FMF - Friends of Minerals Forum, discussion and message board

[chz9392000]

Many thanks!

[Pete Modreski]

I don't see any answers yet, so I'll give you one. You have a "rock", not any pure mineral, so a lot of the tests one tries to do (streak, hardness, etc.) give "mixed" or ambiguous results when the material you're testing is not one, but a mixture of minerals.

It is always difficult to tell too much from just a photo, but what you have appears to be either an igneous rock, or a metamorphosed igneous rock. There is of course, as one learns when you study rocks and minerals, a complete gradation between rocks that are igneous and metamorphic, as to the degree of metamorphism. And likewise, your rock is "something like a granite", but it could be one of the related kinds of igneous rocks, such as granodiorite; again, there are complete gradations. From the overall slightly dark color and the evident lack of any obvious pink or white potassium feldspar, your rock is most likely something more mafic (darker colored) than granite; so, granodiorite or diorite. Probably composed of a mixture of plagioclase feldspar, quartz, and biotite, and other minerals.

So, your rock, a granitic or dioritic gneiss, or a gneissic granitic or dioritic rock. Something like that!

Cheers, Pete

[chz9392000]

Many thanks for the insight! I guessed it may be a schist or gneiss however really not sure. Your information is very helpful.