Monday, November 11, 2013

Sedimentary Petrology

I was doing really well with my 30 minutes of blogging per day, right up until the class I TA started the 4-week section on sedimentary petrology.  Prior to teaching these labs, I hadn't looked at a thin section since the nineties, and even then I was terrible at it.  So I've had to devote a huge chunk of my time to re-learning how to use an optical microscope and the fundamentals of sedimentary petrology. Not that this is a bad thing to review-- I've always been very self-conscious about my level of comfort with a thin section.

I've kept these labs very basic, because about 2/3 of my students haven't even taken mineralogy yet. No measuring angles of extinction, no interference color charts, just visual differentiation between a few key minerals, sedimentary textures, provenance, and diagenesis.

Rather than dig out the department camera that goes with the microscope (it was getting a lot of use in October as people prepared for GSA), I used my smartphone camera to get photos of some of the slides we were using in lab. It's tricky, and the photos aren't high quality, but they are good enough for learning and for reviewing the slides with the students. Here are a few of my favorite thin section photos from the last month or so of teaching. Most of these are at 4x magnification. A few are at 10x magnification.

Glauconite under XPL
Glauconite under PPL

Precambrian impactite? Great examples of feldspar twinning here.

Tahitian black beach sand (mostly olivine)
Silicified oolite under XPL, with calcite exhibiting twinning.

The same silicifed oolite under PPL.
The calcite is showing beautiful rhombohedral cleavage.

Dolostone under PPL
The same dolostone under XPL
Oolitic Limestone under PPL
I don't remember which sample this photo is from (I think it's a carbonate).
I assume this is some sort of recrystallization. Please feel free to comment if you know otherwise.