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Pete Richards
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Joined: 29 Dec 2008
Posts: 830
Location: Northeast Ohio
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Posted: Nov 10, 2016 14:25 Post subject: Colorless crystals with sturmanite |
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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....
Mineral: | Calcite |
Locality: | N'Chwaning II Mine, N'Chwaning mining area, Kuruman, Kalahari manganese field (KMF), Northern Cape Province, South Africa | |
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Dimensions: | Crystals are about 0.8 mm tall if complete |
Description: |
Colorless, horizontally-striated calcite somewhat mimicking quartz |
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11668 Time(s) |
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Mineral: | Calcite |
Locality: | N'Chwaning II Mine, N'Chwaning mining area, Kuruman, Kalahari manganese field (KMF), Northern Cape Province, South Africa | |
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11623 Time(s) |
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Mineral: | Calcite |
Locality: | N'Chwaning II Mine, N'Chwaning mining area, Kuruman, Kalahari manganese field (KMF), Northern Cape Province, South Africa | |
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11666 Time(s) |
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Interpretation of calcite morphology, using only well known, common crystal forms for calcite. |
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11662 Time(s) |
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_________________ Collecting and studying crystals with interesting habits, twinning, and epitaxy |
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Roger Warin
Joined: 23 Jan 2013
Posts: 1180
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Posted: Nov 11, 2016 08:40 Post subject: Re: Colorless crystals with sturmanite |
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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
Site Admin
Joined: 29 Dec 2008
Posts: 830
Location: Northeast Ohio
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Posted: Nov 11, 2016 10:38 Post subject: Re: Colorless crystals with sturmanite |
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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!
Description: |
Calcite, Erie County, Ohio. Drawing of an unusual specimen that has {883} and {211} about equally developed. |
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11631 Time(s) |
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Mineral: | Calcite |
Locality: | Erie County, Ohio, USA | |
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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. |
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11553 Time(s) |
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_________________ Collecting and studying crystals with interesting habits, twinning, and epitaxy |
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Debbie Woolf
Joined: 09 Feb 2009
Posts: 168
Location: Kent
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Posted: Nov 11, 2016 13:02 Post subject: Re: Colorless crystals with sturmanite |
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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
Joined: 23 Jan 2013
Posts: 1180
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Posted: Nov 11, 2016 23:00 Post subject: Re: Colorless crystals with sturmanite |
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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
Joined: 08 Nov 2016
Posts: 3
Location: paris
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Posted: Nov 24, 2016 22:13 Post subject: Re: Colorless crystals with sturmanite |
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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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Roger Warin
Joined: 23 Jan 2013
Posts: 1180
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Posted: Nov 25, 2016 11:29 Post subject: Re: Colorless crystals with sturmanite |
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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)
Mineral: | Calcite |
Locality: | Rhisnes, Namur District, Namur Province, Wallonia, Belgium | |
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11175 Time(s) |
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Mineral: | Sketch |
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11136 Time(s) |
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Mineral: | Calcite |
Locality: | Rhisnes, Namur District, Namur Province, Wallonia, Belgium | |
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with Cesàro's annotations.e2 {100}a1 {001}p {101}L {883} |
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11187 Time(s) |
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Mineral: | Sketch |
Description: |
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11142 Time(s) |
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