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Dany Mabillard




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PostPosted: Jan 28, 2021 11:30    Post subject: Help needed  

Hello everybody,

I need some help to confirm or invalidate the following mineral. If you also have a more precise location, I'll be very grateful.

Thanks in advance and keep safe.

Dany.



Spessartine - Microcline no 209 - FMF.jpg
 Mineral: Spessartine
 Locality:
China
 Dimensions: 49 mm x 40 mm x 26 mm
 Description:
For the vendor, it's on Orthoclase but I think is on Microcline. What do you think?
 Viewed:  914 Time(s)

Spessartine - Microcline no 209 - FMF.jpg


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Volkmar Stingl




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PostPosted: Jan 28, 2021 11:40    Post subject: Re: Help needed  

Hello Dany,

this is a specimen with smoky quartz, spessartine, and orthoclase from Tongbei, Yunxiao Co., Zhangzhou Prefecture, Fujian Province in China.

Volkmar
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Michael Shaw
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PostPosted: Jan 28, 2021 13:50    Post subject: Re: Help needed  

Dany,

As you suggest, the feldspar that that is associated with the Spessartine and Smoky Quartz is Microcline, not Orthoclase. It was originally reported to be Orthoclase, but this has been proven wrong.
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Dany Mabillard




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PostPosted: Jan 29, 2021 11:00    Post subject: Re: Help needed  

Volkmar Stingl wrote:
Hello Dany,

this is a specimen with smoky quartz, spessartine, and orthoclase from Tongbei, Yunxiao Co., Zhangzhou Prefecture, Fujian Province in China.

Volkmar


Thank you for your help. I've checked the location on Mindat and there is this note: The feldspar associated with the spessartine is microcline, not orthoclase as originally reported.
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Dany Mabillard




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PostPosted: Jan 29, 2021 11:26    Post subject: Re: Help needed  

Michael Shaw wrote:
Dany,

As you suggest, the feldspar that that is associated with the Spessartine and Smoky Quartz is Microcline, not Orthoclase. It was originally reported to be Orthoclase, but this has been proven wrong.


Thank you for your help.
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alfredo
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PostPosted: Jan 29, 2021 15:29    Post subject: Re: Help needed  

Let's keep in mind that the differences between orthoclase and microcline are so small that they are only kept as separate species because of historical inertia. If they were described as new species nowadays, under current IMA rules, they'd just be a single species. So, in my own collection I label them "K-feldspar" and don't give a hoot which one of them it is, orthoclase or microcline.
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Pete Richards
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PostPosted: Jan 29, 2021 16:00    Post subject: Re: Help needed  

On top of that, orthoclase becomes microcline as it cools down, if it has time. And there is debate about whether there is such a thing as primary microcline - i.e. microcline that did not start out as orthoclase. The transition from orthoclase to microcline leads to a characteristic kind of twinning called cross-hatch or tartan twinning; if it is present the mineral is microcline, but this may require thin sections and microscopy to determine. If it is absent, the mineral could be orthoclase or it still possibly could be microcline - the elusive primary microcline. Probably most if not all k-spar crystals start out as orthoclase, probably most wind up as microcline pseudomorphs after orthoclase.

This ignores sanidine and adularia, other important forms of potassium feldspar. It's complex enough already!

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Collecting and studying crystals with interesting habits, twinning, and epitaxy
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Dany Mabillard




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PostPosted: Jan 31, 2021 11:34    Post subject: Re: Help needed  

alfredo wrote:
Let's keep in mind that the differences between orthoclase and microcline are so small that they are only kept as separate species because of historical inertia. If they were described as new species nowadays, under current IMA rules, they'd just be a single species. So, in my own collection I label them "K-feldspar" and don't give a hoot which one of them it is, orthoclase or microcline.


Thank you. I'll use K Feldspar.
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Dany Mabillard




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PostPosted: Jan 31, 2021 11:34    Post subject: Re: Help needed  

Pete Richards wrote:
On top of that, orthoclase becomes microcline as it cools down, if it has time. And there is debate about whether there is such a thing as primary microcline - i.e. microcline that did not start out as orthoclase. The transition from orthoclase to microcline leads to a characteristic kind of twinning called cross-hatch or tartan twinning; if it is present the mineral is microcline, but this may require thin sections and microscopy to determine. If it is absent, the mineral could be orthoclase or it still possibly could be microcline - the elusive primary microcline. Probably most if not all k-spar crystals start out as orthoclase, probably most wind up as microcline pseudomorphs after orthoclase.

This ignores sanadine and adularia, other important forms of potassium feldspar. It's complex enough already!



Thank you for your explanation.
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