X-ray Amorphous Material Research Articles

Systematic variations of sialic and volcanic detritus in piston cores from the Red Sea

The 87Sr 86Sr ratios and Sr concentrations of the non-carbonate fractions of sediment from two cores taken in the median valley of the Red Sea indicate that the detrital fractions are mixtures of two components originating from old sialic and young volcanic rocks of the surrounding land areas. The mixing equations were derived from the data and were used to estimate the Sr concentrations of the two components. The volcanogenic detritus has a Sr content between 770 and 800 ppm while the component derived from old sialic rocks contains from 50 to 70 ppm. The volcanic component consists primarily of unweathered particles of alkali basalt and volcanic glass while the sialic component is represented by clay minerals and X-ray amorphous material. Systematic variations of the concentration of volcanogenic detritus were used to define layers of sediment which correlate over a distance of about 200 km separating the two cores. The concentrations of quartz, feldspar, amphibole, illite, kaolinite and chlorite were found to vary systematically with the content of volcanogenic detritus as calculated from the Sr data. The carbonate concentrations of the sediment range from 33 to 87% and appear to increase in a southerly direction. The rate of carbonate deposition increased at times of more efficient input of sediment derived from old sialic rocks. This relationship is explained by the hypothesis that both were influenced indirectly by brief warming trends during the Würm glaciation. The occurrence of sediment layers enriched in sediment derived from old sialic rocks correlates approximately with interstadials of the Würm glaciation in northern Europe. Increased input of volcanogenic detritus occurred around 37,000 and 26,000yr ago and during the past 5000 yr. A plot of 87Sr 86Sr and 87Rb 86Sr ratios defines a line on the Rb-Sr isochron diagram yielding a fictitious date of about 230 million yr.

The thermal decomposition of amosite

SummaryWhen amosite (fibrous grunerite, Fe5·5Mg1·5Si8O22(OH)2), is heated in argon or nitrogen, physically combined water is lost up to 500–700° C. Above 500° C (static) or 700° C (dynamic), dehydroxylation occurs endothermically, giving a pyroxene as the main product. Under dynamic heating conditions, part of the hydroxyl water is lost as hydrogen, with concurrent oxidation of the iron. At about 1000° C the pyroxene is decomposed to olivine and cristobalite; at about 1100° C melting begins. In oxygen or air, physically combined water is again lost below 500–700° C. At 350–1200° C a sequence of overlapping dehydrogenation, oxygen absorption, and dehydroxylation reactions occurs, which gives rise to a broad exotherm on the d.t.a. curve. The main products (for static heating conditions) are an oxyamphibole at 350–800° C, and a spinel, hematite, a pyroxene, and X-ray amorphous material at 800–1100° C. Silica crystallizes as cristobalite at 1100–1350° C, and as tridymite at 1450° C. Most of the products in either neutral or oxidizing atmospheres are formed topotactically. The mechanisms and rates of the reactions are discussed, and the problem of determining the chemically combined water in amosite and other minerals of similar composition is considered.