Chemical changes accompanying spherulitic crystallization in rhyolitic lavas, Central Volcanic Region, New Zealand

SummarySpherulitic devitrification is a post-eruptive process affecting many rhyolitic lavas of the Taupo region. The spherulites consist of cryptocrystalline intergrowths of α-cristobalite and alkali feldspar (calcic anorthoclase), with minute granules of magnetite, hematite, and secondary (?)goethite. The effects of chemical fractionation occurring during progressive spherulite growth has been studied from a suite of samples from the Aratiatia rhyolite. The most significant effect is the progressive enrichment of both bulk spherulite compositions and the coexisting residual glass in potash with increasing spherulite development. This effect is due both to the very low potash in the earliest formed spherulites and to the consistently higher Na/K ratios of the spherulites relative to the total rock compositions. These differences progressively decrease with increasing spherulite crystallization. The bulk rock compositions, however, evidently remain essentially constant. The degree of potash enrichment in the residual glasses during advanced stages of devitrification is greater than expected by reference to the ternary feldspar and quartz-feldspar systems. This post-eruptive alkali fractionation during spherulite formation is superimposed on the pre-eruptive phenocryst-liquid fractionation.

The characterization of the humic complexes of soil organic matter

1. During the course of the alkali fractionation and the preparation of the humic precipitates, the qualitative characters of the fractions were followed by employing a standard hydrolysis with dilute acid. Humic acid itself was shown to be unhydrolysed by this reagent, and, therefore, the extent of hydrolysis of a humic fraction can be used as a criterion of purity.2. Studies on the fractionation of the humic precipitate with aqueous, anhydrous, and alcoholic bases, led to the following conclusions:(a) The humic precipitate as normally prepared i s always a mixture of true humic acid with varying amounts of co-precipitated material of a non-humic nature.(b) This non-humic material is not an integral part of the humic molecules, it is merely co-precipitated and adsorbed contaminants.(c) The non-humic material is differentiated from the humic acids by the following properties: (i) it is hydrolysable with acid; (ii) it does not give the characteristic nitro-humic compound on nitration.