Crystallography and Mineralogy. By Kiyoshi Fujino. Ky\bf\overline oritsu, 2015. Hardcover, X+180. Price Yen 3600. ISBN 978-4-320-04719-8.
Crystallography and Mineralogy (kesshogaku kobutsugaku), 11th volume in the series Introduction to Modern Earth Science published by the Japanese company Kykoritsu, is a small textbook composed of 11 chapters, three appendices, two appendix tables and one plate, that clearly targets students and young researchers in Earth and Planetary Sciences with a practical approach to the investigation of mineral samples. Given the small size of the book, the topics are necessarily presented in a rather concise way and selected from what could have been a wider choice. The text is well written with a pedagogical approach: a beginner reading this book will have a rather clear idea of the general features of the samples and of what information (s)he may get from the main experimental techniques: for a more in-depth approach one will however need to move to more specialistic textbooks. The first two chapters, which span only seven pages, summarize the type of samples that are the focus of the treatment and the techniques used for their chemical and structural investigation. Geometric crystallography is the object of chapter 3 (‘geometry and symmetry of a crystal’, 18 pages), which is the weakest in this book. It makes constant reference to the 1972 edition of International Tables for X-ray Crystallography; for a text appearing in 2015, the choice of such an outdated reference is hard to understand and certainly not justified. A number of imprecisions affect this chapter which, because of the very limited space, reduces to little more than a list of definitions. Note in Table 3.1 we read about a ‘trigonal lattice’ (sampo koshi), which simply does not exist, trigonal being a word reserved for a crystal system: the ‘other’ lattice in the hexagonal family is rhombohedral (hishigata koshi). On the same page, we read that the symmetry of the lattice leads to a classification in a crystal system (shkokei), which is untrue, the result being lattice systems (koshikei). The same mistakes are repeated on page 18, whereas the figure on p. 19 correctly shows a rhombohedral cell: the contradiction should be selfevident. On p. 12 the definition of geometric element is incorrectly applied to the symmetry element. In the next page a curious listing of eight ‘independent symmetry elements’ is given, which does not make much sense. Firstly, if the list really concerns symmetry elements, as suggested by the fact that 3 and 6 are missing (being considered the combination of 3 and 1 and of 3 and m perpendicular to it, respectively), then the identity should not appear because it obviously has no ‘element’. Secondly, if the list includes symmetry operations instead, as suggested by the presence of the identity, then one could list only 6 and 1, the repeated application of either or both of them generating all the ten types of operations in crystallography point groups. Whatever interpretation is adopted, the statement remains incorrect. Fig. 3.5 in the following two pages gives the stereographic projection of the 32 types of crystallographic point groups where, however, the cubic centrosymmetric groups are given as m3 and m3m instead of m3 and m3m (same problem in Table 1 of the appendix): an unjustifiable remnant of prehistoric times when typographical issues primed over consistency (3 includes 3 as even powers, whereas the opposite is not true). Table 3.3 in the next page gives [100] instead of [110] as the tertiary symmetry direction of tetragonal crystals; furthermore, only one direction (square brackets) is given instead of a representative set of equivalent directions (angular brackets) for cubic and uniaxial crystals, and the direction chosen does not follow international standards: this certainly comes from the questionable decision of choosing as a reference an old mineralogy book by Morimoto et al. (1975) instead of International Tables. No time to breathe, another imprecision follows immediately: the orthographic ISSN 2052-5206
Absolute-structure determination: past, present and future.
Single-crystal X-ray crystallography is the major analytical technique in use today for absolute-configuration determination. The origins of absolute-structure determination, starting from Friedel's 1913 proof that the intensities of the opposites hkl and hkl are identical, are traced. The important structural principles derived from the study of chiral, but pseudo-mirror symmetric, methyprylon are described. For the present time, the use of the average and difference intensities of the opposites hkl and hkl are stressed. This leads to the use of Friedif, of 2 AD and selected D plots, of Rmerge and the D-Patterson. The best techniques for absolute-structure determination in the future are described. Some advice to the scientific community concludes the paper.
Equilibrium Cross-section of Meandering and Braided Rivers
THERE are two simple models which describe the equilibrium state of a channel cross-section with constant conditions, one static, the other dynamic. In the first case equilibrium is said to hold when all grains on the bed are just at the threshold of movement1. Because movement is produced by a combination of fluid drag and the downslope gravity component and because the fluid drag is greatest in the centre of the channel, the channel slopes can increase up the sides to compensate for the decrease in drag. The resulting profile is roughly a parabola1 and is very similar to many natural channel forms. This static model is particularly relevant to the design of some canals where discharges are fairly constant and the bed material is normally immobile. Natural rivers transport sediment, however, and the mean channel form develops at high stages when nearly all the bed material is in intermittent motion. When sediment is transported as bed load it must be continually tending to creep downslope into the centre of the channel under the influence of gravity. If, however, the channel maintains a constant profile there must be an equal and opposite tendency at all points for the sediment to move sideways upslope. Apart from buoyant upthrust, the only other force acting on the grains is the fluid drag acting in the direction of the water flow, leading to the second model. When river channels are in dynamic equilibrium the flow along the bed has a sideways component up the channel banks which exactly compensates the tendency of the sediment to move downslope under the influence of gravity. As water flowing sideways along the bed must return to the channel centre along the surface this implies that the river has a double spiral secondary flow (Fig. 1). Although double spiral flow in straight channels has been demonstrated2,3, single spiral flow in meander bends is better known1. As, however, the sense of the spiral changes from one bend to the next, it follows from considerations of continuity and symmetry that in the straight reaches between bends there is a transitional double spiral structure (Fig. 1 C). In short the secondary flow in river channels is a double spiral which is asymmetrically developed in bends so that one vortex momentarily excludes the other at the crest of the meander. The two spirals adjoin where the flow attaches itself to the bed and diverges sideways; this line is also the thalweg or deepest part of the channel. It is no coincidence that the same hypothesis of double spiral flow is commonly used to explain the origin of many sand ridge bed configurations formed parallel to the flow, such as primary current lineation, sand ribbons, tidal current ridges and longitudinal aeolian dunes and draas4,5. River channels are analogous to the troughs between longitudinal sand ridges in certain other respects too: they arise spontaneously during sediment transport, reach a dynamic equilibrium by interaction with the flow and are associated with transverse structures, forming meander bars, that migrate downstream. In short, there is a good case for saying that the entire form of a river channel is a bed configuration sensu stricto.