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ID help with iron aluminum silicate

Posted by Tom Mortimer  
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Tom Mortimer June 11, 2019 08:59PM


This mineral presents as 1 mm radial structure spheres, found in miarolitic cavities from the Moose Pocket, Albany, NH. I have five EDS analyses of these over a twenty year time frame. All show a Fe, Al silicate with a bit of potassium. The earliest was by Dr. Eugene Foord of the USGS in 1997. A suggestion given with an Excalibur analysis suggested the macaulayite species, mindat formula (Fe,Al)24Si4O43(OH)2 . Two EDS polished grain analyses this month with an instrument with good light element detection indicated K:Fe:Al:Si APFU ratios of 1:6:3:12. Other than oxygen, no other elements were detected. These Fe+Al to Si ratios are way too low for macaulayite. Searching the IMA data base for Fe,Al, Si minerals (with/without K) , I came up with nothing close to my measured ratios. Visually all five EDS plots (from four different instruments) are very similar in Fe:Al:Si peak ratios and all show a small but detectable amount of potassium.
Suggestions welcome.
Tom Mortimer
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Frank K. Mazdab June 12, 2019 06:02AM
The element ratio and radial morphology of the crystal clusters are consistent with stilpnomelane.

Since Al in stilpnomelane can be distributed between both the VIM and IVT sites, the observed 1:6:3:12 ratio of K:Fe:Al:Si is roughly consistent with the nominal 1:8:12 ratio of K:(Fe+VIAl):(Si+IVAl).

Although stilpnomelane isn't typically thought to be a "granite" mineral, it is noted from a geologically-similar "rotted" anorogenic granite from Wisconsin: https://www.mindat.org/loc-26807.html . A stilpnomelane from this environment would likely have little to no Mg, and should be close to one or more of the the "ferro-stilpnomelane", "ferri-stilpnomelane" or even "ferro-ferri-stilpnomelane" hypothetical end-members.



Edited 3 time(s). Last edit at 06/12/2019 06:30AM by Frank K. Mazdab.
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Clifford Trebilcock June 12, 2019 12:06PM
Hi Tom,

Check out Chalcodite-stilpnomeline info on Mindat.

Cliff
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Tom Mortimer June 12, 2019 03:09PM
Frank,
I firmly believe you are correct ! This is most helpful.
This is a most interesting revelation !
Stilpnomelane makes sense. Harvard /Carl Francis found it at the Lovejoy Quarry, Conway, NH. see my:
https://www.mindatnh.org/Stilpnomelane%20sheet.html

https://www.mindatnh.org/Stilpnomelane%20Gallery.html   with analysis
Where I went astray is in my IMA mineral chemistry search, I *** assumed *** because there was so much aluminum, that it was "essential." ... which it is not for stilpnomelane.
I have always been suspicious of the NH macaulayite: see my 
https://www.mindatnh.org/Macaulayite%20sheet.html

https://www.mindatnh.org/species%20data/Macaulayite%20data.html

I am now convinced the NH macaulayite is actually stilpnomelane. The Lovejoy stilpnomelane is quite micaeous (in high magnification). This Moose Pocket mineral appears more acicular-stellate. Some of the mindat photos of stilpnomelane are quite similar to this Moose Pocket mineral.
Tom
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