Common challenges associated with grade uncertainty involve failing to meet decisive operational targets, which include (among others) the following: ore tonnage sent to the mill, total metal processed at the mill, blending requirements on ore feed, total waste tonnage mined, maximum allowable proportion of potentially deleterious materials (e.g., toxic elements such as arsenic). These challenges reflect, to an important extent, the uncertainty involved in defining precisely the mineral grades in an ore deposit. <...>

ALEXANDER, R. J. & HARPER, G. D. The Josephine ophiolite: and ancient analogue for slowto intermediate-spreading oceanic ridges
MACLEOD, C. J. & ROTHERY, D. A. Ridge axial segmentation in the Oman ophiolite: evidence from along-strike variations in the sheeted dyke complex
SEAREE, R. C. The volcano-tectonic setting of oceanic lithosphere generation
RUSBY, R. I. GLORIA and other geophysical studies of the tectonic pattern and history of the Easter Microplate, Southeast Pacific
KARSON, J. A. & WINTERS, A. T. Along-axis variations in tectonic extension and accommodation zones in the MARK Area, Mid-Atlantic Ridge 23°N latitude
TAYLOR, R. N., MVRTON, B. J. & NESBITr, R. W. Chemical transects across intra-oceanic arcs: implications for the tectonic setting of ophiolites

Austin F. Rogers, the senior author of the first two editions of this text and professor emeritus in mineralogy at Stanford University, passed away at Berkeley, California, in April, 1957. His wise counsel as a former professor and his judgment as a mineralogist have been greatly missed during this revision. On the other hand, many readers, particularly those most familiar with Professor Rogers and his work, will recognize the influence of his teaching and will remember portions of the text which remain unchanged in the third edition.

The most striking features of crystals are their symmetric, polyhedral forms. Federov said that crystals “flash-forth their symmetry” (Shubnikov and Kopstik, 1974), a strong allusion to the sharp, highly reflecting surfaces of well-formed crystals prized by enthusiasts. We learned from Häuy that the shapes of crystals reflect the arrangements of the particles from which they are constructed. Today, these arrangements are routinely deduced from x-ray diffraction patterns. In the 19th Century, prior to the discovery of x-rays, the perturbations to the state of visible light traversing a crystal were used to assay symmetry.

Ideally, the symmetry of a crystal that an investigator would deduce from shape, from the transmission of visible light, from the scattering of x-rays, or from any other method, should be the same. However, as early as 1818, David Brewster in Edinburgh first noticed that some crystals appeared to have lower optical symmetries than morphological symmetries (Brewster, 1818b). <...>