Rock composition, density and a variation diagram

SummaryThe relationship between silicate rock density and composition has been investigated and empirical formulae have been obtained that give a factor equivalent to a ‘normative’ density. Comparison between this factor and measured density may provide useful diagrams assisting the interpretation of metamorphic petrogenesis. The factor itself is a viable differentiation index aiding comparison between the compositions and liquidus temperatures of volcanic suites.

Kaersutite-bearing plutonic xenoliths from Tristan da Cunha, South Atlantic

SummaryThe xenoliths, which are gabbroic in character, consist of kaersutite, titanaugite, plagioclase, and iron ore in variable amounts. Biotite and olivine are also occasionally present. New analyses and trace element data are given for two xenoliths and their coexisting kaersutite, titanaugite, and plagioclase. After discussing several possible origins with the aid of a principal latent vector variation diagram, it is concluded that they represent the accumulation of phases that crystallized from the Tristan magma at depths between 5 and 25 km. Evidence is also presented to show that the parental Tristan magma may be of trachybasalt composition.

Axinites from the contact skarns of the Meldon aplite, Devonshire, England

SummaryChemical analyses, optical properties, and specific gravities are presented for nine axinites. A reciprocal substitution relationship is demonstrated between Mg and Mn2+, and a linear variation diagram is constructed between refractive indices and the (Fe2++Fe3++Mn2++Ti) ions calculated on the basis of 16(O,OH). The composition of these axinites is independent of the composition of their host rocks; their colour appears to be independent of the Fe2+/Mn2+ ratio for the range of iron and manganese values of the specimens investigated.

Three-Dimensional Variation Diagrams for Control of Calculations in Optimum Design

This paper presents and illustrates a graphical technique for the control of calculations in the procedure of optimum mechanical design. This three-dimensional variation diagram technique minimizes the number of calculations required for the determination of the optimum design, and the general technique is applicable to many problems. Specific practical examples are given for the optimum design of a torsion bar and for the optimum design of a gearset.

An X-ray powder method for the estimation of (K, Ba) feldspars

SummaryExamination of previous physical methods of estimating the barium-content of these feldspars showed that they are generally unsatisfactory. A new variation diagram is constructed correlating BaO content and values of d132 determined from diffractometer records. It is estimated that the BaO content of these feldspars can normally be determined to about 2–3% by using this diagram.Some of the problems of the mineralogy of the series are reviewed in the light of the experimental results, in particular the effect of polymorphic changes on the variation diagram. Some experimental evidence of the effect of different structural states upon the value of d132 is obtained, and the limitations imposed upon the method by the existence of such states are discussed.

On the Differentiation of the Glen More Ring-dyke, Mull

SummaryNew analyses are given of the Glen More Ring-dyke. The basic end of the variation diagram is found to differ considerably from that of the Normal Magma Series. The magma variation is explained by gravitative sinking of augite and iron-ore (and plagioclase), the olivine of the Normal Series playing only a subordinate part. Various objections are brought forward to the explanation of the magmas by hybridization. The peculiar microscopic aspect of the intermediate rocks is tentatively attributed to pneumatolytic emanations from the lower reaches of the column. It is shown that a gradual transition from basic to acid already existed before the final consolidation of the magma took place.