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| Search found 14 matches |
| Author | Message |
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 Topic: Forms Involved in Titanite Crystal |
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| Mark Holtkamp Replies: 19 Views: 21088 |
 Forum: Minerals and Mineralogy Posted: Jan 27, 2022 10:00 Subject: Re: Forms Involved in Titanite Crystal |
| Josele, the parallel extinction doesn't prove it is biaxial, it could still be uniaxial. But it is consistent with Pete's interpretation. If the photograps were taken along the b-axis, you would expec ... | |
| Topic: Forms Involved in Titanite Crystal |
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| Mark Holtkamp Replies: 19 Views: 21088 |
Forum: Minerals and Mineralogy Posted: Jan 23, 2022 09:22 Subject: Re: Forms Involved in Titanite Crystal |
| Nice, I think this settles it and Pete's first scheme is the correct one. The plechroism also fits, if the polarisation of you screen is horizontal.
What colors do you see if you turn the crystal 90 ... |
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| Topic: Forms Involved in Titanite Crystal |
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| Mark Holtkamp Replies: 19 Views: 21088 |
Forum: Minerals and Mineralogy Posted: Jan 22, 2022 08:30 Subject: Re: Forms Involved in Titanite Crystal |
| Josele, you can just use the cell of Zachariasen (with beta=119°43'), where the best cleavage is {110} and the common twin plane {100}. If you have a textbook with the same cleavage en twin plane you ... | |
| Topic: Forms Involved in Titanite Crystal |
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| Mark Holtkamp Replies: 19 Views: 21088 |
Forum: Minerals and Mineralogy Posted: Jan 20, 2022 16:22 Subject: Re: Forms Involved in Titanite Crystal |
| Hi guys
The 'old' unit cell in Mindat is from Zachariasen (1930). For the full reference see the link on Mindat's titanite page to: Speer J A, Gibbs G V (1976) (below the crystal structure applet). ... |
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| Topic: Hematite Confirmation and Location |
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| Mark Holtkamp Replies: 8 Views: 10866 |
Forum: What is it? - Where is it from? Posted: Mar 23, 2020 02:39 Subject: Re: Hematite Confirmation and Location |
| These crystals are a combination of a (trigonal) rhombohedron and a pinacoid. Quite common for hematite I think. | |
| Topic: Pseudoctahedral Calcite |
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| Mark Holtkamp Replies: 11 Views: 31835 |
Forum: Rockhounding trips Posted: Nov 07, 2016 07:14 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.
Th ... |
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| Topic: Classification of crystallographic forms - any references? |
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| Mark Holtkamp Replies: 21 Views: 59530 |
Forum: Mineralogical Literature Posted: Feb 23, 2015 16:31 Subject: Re: Classification of crystallographic forms - any references? |
| Yes that is what he does. The projections of the holohedral classes are divided in triangles by the symmetry planes. So you have 48 triangles in the cubic system, but none in the monoclinic and tricli ... | |
| Topic: Classification of crystallographic forms - any references? |
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| Mark Holtkamp Replies: 21 Views: 59530 |
Forum: Mineralogical Literature Posted: Feb 22, 2015 16:30 Subject: Re: Classification of crystallographic forms - any references? |
| Hi Pete,
Yes you're right. it's the same principle. I didn't realise this can be done for all pointgroups. Mark. |
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| Topic: Classification of crystallographic forms - any references? |
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| Mark Holtkamp Replies: 21 Views: 59530 |
Forum: Mineralogical Literature Posted: Feb 22, 2015 08:57 Subject: Re: Classification of crystallographic forms - any references? |
| Paul Tambuyser in his book 'Kristalmorfologie' (in Dutch) uses an interesting method to systematically derive all crystal forms. I don't know if this method is used by other authors. It is based on di ... | |
| Topic: Classification of crystallographic forms - any references? |
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| Mark Holtkamp Replies: 21 Views: 59530 |
Forum: Mineralogical Literature Posted: Feb 21, 2015 05:54 Subject: Re: Classification of crystallographic forms - any references? |
| Hi Pablo,
So if I understand correctly you are looking for a way to mathematically derive the 48 forms, or at least prove there are exactly 48 (or 47) My mathematics are not good enough to do eith ... |
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| Topic: Brookite - Crystal habit. |
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| Mark Holtkamp Replies: 1 Views: 7690 |
Forum: Minerals and Mineralogy Posted: Sep 05, 2014 04:29 Subject: Re: Brookite - Crystal habit. |
| Hi Roger,
See this discussion on Mindat: https://www.mindat.org/mesg-9-15201.html (Link normalized by FMF) Mark. |
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| Topic: Tetrahedron [111] penetration twin |
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| Mark Holtkamp Replies: 17 Views: 85715 |
Forum: Minerals and Mineralogy Posted: Dec 29, 2013 10:08 Subject: Re: Tetrahedron [111] penetration twin |
| Thank jou Josele also for spotting the error. You were right after all, it is a twin on [100] !
Mark. |
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| Topic: Tetrahedron [111] penetration twin |
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| Mark Holtkamp Replies: 17 Views: 85715 |
Forum: Minerals and Mineralogy Posted: Dec 27, 2013 17:15 Subject: Re: Tetrahedron [111] penetration twin |
| After more reading I now believe that the tetrahedrite {100} twin doesn't occur in recent literature because it doesn't exist. The images in Strunz and Dana probably were copied from old references a ... | |
| Topic: Tetrahedron [111] penetration twin |
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| Mark Holtkamp Replies: 17 Views: 85715 |
Forum: Minerals and Mineralogy Posted: Dec 27, 2013 11:33 Subject: Re: Tetrahedron [111] penetration twin |
| Hi all,
Sorry for the confusion and the late reply. The drawings in the original post are from my website, after a bit of research I think the crystal in the first drawing is not a twin on [111] at ... |
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