4/30/2008

Rock of the Week #4

1. Name two minerals present
2. Name this rock type
3. Give an example of a locality where this type of rock can be found:



I'll give you a hint: that's not actinolite!

And the solution to Rock #3:

Copper minerals are fun because they are bright coloured, and the colour can tell you what mineral it is (at least, it is a more reliable indicator than with most other minerals). The best-known copper minerals have distinctive colours (for example, deep bright/dark green stripes for Malachite, deep blue-violet to blue-black for Azurite).

This rock is from Francistown, Botswana, and is mostly quartz. In this rock, light blue-aqua colour is characteristic of chrysocolla [(Cu,Al)2H2Si2O5(OH)4 •nH2O]. Chrysocolla forms in the oxidation zones of copper ore bodies. The lighter coloured, massive greenish-yellowish crust on the rock has a distinctively different colour and fizzes vigorously in HCl. It is either the unusual mineral rosasite [(Cu,Zn)2CO3(OH)2], which also occurs in the oxidation zone of copper/zinc/silver deposits (such as the Francistown locality), or it is a calcite crust with unknown copper minerals in. You will get a point for either answer as I am not sure myself!

Thanks to Catherine Curtis for the interesting rock!

1. (2 pts) blue-green minerals?
: mention of copper (1 pt)
: chrysocolla, rosasite or calcite (1 pt)

2. (1 pt) How was it formed?
: Oxidation or precipitation from water got a point.

AWARDS OF THE WEEK:

FIRST PLACE: Shirley Whitmore for:

1. Mildew
2. Left in a moist area

The “awards of the week” seem to be turning into the “most sarcastic of the week”, and Shirley certainly has the lead!

Please note that I am going off to sea for a month so May's RoW will be auto posted. Please don't be discouraged from commenting however! I'll be checking in when I can.

3 comments:

Ron Schott said...

I'd like to petition for a view of a weathered surface of this rock. (It's much easier to distinguish the high-T ferromagnesian minerals when they're weathered... or in thin section!)

andrew said...

Not actinolite? Now I'm stumped.

Ishmael said...

I'm going to guess:
It's an igneous rock.
It has quartz and granite.
It's often found on the ground.