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Identity HelpID help with iron aluminum silicate
11th Jun 2019 20:59 UTCTom Mortimer Expert
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
12th Jun 2019 06:02 UTCFrank K. Mazdab 🌟 Manager
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.
12th Jun 2019 09:52 UTCTom Mortimer Expert
12th Jun 2019 12:06 UTCClifford Trebilcock
Check out Chalcodite-stilpnomeline info on Mindat.
Cliff
12th Jun 2019 15:09 UTCTom Mortimer Expert
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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Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2024, except where stated. Most political location boundaries are © OpenStreetMap contributors. Mindat.org relies on the contributions of thousands of members and supporters. Founded in 2000 by Jolyon Ralph.
Privacy Policy - Terms & Conditions - Contact Us / DMCA issues - Report a bug/vulnerability Current server date and time: March 29, 2024 12:58:36