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Pseudoctahedral Calcite
  
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dontgogreen




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PostPosted: Nov 06, 2016 21:01    Post subject: Pseudoctahedral Calcite  

There is a large roadcut in West Virginia along Corridor H where I have found a variety of minerals such as quartz, calcite, strontianite, and sulfur. They are not particularly flashy or large specimens, in fact a prominent collector described them as "homely", but I did notice that many of the calcites have a psuedoctahedral habit. Can anyone comment on the occurrence of this habit in calcite?


calcite1.jpg
 Mineral: Calcite
 Locality:
Corridor H roadway, West Virginia, USA
 Dimensions: Crystal = 4mm
 Description:
 Viewed:  2299 Time(s)

calcite1.jpg


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dontgogreen




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PostPosted: Nov 06, 2016 21:02    Post subject: Re: Pseudoctahedral Calcite  

A second specimen


calcite2.jpg
 Mineral: Calcite
 Locality:
Corridor H roadway, West Virginia, USA
 Dimensions: Crystal = 4mm
 Description:
 Viewed:  2289 Time(s)

calcite2.jpg


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dontgogreen




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PostPosted: Nov 06, 2016 21:03    Post subject: Re: Pseudoctahedral Calcite  

A third


calcite3.jpg
 Mineral: Calcite
 Locality:
Corridor H roadway, West Virginia, USA
 Dimensions: Crystal = 4mm
 Description:
 Viewed:  2285 Time(s)

calcite3.jpg


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dontgogreen




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PostPosted: Nov 06, 2016 21:10    Post subject: Re: Pseudoctahedral Calcite  

These calcites occur in a vuggy limestone, about one foot in thickness that dips about 45 degrees. The area has undergone quite a bit of deformation in the form of folding. There are several of these calcite-producing layers at this particular outcrop, however the majority of the limestone and shales here are barren of minerals. This particular layer produces the pseudoctahedral calcites in association with powdery strontianite. Some small strontianite sprays were located nearby, but not within this layer where it was usually in massive pods of about 1-3in.


calcite4.jpg
 Mineral: Calcite
 Locality:
Corridor H roadway, West Virginia, USA
 Dimensions: 5.1cm
 Description:
 Viewed:  2275 Time(s)

calcite4.jpg


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Mark Holtkamp




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PostPosted: Nov 07, 2016 07:14    Post subject: Re: Pseudoctahedral Calcite  

Long time ago there was an article in Lapis magazine about a pseudo-octahedral crystal from the Juchem quarry in Germany. I also found one for sale on minfind, from Aussig in the Czech Republic.

These crystals are probably a combination of a suitable rhombohedron (e.g. {301} and the base pinacoid.

http://www.smorf.nl/draw.php?json=q_a1UtI1NtLPVdLxs_LXCbCqdrQy1HECYmcrAz0LUxOdQCtLA50gEBFsZWhkUKsTYhVd7WFloOMNxD5AhS5WhrU6QBFjFJHYWgA=
(link normalized by FMF)

Cheers, Mark
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Pete Richards
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PostPosted: Nov 07, 2016 09:01    Post subject: Re: Pseudoctahedral Calcite  

Pseudo-octahedral clacite has also been found in other places. There was an article in the last 10 years about an Australian example (sorry not to have a better citation). Pseudo-octahedral calcite crystals were found in a boulder at the Minesota Mine in Michigan's Upper Peninsula, and described in Rocks & Minerals by Pete Rodewald and me. See R. Peter Richards & Peter Rodewald (2008) Pseudo-Octahedral Calcite from The Minesota Mine, Ontonagon County, Michigan, Rocks & Minerals, 83:4, 308-313, DOI: 10.3200/RMIN.83.4.308-313.

These crystals turned out to be a complex combination of four forms. The summary figure is appended below.



Calcite.jpg
 Mineral: Calcite
 Locality:
Minesota Mine (Minnesota Mine), Rockland, Ontonagon County, Michigan, USA
 Description:
 Viewed:  2112 Time(s)

Calcite.jpg



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prcantos
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PostPosted: Nov 07, 2016 13:06    Post subject: Re: Pseudoctahedral Calcite  

I can provide a graphical approach where the rhombohedron {301} is changed into a pseudoctahedron by a combination with the pinacoid {001}, according to

Mark Holtkamp wrote:
These crystals are probably a combination of a suitable rhombohedron (e.g. {301} and the base pinacoid.



CalcitePseudoctahedron.jpg
 Description:
 Viewed:  2048 Time(s)

CalcitePseudoctahedron.jpg



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dontgogreen




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PostPosted: Nov 07, 2016 13:16    Post subject: Re: Pseudoctahedral Calcite  

The crystal drawings shown by Mr. Richards are more similar to a true octahedron than the crystals I have found. Each of these pseudo-octahedral crystals has one face that is terminated by a very low-angle rhombohedron, but since the crystals are almost equant in dimension, they appear octahedral from many angles.
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Pete Richards
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PostPosted: Nov 07, 2016 17:50    Post subject: Re: Pseudoctahedral Calcite  

As a purely geometric exercise, there are a number of possible ways to make a calcite crystal that looks somewhat like an octahedron. Some of these are unlikely, if the forms used to make them are rare or unknown on calcite. Palache's table of calcite forms, though dating from the 1940's, is the Bible for calcite forms. He does not list {30-31} as a known form, which is problematic for its proposed use in this discussion. He does list {03-31} which is identical in "steepness" but rotated by 60° relative to {30-31}; it could contribute to an "octahedral" crystal of the same appearance, but it is also an uncommon form.
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