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dontgogreen
Joined: 02 Aug 2015
Posts: 79
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Posted: Nov 06, 2016 21:01 Post subject: Pseudoctahedral Calcite |
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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 pseudoctahedral habit. Can anyone comment on the occurrence of this habit in calcite?
Mineral: | Calcite |
Locality: | Corridor H roadway, Scherr, Grant County, West Virginia, USA | |
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Dimensions: | Crystal = 4mm |
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dontgogreen
Joined: 02 Aug 2015
Posts: 79
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Posted: Nov 06, 2016 21:02 Post subject: Re: Pseudoctahedral Calcite |
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A second specimen
Mineral: | Calcite |
Locality: | Corridor H roadway, Scherr, Grant County, West Virginia, USA | |
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Dimensions: | Crystal = 4mm |
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26747 Time(s) |
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dontgogreen
Joined: 02 Aug 2015
Posts: 79
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Posted: Nov 06, 2016 21:03 Post subject: Re: Pseudoctahedral Calcite |
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A third
Mineral: | Calcite |
Locality: | Corridor H roadway, Scherr, Grant County, West Virginia, USA | |
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Dimensions: | Crystal = 4mm |
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dontgogreen
Joined: 02 Aug 2015
Posts: 79
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Posted: Nov 06, 2016 21:10 Post subject: Re: Pseudoctahedral Calcite |
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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.
Mineral: | Calcite |
Locality: | Corridor H roadway, Scherr, Grant County, West Virginia, USA | |
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Dimensions: | 5.1cm |
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Mark Holtkamp
Joined: 26 Dec 2013
Posts: 17
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Pete Richards
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Joined: 29 Dec 2008
Posts: 828
Location: Northeast Ohio
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Posted: Nov 07, 2016 09:01 Post subject: Re: Pseudoctahedral Calcite |
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Pseudo-octahedral calcite 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.
Mineral: | Calcite |
Locality: | Minesota Mine (Minnesota Mine), Rockland, Ontonagon County, Michigan, USA | |
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_________________ Collecting and studying crystals with interesting habits, twinning, and epitaxy |
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prcantos
Site Admin
Joined: 17 Apr 2012
Posts: 243
Location: Granada (Spain)
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Posted: Nov 07, 2016 13:06 Post subject: Re: Pseudoctahedral Calcite |
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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. |
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_________________ Pablo Rodríguez Cantos
Λίθον˛ον απεδοκίμασαν˛οι οικοδομουντες |
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dontgogreen
Joined: 02 Aug 2015
Posts: 79
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Posted: Nov 07, 2016 13:16 Post subject: Re: Pseudoctahedral Calcite |
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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
Site Admin
Joined: 29 Dec 2008
Posts: 828
Location: Northeast Ohio
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Posted: Nov 07, 2016 17:50 Post subject: Re: Pseudoctahedral Calcite |
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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.
_________________ Collecting and studying crystals with interesting habits, twinning, and epitaxy |
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dontgogreen
Joined: 02 Aug 2015
Posts: 79
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Posted: Nov 29, 2018 11:14 Post subject: Re: Pseudoctahedral Calcite |
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We made another trip back to this area to see what had turned up. It took about half an hour to gather a large quantity of grey vuggy limestone riddled with calcite crystals. Many of the large pieces make excellent bookends and decorator pieces. Under the microscope, the crystals exhibit different, more complex forms than those collected previously. Additionally, breaking open some massive vein material revealed native sulfur and fluorite (mostly broken) associated with orange iron staining, probably from the dissolution of sulfides. The occurrence of native sulfur and purple fluorite makes me wonder if perhaps a pyrite crystal was weathered, allowing the sulfur to crystallize in its native state and allowing the iron to color the fluorite. This is just speculation.
I also learned that a large cave had been opened near this area several years ago during road construction. The team that explored it took lots of photos which can be seen here:
https://www.tristategrotto.net/Bob/Scherr%20Crystal%20Cave/album/index.html
(link normalized by FMF)
After exploration, the cave was sealed, and its whereabouts are unknown to the public. From the available info, I gather that it is quite close by to this locality.
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near Scherr, West Virginia |
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Mineral: | Calcite |
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near Scherr, West Virginia FOV 1 cm |
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Mineral: | Calcite |
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near Scherr, West Virginia 30 cm |
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Mineral: | Sulphur, fluorite, calcite |
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near Scherr, West Virginia ~15 cm each |
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makokerowill
Joined: 29 Nov 2018
Posts: 5
Location: Córdoba
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Posted: Nov 29, 2018 13:07 Post subject: Re: Pseudoctahedral Calcite |
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Buenas tardes amigos !
Conocéis algún yacimiento español donde hayan aparecido calcitas pseudo-octaédricas ?Saludos. dontgogreen wrote: | 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? |
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Jordi Fabre
Overall coordinator of the Forum
Joined: 07 Aug 2006
Posts: 4902
Location: Barcelona
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Posted: Nov 29, 2018 13:10 Post subject: Re: Pseudoctahedral Calcite |
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makokerowill wrote: | Buenas tardes amigos !
Conocéis algún yacimiento español donde hayan aparecido calcitas pseudo-octaédricas ?Saludos. dontgogreen wrote: | 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? |
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This is an English language Forum. Please ask your question in the Spanish side. Thank you.
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