Rock of the Week #9

Here's a real beauty. In my low-grade, brittle line of work I don't often get to admire such prettiness in my sample collection. But I digress...
1. Name two of the many minerals present in this rock.
2. What is the name of this rock type?
3. How is this rock type formed?

Once again if you can answer 1. you can figure out 2, which should lead to 3. Both cut slab and weathered side are shown - sorry for the saw marks. The third photo is the most true to colour.

Solution for Rock of the Week #8:
This is a fine-grained, blueschist facies metabasalt from the Franciscan Complex, California, USA. It was collected at the Russian River, California where it was lying as a large knocker (exotic block) in a tectonic mélange. Mineralogy is glaucophane+garnet+epidote+albite+pyrite, consistent with an overall composition of hydrated basalt.

1. (1 pt) Rounded red mineral?
: 1 point given for garnet.

2. (1 pt) Fine-grained matrix?
: 1 point for glaucophane or sodic amphibole.

3. (1 pt) Rock name or metamorphic grade:
: 1 point for blueschist.


Ron Schott said...

If I'm not mistaken this looks like a skarn (from the Llano Uplift in Texas?). The brown mineral is vesuvianite (idocrase) and the white mineral is wollastonite. There's probably also some quartz in there. The green mineral might be diopside. Skarns such as this are formed by metasomatism of calcareous rocks by fluids off (most commonly) granitic intrusions.

Lost Geologist said...

I think Ron is right!

Silver Fox said...

I'd go with something skarnish, also. [Disclaimer: "skarnish" is not a technical term, and will not do well on any tests.]

aa said...