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Cristobalite with Osumilite or Hortonolite, Sardinia ?
  
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Roger Warin




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PostPosted: May 02, 2017 22:27    Post subject: Cristobalite with Osumilite or Hortonolite, Sardinia ?  

Hi,
Here is an old sample collected in the '70s. It comes from Mount Arci, Oristano, Sardinia.
There are multiple cristobalite crystals accompanied by a rapid growth of osumilite on grey volcanic matrix.
At least that's what I believe, because the label says cristobalite with hortonolite. This last mineral is a pyroxene of the series Forsterite-Fayalite with 30-50% Fo. It’s also possible.
What do you think ? Thanks.
Roger.



Cristobalite#1A&B2.jpg
 Mineral: Cristobalite
 Locality:
Funtanafigu Quarry, Marrubiu, Arci Mountain, Oristano Province, Sardinia/Sardegna, Italy
 Description:
 Viewed:  1197 Time(s)

Cristobalite#1A&B2.jpg



Cristobalite#2_R.jpg
 Mineral: Cristobalite
 Locality:
Funtanafigu Quarry, Marrubiu, Arci Mountain, Oristano Province, Sardinia/Sardegna, Italy
 Description:
 Viewed:  1197 Time(s)

Cristobalite#2_R.jpg



Osumilite#3A&B_R.jpg
 Mineral: Osumilite or Hortonolite ?
 Locality:
Funtanafigu Quarry, Marrubiu, Arci Mountain, Oristano Province, Sardinia/Sardegna, Italy
 Description:
That's the question...
 Viewed:  1195 Time(s)

Osumilite#3A&B_R.jpg


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marco campos-venuti




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PostPosted: May 03, 2017 03:42    Post subject: Re: Cristobalite with osumilite or hortonolite, Sardinia ?  

This is not osumilite.
Osumilite is very constant in this deposit, is black, hexagonal and shows triaxial striations on pinacoid.
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Roger Warin




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PostPosted: May 03, 2017 03:46    Post subject: Re: Cristobalite with osumilite or hortonolite, Sardinia ?  

Many thanks Marco.
Roger.
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marco campos-venuti




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PostPosted: May 03, 2017 04:02    Post subject: Re: Cristobalite with Osumilite or Hortonolite, Sardinia ?  

I would say you have tridymite instead cristobalite. It is hexagonal.
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Jordi Fabre
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PostPosted: May 03, 2017 04:12    Post subject: Re: Cristobalite with Osumilite or Hortonolite, Sardinia ?  

marco campos-venuti wrote:
I would say you have tridymite instead cristobalite. It is hexagonal.

I agree, and the brown crystals looks to me like Cordierite...
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marco campos-venuti




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PostPosted: May 03, 2017 07:31    Post subject: Re: Cristobalite with Osumilite or Hortonolite, Sardinia ?  

Actually cordierite from this area is quite different. It shows elongated hexagonal prisms. The color is similar, but shape is not. I didn't found any citation for hortonolite, but why not?
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Pete Richards
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PostPosted: May 03, 2017 08:24    Post subject: Re: Cristobalite with Osumilite or Hortonolite, Sardinia ?  

It looks as if there may be several of the characteristic V-shaped twins of tridymite near the middle of the first image.
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Collecting and studying crystals with interesting habits, twinning, and epitaxy
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Vinoterapia




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PostPosted: May 03, 2017 10:07    Post subject: Re: Cristobalite with Osumilite or Hortonolite, Sardinia ?  

Hi Roger, in your note you mentioned: " This last mineral is a pyroxene of the series Forsterite-Fayalite with 30-50% Fo."

I just need a clarification. Is the Forsterite-Fayalite series included now into the pyroxenes group, and not any longer as compositional differences of the Olivine?
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marco campos-venuti




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PostPosted: May 03, 2017 12:15    Post subject: Re: Cristobalite with Osumilite or Hortonolite, Sardinia ?  

I found a very similar fayalite in mindat from Coso Hot Springs, Inyo Co., California.
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