11243 Time(s)

calcite.jpg



calcite 7.jpg
 Mineral: Calcite
 Locality:
Erie County, Ohio, USA
 Dimensions: Main crystal 2,5 cm tall
 Description:
Crystal strongly dominated by {883}. Most crystals would have a larger termination, making them less elongate in aspect.
 Viewed:  11170 Time(s)

calcite 7.jpg



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Debbie Woolf




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PostPosted: Nov 11, 2016 13:02    Post subject: Re: Colorless crystals with sturmanite  

Very interesting topic. I can add, in 1998 a handful of unique specimens were collected from a cavity in the footwall at N'Chwaning II. Prismatic, elongated, highly lustrous, colorless to pale blue calcites form the matrix to bright yellow ettringite that may be scattered in, on and around the calcites. (Cairncross 2012)
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Roger Warin




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PostPosted: Nov 11, 2016 23:00    Post subject: Re: Colorless crystals with sturmanite  

Hi Pete,
I still contemplate the crystal clearness of Erie County. Thank you for showing it.
The comments describe this crystallization well.
Roger.
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cptbtptp248




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PostPosted: Nov 24, 2016 22:13    Post subject: Re: Colorless crystals with sturmanite  

Hi Pete. Thank you for showing it. Very interesting topic. Like you I am a lover of calcite. I observe that as in Rhisnes,the rhombohedron,and the pinacoid terminate the N'Chwaning crystal.The earth is our homeland and all materials we need to live with come from nature.
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PostPosted: Nov 25, 2016 11:29    Post subject: Re: Colorless crystals with sturmanite  

Hi,
In Rhisnes, calcite dipyramids L {883} (morphological cell indices) can be found twinned or not.
The composition plan is (001) although Cesàro also highlighted the (100) plane, these 2 planes of compositions existing in the same twinned crystal.
(morphological cell indices)



161-Calc-rhisnes#2207-Cesaro_R.jpg
 Mineral: Calcite
 Locality:
Rhisnes, Namur District, Namur Province, Wallonia, Belgium
 Description:
 Viewed:  10788 Time(s)

161-Calc-rhisnes#2207-Cesaro_R.jpg



162-Isoscel-tw.jpg
 Mineral: Sketch
 Description:
 Viewed:  10754 Time(s)

162-Isoscel-tw.jpg



163-Rhisnes#2235-a1-e2b_R.jpg
 Mineral: Calcite
 Locality:
Rhisnes, Namur District, Namur Province, Wallonia, Belgium
 Description:
with Cesàro's annotations.e2 {100}a1 {001}p {101}L {883}
 Viewed:  10801 Time(s)

163-Rhisnes#2235-a1-e2b_R.jpg



164-Buttgen-Rhisnes-168.jpg
 Mineral: Sketch
 Description:
 Viewed:  10758 Time(s)

164-Buttgen-Rhisnes-168.jpg


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