MicROCKScopic-Plagioclase Twins

Bernardo Cesare

Description of Bernardo Cesare images:

Along with an exciting geological history, every piece of rock hides a universe of colors and shapes that can be disclosed with a microscope utilizing polarized light. Sliced down to 30 microns, a rock becomes transparent: such “thin sections” are the ordinary subjects studied by petrographers to obtain useful scientific information rather than to unravel geological secrets. At MicROCKScopica, www.microckscopica.org, rocks are photographed to reveal their inner beauty, grace and elegance. These photomicrographs provide feelings of the ordered, geometric development of crystals and also the opposite: the chaos and irregularity that characterize the natural processes of rock formation and evolution.

Technical Details: photomicrographs were taken with a camera (at the beginning a 35mm reflex, later a dedicated digital) mounted on a polarizing optical microscope, in which polarized light is transmitted through (not reflected by) the rock or mineral. The microscopes used were a Zeiss Axioscop 40 Pol or a Olympus BH-2; the cameras were either a Zeiss Axiocam MRc5 digital or a Olympus SRL dedicated film camera. Except for single crystals (e.g. Gypsum), all the subjects are thin sections of rocks of metamorphic or magmatic origin. The size of subjects (width of view) varies in the range 0.2 - 2.0mm, corresponding to 2.5X to 20X lens magnification. Most photos were taken with two crossed polarizers and quartz red tint plate, rotated so as to reach the preferred interference colors.

Description of MicROCKScopic-Plagioclase Twins

Plagioclase feldspars are commonly twinned. It is almost a rule. In this case more twinned crystals are compenetrated and form a cluster called glomerocryst, typical of volcanic rocks. In fact, this photo has been taken from a sample of andesite from Carboneras (SE Spain). This rock was erupted approximately 15 million years ago, and the magma from which it originates formed by the melting of the uprising Earth's mantle, during the opening of the western Mediterranean. The twinned plagioclase is set in a fine-grained darker matrix, that denotes the fast cooling of the lava after submarine eruption.