Spindle-stage methods combined with computer and statistical analysis of the resultant data promise a significant upgrading and expansion of the present optical data base for crystalline compounds. For example, the accurate crystal orientation achievable with a spindle stage, combined with a simple statistical treatment of double-variation data permit a crystal’s principal indices to be determined to (ca.) 0.0005 for any visible wavelength and, with lesser accuracy, for near infrared and ultraviolet wavelengths.

Disper, a subroutine recently added to the now modified Bloss and Riess (1973) program, provides quantitative measurements of dispersion for a biaxial crystal’s five optic vectors—namely, the two optic axes plus X, Y, and Z—if the crystal’s extinction data for up to four different wavelengths are measured. After the Bloss-Riess solution for each wavelength, Disper calculates (1) the angular change in each vector, from one wavelength to another, and(2) the confidence level for rejecting the null hypothesis of non-dispersion. Extinction data for 433 and 666 nm wavelengths permitted rejection, at the 0.90 confidence level, of nondispersion for all optic vectors except the obtuse bisectrix for an adularia from St. Gotthard, Switzerland, and for an albite from Tiburon, California. By contrast, an albite from Amelia Courthouse, Virginia, exhibited dispersion for the obtuse bisectrix as well. This dispersional difference between the two albites is puzzling. They agree in 2VD by the Bloss-Riess method [77.1° (esd 0.3°), Tiburon; 76.9° (esd 0.3°), Amelia], and microprobe analyses indicate almost identical compositions. Indeed, by analogy to orthoclase and adularia, the slightly higher K2O in the Amelia specimen would presumably suppress rather than foster dispersion of the optic normal. Does this subtle optical difference reflect different petrogenetic histories?

By determining the crystal’s extinction positions with a photomultiplier or IR detector, 2V and the orientation of the five optic vectors become readily determinable (1) for non-visible wavelengths and/or (2) for the crystal at temperatures up to 1000°C or more. Such photometric measurements of extinction at 540 nm and 900 nm were made for a polished cylinder of Mijakejima anorthite while its temperature in degrees Celsius was held at 25, 200, 300, 350, 400, 500, 600, 700, 750, 800, 850—and during subsequent cooling—at 600, 500, 350, 200, and 25. Because extinctions were measured with the crystal cylinder in air, the Bloss-Riess solutions of the data for each temperature incorporated a small systematic error. Nevertheless, a plot of 2V versus temperature disclosed a well-defined minimum near 350°C, particularly for 900 nm. Highly magnified stereographic plots of positional change with temperature for each of the five optic vectors, particularly for 900 nm, usually showed sharp inflections near 350° during heating and during cooling. Thus, the gradual disappearance of the c reflections (h + k even; l odd) that accompanies the continuous transition from ‘primitive’ to ‘body-centered’ anorthite as it is heated from 25°C to (ca.) 350°C is accompanied by measurable optical changes. These optical changes monitor the temperatures at which thermally-induced structural changes occur or achieve completion.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.