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Microline in Basalt from St. Louis Mine, Michigan?

Posted by Robert Darabos  
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Robert Darabos October 11, 2018 02:05AM
Basalt from the St. Louis Mine, Laurium, Michigan.z
Not too many options from this location, but wanted to make sure about the reddish-orange crystal in the first image.
Best options on Mindat are Microline or Laumontite.

I had assumed the rest are Pumpeyllite (green) and agate (reddish circles) with quartz.

Any one know this material?






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Frank K. Mazdab October 11, 2018 03:53AM
I'm not familiar with this particular locality, but microcline lining amygdules in zeolite-facies, very-low-grade (meta)-basalt would be an unusual occurrence. If the red/pink material is indeed feldspar, be aware that it need not be K-spar. Indeed, alteration albite with finely-dispersed hematite can also be pink or red, and would probably be more consistent with the bulk composition and metamorphic grade.

If the reddish mineral isn't a feldspar, a zeolite such as stilbite can also commonly be that color and would also be consistent with the setting.



Edited 1 time(s). Last edit at 10/11/2018 03:56AM by Frank K. Mazdab.
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Doug Daniels October 11, 2018 05:21AM
I would agree that it is unlikely to be microcline - not associated with basalt. I would think even if metamorphosed. Maybe another feldspar (but then, why aren't there more of them?), or one of the zeolites (with which I'm no expert).
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Larry Maltby October 11, 2018 12:04PM
Red (orange) microcline is very common in the Keweenaw Peninsula of Michigan. It is often found in vesicles in basalt.

Here is a reference; http://www.minsocam.org/msa/collectors_corner/arc/mimicrocline.htm
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Frank K. Mazdab October 11, 2018 09:01PM
Interesting old paper, and as the author notes in the first sentence, the occurrence of microcline in this environment "... has not been appreciated"... touché! I suppose once you leave the realm of isochemical metamorphism and consider metasomatism, all bets are off about expected element associations... then any conceivable combination of elements may be permissible as long as either the host rock or the inbound fluid has the appropriate components.

That said, the photos above don't seem to match with the author's own observations of the occurrences (small: "generally less than a few mm", and late: "... the bulk of these minerals [amydgule-filling minerals] was formed later than the feldspar"), although his summary of some of the earlier descriptions seems more permissive.

So, what are those reddish crystals? Well, maybe they are indeed microcline... or maybe they are something else... :-)
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Paul Brandes October 11, 2018 09:57PM
Those look very much like microcline crystals that are found in scattered amounts along the Keweenaw in amygdules with pumpellyite and quartz. They tend to be rare from St. Louis, however.
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Larry Maltby October 11, 2018 10:46PM
Paul makes a good point about the fact that red microcline is rare at the St. Louis. It is interesting to note that at the Laurium Mine, just a few miles away, it is abundant. The mineralogy of the St. Louis Mine is illustrated in this article that I prepared some years ago:

https://www.mindat.org/article.php/1801/+St.+Louis+Mine%2C+Houghton+Co.+Michigan

There are many questions still open and I hope that someday a professional study of the St. Louis will be made. It seems to me that it is a perfect subject for a master’s thesis.
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Robert Darabos October 13, 2018 03:59AM
The dealer I bought the material from could have mislabeled Laurium material at St. Louis, I guess. I have maybe 10 or 20 large basalt speimens and almost every one has at least one section of the microline.
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