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Mystery Rock / Mineral Found While Detecting


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Update:  I filed it with a triangular (saw sharpening) file.  It cut easily leaving a metallic luster.  Conclusion:  metallic and very likely an aluminum alloy.  It is surprising to me how it weathered to look like a rock, but I think that is the most likely explanation of its current condition, not some intentional/artificial working.  The area I found it shows signs of backfill and it's rather thick, sticky (when wet) clay.  That is a common backfill in my area, and backfilling is apparently a lucrative vocation/profession here with prolific government support/contracts.  Every site I've hunted (public schools and multiple muni- and county parks) shows lots of signs of backfill/resurfacing, much to my (detecting) dismay.

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  • 1 month later...

One last(?) update:  let's start with a photo --

mystery_mineral_2.thumb.JPG.968f6facb26af77fce3f809ea03c00a6.JPG

(Notice I snuck in one of my recent silver dime finds.  :biggrin:)  The original mystery rock is on the right.  The other two are also recent finds, both also turning out to be aluminum.

My initial confusion about how aluminum could turn out to look like a rock was flawed.  I review that my argument was that since ring-and-beavertail pulltabs in the ground for ~50 years often show no signs of corrosion (other than the standard light oxidation layer -- dulling the luster) was over simplified.

Most aluminum used today is in alloys.  Exceptions are evaporative coatings and some (maybe most or all) aluminum foils used for cooking -- e.g. rolls you can buy at every grocery store.  But there are many alloys used for multiple purposes.  The fact that some alloys don't corrode easily doesn't mean other alloys don't either.  IMO that's what is happening to the specimens in the photo.

Another example of this variable corrosion of alloys occurs with iron alloys.  Some steels corrode rather easily.  Others (e.g. many stainless steels) are quite resistant to corrosion.  And pure iron is somewhere in between, I think.

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Being in a fire might give it that lump shape and change it's property to corrosion.

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At an SG of 2.65 it has to be mainly quartz and it does look like a quartz-rich igneous or metamorphic rock. My guess is that it has a disseminated conductor as a minor component that's causing the response. Maybe a graphitic gneiss or something granitic with some sulfide a.k.a. "hot rock". Steve has an old post from Dove Creek Alaska showing quartz monzonite and great chunks of gold running through it. You might get lucky.......

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I missed this post, it just popped up on the sidebar right now though and I saw it.

Metals when heated can alter hardness, even if not alloyed. Some by 2x or more, so Moh's is not a definitive test for slags and whatnot. You already solved it, but I agree, your tests indicate aluminum slag. :smile:

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