Chlorite Crystals Research Articles

X-Ray Diffraction Criteria for the Characterization of Chloritic Material in Sediments

AbstractRecent surveys of clay mineral assemblages in argillaceous sediments have indicated that chloritic minerals are of more frequent occurrence and of greater significance than was formerly recognized.The wide latitude of compositional variations within the chlorite crystallization scheme introduces enough complexity into the x-ray diffraction identification and characterization of these minerals that a detailed description of a few typical clay mineral associations seems justified.For the simplest cases, in which resolution of diffraction features from entirely chloritic grains can actually be demonstrated, analyses may be highly reliable. Further, for many unresolved instances, simple heat treatments effect reliable identifications.For more complicated assemblages, in which coherently scattering domains are mixtures within themselves, individually planned heating or swelling reactions may become necessary.Typical natural examples are illustrated, for which apparently proper distinctions have been made for characterizing associations on mono- and mixed-clay mineral grains.

Metamorphism in the Upper Devonian of N. Cornwall

The geological succession and structure of the district are briefly summarized. Evidence of stress conditions becomes increasingly abundant as one passes from south to north towards the boundary of the Carboniferous.The thermal alteration due to the Bodmin Moor granite is definitely delimited by tracing the outer limit of spotting in normal pelitic sediments.Petrographical study of phyllites from outside this limit shows considerable mineralogical reconstruction in the sediments, with crystallization of white mica, chlorite and chloritoid; in some instances garnet is developed. The distribution of these types shows no relationship to the granite outcrop, but accords with the main structural features of the area.Analogous changes are prominent in the igneous horizons, with the formation of schists of the general type of prasinite.Within the aureole thermal effects can be traced superposed on the earlier dynamic changes.From these lines of inquiry it is concluded that many of the changes in the phyllites, notably the formation of porphyroblastic white mica, chlorite and chloritoid, hitherto ascribed with greater or less finality to the action of the granite, are due to an earlier dynamic metamorphism.

XX. Explanation of a Supplementary Plate to the Paper on Vegetable Remains preserved in Chalcedony, printed in the Second Volume of the Transactions of the Geological Society.

Fig. 1. * This figure is given for the purpose of explaining the mode in which the fibrous disposition of chlorite is effected. The crystallized arrangement which is here so visible is generally observed in those chlorite fibres which occur in transparent quartz. I have never observed it in chalcedony, yet there is little doubt that a similar disposition is equally the cause of the fibrous appearance so common in the specimens which have been figured in the former plates, the chief differences consisting in the smaller size and less perfect crystallization of the chlorite scales. Fig. 2. The crystals of chlorite magnified. Fig. 3. Although in the former plates I figured some specimens which I imagined to be confervæ, I had not then procured any example in which that regularity of structure, which is so conspicuous in some of the plants of this family, could be observed. The present figure exhibits that regularity in a striking manner. Fig. 4. This figure affords a second example of that regular ramification which botanists will immediately recognize as characterizing many of the plants of this tribe; although for the reasons already assigned in the paper, it will be fruitless to attempt to discover the species.