Optic properties of centimeter-sized crystals determined in air with the spindle stage using EXCALIBRW

Abstract

Extinction data sets for four centimeter-sized anisotropic crystals were collected in air with standard spindle-stage methods and submitted to a new Windows-based version of EXCALIBR, termed EXCALIBRW. EXCALIBRW solved these data sets with varying degrees of accuracy related to the external shape of the crystal: the more rounded the crystal, the more precise the results. For an olivine crystal ground into a sphere, the results were similar to those obtained for a crystal immersed in an index-matching fl uid. However, even for samples bounded with growth or cleavage faces, the program determined the orientation of the optical indicatrix and 2V with an error of only 1−2°. Thus, this logical extension of spindle-stage methods is helpful: (1) to orient centimeter-size single crystals for various types of mineralogical measurements (e.g., spectroscopy or diffusion studies in which it might be undesirable to place the sample in a liquid); (2) as a non-destructive means of identifying gemstones based upon a determination of their optical class (i.e., isotropic vs. uniaxial vs. biaxial); and (3) for optical characterization by determination of 2V. In addition, the newest version of EXCALIBR is easier to use, mathematically more robust in its solution algorithms, and provides solutions for crystals in less favorable orientations than the earlier versions of EXCALIBR.

The atomic arrangement of merrillite from the Fra Mauro Formation, Apollo 14 lunar mission: The first structure of merrillite from the Moon

“…Graphical output from the newest version of EXCALIBR (Gunter et al 2005) confi rmed merrillite to be uniaxial, which is in agreement with the structural refi nement described herein. Bloss (1981) and Gunter et al (2005) both pointed out that stereographic projections of the extinction curves for uniaxial minerals result in a great circle projection that coincides with the one circular section of the uniaxial indicatrix, and its pole plots on the other extension curve and locates the single optic axis. Such a relationship was confi rmed for merrillite.…”

“…In addition to the graphical output, EXCALIBRW (Gunter et al 2005) also provides the spindle stage and microscope stage settings to determine n ε and n ω for merrillite, as well as to determine the optic sign by use of an interference plate to ascertain the relative magnitude of the two principal indices of refraction. In the case of merrillite, n ε < n ω , and thus merrillite is uniaxial (-).…”

Kircherite, a new mineral of the cancrinite-sodalite group with a 36-layer stacking sequence: Occurrence and crystal structure

“…The refractive indices were determined by the double variation method (see Su et al 1987 and references therein) and the grain was oriented with the spindle stage so as to measure the refractive indices (Gunter et al 2005 and references therein).…”

GUNTERITE, Na4(H2O)16(H2V10O28){middle dot}6H2O, A NEW MINERAL SPECIES WITH A DOUBLY-PROTONATED DECAVANADATE POLYANION: CRYSTAL STRUCTURE AND DESCRIPTIVE MINERALOGY

“…The value of 2V was measured using conoscopy, which yielded 77(2)°, and determined using extinction data with ExcAliBRw (Gunter et al 2004(Gunter et al , 2005, which yielded 79(2)°. The indices of refraction a and b, determined in white light, are 1.735(5) and 1.770(5), respectively.…”