High-order plates for the microscopic examination of mineral grains

Abstract

For the examination of grains with high birefringence the usual types of wedge or plate often fail to give clear indications of optical sign. In such cases plates up to the 20th order on Newton's scale (relative retardation about 10μ) may be employed.Determination of sign of length of a crystal—A detrital crystal of zircon (length positive, i.e. slow) about 300-400μ thick observed between crossed nicels in conjunction with a 15-20th-order plate will illustrate this test. When the length of the crystal is parallel to the fast direction of the plate (fig. 1 A) there is compensation, and colour bands are seen. Without the plate, or with the crystal rotated through 90° (fig. 1 B), only high-order whites are seen. A crystal of high birefringence with length negative (e.g. a bipyramid of anatase) will give colour bands in position B, and none in position A.