Sunday, December 16, 2007

Trophy rocks

My first trophy rock is a 4 kg boulder of the Lavras conglomerate from the Tombador formation of the mid-Proterozoic Espinha├žo Supergroup. This is a well-rounded, poorly sorted conglomerate with a clast size of 1-9 cm in this hand sample. The clasts are dominated by a pink arkose, with white and green arenites also present. The rock is clast- supported, and some clasts show evidence of minor pressure dissolution along their contacts. The matrix is slightly ferruginized. This rock is the purported source rock for the diamonds for the Lencois diamonds workings in Bahia state, Brazil.

My second trophy rock is a migmatite from the Wyoming craton. The strongly folded mesosome consists mostly of biotite and amphibole, with a minor opaque oxide phase. The leucosome follows the folds of the rock, but has equant, undeformed coarse-grained K-spar and quartz crystals, suggesting that crystallization was post-deformational.

The third trophy rock, part A, is a 45cm Cambrian stromatolite colony from an undisclosed location in Australia’s Georgoina basin. Part B is a smaller fragment of a contemporaneous stromatolite colony, with which has been progressively silicified, showing laminar color variations caused by trace elements in the chalcedony.

Unlike Brian’s rocks, these specimens are widely separated in space and time.


BrianR said...

i don't see any photos

Dr. Lemming said...

So my descriptions are lacking? Inadequate?

Sabine said...

My favorite is a skarn that I brought back (as much as I could carry and convince other people to carry for me, hehe) from a mine near my field camp in Colorado. It contains beautiful, rich green epidote crystals and pale bluish-green radial actinolite. There are also some small calcite crystals in it. I have a few huge chunks in the house and small ones all over the place.

C W Magee said...

I'll take pictures whenever I go home (currently somewhere else for work).

Ben said...

"Ferruginized" isn't a word - you made that up. You made up lots of those words. And I reckon you got the other ones from this guy.

Dr. Lemming said...

If "Ferruginized" isn't a word, then what do you think this guy is studying?

You ought not to slander your neighbors.