The Spindle Stage: Principles and Practice

Abstract

When I published my first paper [Rosenfeld, 1950], one on what is now called the spindle stage, describing a simple device and conoscopic method for both orientation and measurement of the principal refractive indices of an optically anisotropic crystal, the existing determinative methods were clumsy, slow, and subject to error. At that time, refractive indices, inadequate though they appear in hindsight, played a large role in determining the compositions of nonopaque crystalline materials; and any improvement in optical methodology was welcome. The advent of the electron microprobe in the early sixties, capable of rapid and accurate chemical analysis, largely displaced the methods of chemical analysis relying on measurement of refractive indices. This change was reflected in many courses in optical mineralogy by deemphasis of the use of smelly and toxic refractive index liquids. The time freed was used for more intensive study of thin sections, a necessary kind of study for the formulation of penological problems if the electron probe is to be used effectively. However, the probe did not make use of the spindle stage totally obsolete for economic reasons and because, while the probe is essentially limited to determination of elemental composition, the optical properties determined with the spindle stage reflect features of the structural state of the mineral being examined and, in many cases, the valence state of contained elements. That knowledge can be useful to the petrologist. Further, for untwinned or singly twinned crystals a well‐designed and well‐constructed spindle stage on a good microscope is inherently superior to the more widely used universal stage because of low cost, accuracy, simpler geometry (with consequent need for few if any corrections in its application), and much greater procedural simplicity. As an example, a reasonably skilled microscopist can use the spindle stage both to determine the composition and to discriminate among structural states for a plagioclase feldspar grain in less than a half‐hour. But even for this last task, the method of the spindle stage would seem to be inferior to that of X ray diffraction with regard to structural state. Thus, in perspective, the method of the spindle stage is elegant where optical properties constitute an end in themselves, but commonly achieves only ‘quick and dirty’ results where penological goals dominate. I believe the petrological literature reflects that perspective even though there are doubtless many papers that reflect elaborate and costly methods when ‘quick and dirty’ results would have been sufficient for the task at hand.