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Identity HelpBlack Granite? Gabbro? etc?

8th Dec 2018 00:21 UTCRobert Darabos

Found some of this material in the mid Michigan area in a popular tourist location.

It was used in gardens, so may have came from anywhere... probably somewhere in Michigan still, would be my guess.

Is this Black Granite? (is that even a thing?) Gabbro? Anorthosite? Etc?

8th Dec 2018 00:42 UTCPaul Brandes 🌟 Manager

Appears like an anorthosite, bit without actually seeing it in person.....

8th Dec 2018 01:30 UTCFrank K. Mazdab 🌟 Manager

could also be gabbro, diorite or something similar. Not sure about Michigan, but there are scattered mafic/intermediate intrusives in the Upper Midwest... the Duluth gabbro comes to mind.

8th Dec 2018 03:29 UTCDoug Daniels

Note that "black granite" is more a commercial term... Who wants to buy a countertop made of GABBRO ? Let alone ANORTHOSITE, or DIORITE. Not very enticing names commercially. At least most have heard of granite, it's a great rock for various purposes, but usually white (ok could be stained reddish); calling it black makes it ok, I guess. (Had this argument with the late mother-in-law years ago, when they installed new countertops made of this).

8th Dec 2018 04:14 UTCFrank K. Mazdab 🌟 Manager

You're right Doug... "granite" or "marble" seem to be the building industry's trade names for any just about all decorative building stones, regardless of what a petrologist would otherwise call them.

The strange thing, though, is that according to the QAPF naming diagram for intrusive igneous rocks, you can have up to 90% mafic minerals in an otherwise feldspar-bearing igneous rock before you have to switch over to one of the other naming diagrams. So it's conceptually possible to have a coarse-grained rock that is 89% black amphibole, 3% plagioclase, 3% quartz and 5% orthoclase, and that rock would still technically be a granite, because only the relative proportions of the 11% total of Q (quartz), A (alkali feldspar) and P (plagioclase) would count towards the name. I have no knowledge of any weird rock anywhere close to such an oddball composition (nor can I imagine a normal igneous process likely to produce one), but nonetheless, that hypothetical rock would indeed be a very "black granite"!

8th Dec 2018 12:52 UTCMark Heintzelman 🌟 Expert

Note: "black granite" was also a diabase. The very popular, extra black "French Creek black granite", was originally cut from a diabase quarry in the same area of the more famous mines. The name is still in use, but I've no idea what the material is or where it comes from today. All just random trade names, implying nothing about composition.

8th Dec 2018 17:40 UTCRobert Darabos

I assumed that "Black Granite" was not a correct name. Thanks for the info on that.

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