New Mexico Mineral Symposium — Abstracts

Many of the research advances in mineralogy since the early 1960's can be related to the advent of new types of instrumentation such as the electron microprobe, the scanning and transmission electron microscopes, and the computer-controlled single-crystal x-ray diffractometer. Many of these new and sophisticated tools allow us to look at minerals on a very fine scale. Quantitative chemical analyses with the electron microprobe can be achieved on areas only a few microns in size. Scanning electron microscopes (SEM) produce magnifications with resolutions of 100 Angstroms or better. Transmission, especially high-resolution transmission electron microscopy (HRTEM), provides us structural images with a resolution of a few Angstroms. Complete crystal-structure determinations are now obtained routinely by computer-controlled single-crystal x-ray diffractometers on crystals with volumes of about 0.003 mm3. Investigations involving one, or several of the above techniques in combination, demonstrate that a homogeneous mineral is a very rare species. What was once considered a homogeneous single phase is now often recognized as a difficult to interpret composite.

Examples of major advances in our understanding of
structural and chemical aspects of common minerals are:
opal, which is now known to consist of spheres in a size range of 1,500 to 3,000 Å that are arranged in some regular scheme of closest packing (Darragh et al., 1976); and plagioclase feldspar, which is not a continuous solid- solution series at low temperature but contains several miscibility gaps, as expressed by the occurrence of fine lamellar structures in "moonstone," in labradorite, and in a
compositional region between An50 and An85 (Smith, 1974).

Evaluation of the exsolution lamellae inside a single metamorphic pyroxene has led Robinson et al. (1977) to a quantitative assessment of its cooling history.

The invention of the diamond-anvil pressure cell has made possible studies of the changes in crystalline materials as a function of high to very high pressure. Such studies should help in an evaluation of the deep mantle region of the earth.

High-resolution transmission microscopy (Veblen et al., 1977) has proven the existence of new types of crystal structures that are closely related to those of biotite, pyroxenes, and amphiboles ("biopyriboles").
And the measurement of strontium isotope ratios in a single rotated garnet crystal from a metamorphic rock has allowed for the first quantitative determination of a crystal growth rate (Christensen et al., 1989).
 

References:

1. Christensen, J. M., Rosenfeld, J. L., and DePaolo, D. J., 1989, Rates of tectonometamorphic processes from rubidium and strontium isotopes in garnet: Science, v. 244, pp. 1465-1469.
2. Darragh, P. J., Gaskin, J. J., and Sanders, J. V., 1976, Opals: Scientific American, v. 234, no. 4, pp. 84-95.
3. Robinson, P., Ross, M., Nord, G. L., Smyth, J. R., and Jaffe, H. W., 1977, Exsolution lamellae in augite and pigeonite-- fossil indicators of lattice parameters at high temperature and pressure: American Mineralogist, v. 62, pp. 857-874.
4. Smith, J. V., 1974, Feldspar minerals, v. 1 and v. 2: Springer??-Verlag, New York.
5. Veblen, D. R., Buseck, P. R., and Burnham, C. W., 1977, Asbestiform chain silicates--new minerals and structural groups: Science, v. 198, no. 4315, pp. 359-365.