Windows in Rocks
May 6, 2012
We recently acquired a surplus Zeiss Standard petrographic microscope for looking at thin sections of rocks. It has three high-power objectives, with space for two more objectives on the turret. For lower power viewing, I still have my Olympus Elgeet POS, which I featured in my 2001 CD-ROM, Collecting and Using Mineral Pigments. Both microscopes were made about 1980. The Olympus is wonderful but was considered a student microscope because of its simple, old-fashioned design. The Zeiss has a wider, brighter field of view, a more versatile design, and higher quality objectives. It was intended for advanced students, industrial use, and research (though fancier research models were also available).
Several features make a petrographic microscope different from a biological microscope. The most obvious are the round stage that can be rotated, and the two polarizing lenses (one above the stage and one below it). This microscope is most commonly used for studying thin sections – transparent slices of rocks – that are are glued to glass slides.
We got a camera attachment so we can take pictures through both microscopes, and I’ve set up part of my shop to make thin sections, which is just basic lapidary work. I learned how to make them in 1987 as an undergraduate geology student (coincidentally, the same semester that I learned how to make cabochons). It takes time and patience, since my method is quite primitive (more on that in another post). Meanwhile, we’re playing with a few slides that were duplicates from my 1992 Master’s thesis.
So what are we looking at?
Photos below are of a high-grade metamorphosed gabbroic anorthosite from the southeastern Adirondacks, NY. Anorthosite is a rock made mainly of plagioclase feldspar with a few minor dark iron-rich minerals. Gem quality iridescent labradorite (“spectrolite”) is from anorthosite from Scandinavia and Madagascar, and slabs of dark anorthositic rocks are among several rock types that are sold as “black granite” for counter tops. Anorthositic rocks are also found on the moon.
I photographed a tiny piece of the thin section on high power using the Zeiss scope. The first picture is in ordinary light. The black grain is two intergrown opaque metallic minerals: magnetite (iron oxide) and ilmenite (titanium oxide). The Fe-Ti oxide has a rim of pale brown titanite. On the left is plagioclase feldspar which has been mostly altered to sericite, a cloudly-looking mixture of clay and fine-grained mica. The green grain in the upper left corner is hornblende. On the right half is a cluster of mica crystals. Their striped or woody-looking pattern is unusual, and means that there are probably two types of mica present, most likely biotite and phlogopite.
When viewed under polarized light (“crossed polars”), the mica shows beautiful interference colors, the plagioclase looks gray (and it’s easier to see the straw-colored sericite clusters), and the titanite looks relatively unchanged.
Here’s a nonpolarized lower power view taken through the Olympus scope. It looks darker than the first photo because the light isn’t as strong, and dark minerals occupy more of the field of view than in the higher power photo.
Petrography – the study of rocks in thin section – is essential to a geologist’s training. Mineral identification is an important aspect, but it also includes recognition of textures, crystallization sequences, alteration and weathering products, and other visual clues to a rock’s compostion and history. The specialized microscope, the array of odd and challenging optical techniques, and the unique vocabulary make petrography one of the most arcane aspects of geoscience. For the student, it has a rarified, initiatory quality that is lacking in the more “real world” geological studies such as mapping or identifying fossils. It is quite demanding, and expertise comes only with experience after studying slides of many different rocks. Some students struggle with it and are happy to leave it behind as soon as possible. But for geologists with an eye for color and pattern and a sensitivity to detail, it becomes a valuable skill and a treasured art form. Many thin sections are extremely beautiful, and each is a window into a rock, revealing secrets that cannot be known any other way.
Old petrography handbooks were illustrated with black and white ink drawings. Atlases with slick color photos were first published in the mid-1980s. Since about 2000, modern computers and digital photography have made it easy to create and share excellent petrographic images. As a student, I was fascinated with the old drawings, and taught myself this rare form of scientific illustration which was rapidly becoming obsolete. Ink is traditional, but some high-contrast thin sections work fine as pencil drawings. Here’s a pencil rendering of the slide pictured above, drawn on a 3.5 inch circle that shows the entire field of view on high power. Compared to the photo, it shows additional Fe-Ti oxide and titanite grains, more mica crystals, and a small cluster of hornblende crystals. Although this type of drawing cannot show color, it is valuable for illustrating relief, which is the extent to which a mineral appears to stand above the others, or stand out in 3D. Relief is an important mineral identification property. This particular slide is a simple example, with minerals that differ strongly in relief, shape, and color. Beginning with the mineral with highest relief, the sequence here is Fe-Ti oxide – titanite – hornblende – mica – plagioclase – sericite.