An experimental study of ordering in sodium-rich alkali feldspars

SummaryVariations in the value of 2θ131 – 2θ13̄1 of two anorthoclase feldspars of compositions Ab95Or5 and Ab90Or10 were investigated by hydrothermal crystallization of gels for long periods in the temperature range 500–950 °C, at water vapour pressures up to 15 000 lb/in.2. The results are compared to those obtained by MacKenzie (1957) for pure albite. It is shown that the variation in 2θ131 – 2θ13̄1 is largely the result of Al-Si order-disorder and that at 700 °C and above the magnitude of the variation in the anorthoclases suggests that the degree of order is like that in albite, but at lower temperatures, particularly below 600 °C, the variation is less, and the anorthoclases are less ordered than albite at the same temperatures. At 600 °C and above the rate of attainment of equilibrium is the same for all three compositions. Below 600 °C Ab95Or5 behaves differently to albite. At low temperatures the equilibrium degree of Al-Si order in mixed Na-K feldspars is a function of composition as well as of temperature of crystallization.

A former association of enstatite and kyanite

SummaryAn enstatite-cordierite-sillimanite-quartz rock from a pyroxenegranulite facies terrain in southern Rhodesia gives evidence of at least two stages of recrystallization : an early, high-pressure recrystallization to form the assemblage enstatite-kyanite-quartz, and a later, ‘retrogressive’ recrystallization during which kyanite inverted to sillimanite, and cordierite was formed by reaction. The rock itself has ratios of R2+ : Al : Si that are virtually those of cordierite ; the recent data of Schreyer (1967) may thus be applied to the occurrence. The early kyanite-enstatite recrystallization occurred under pressures in excess of 10 kb, corresponding to depths greater than those of the continental Moho.

On the electrolysis of molten basalt

SummaryThe effects to be expected from the interaction of possible electric currents at depth with bodies of basaltic magma have been investigated experimentally. Trough-shaped molten specimens were produced in the surface of basalt blocks by heating from above ; the melts were electrolysed and the products analysed chemically.Si, Al, Ti, P, Fe2+, and probably Fe3+ are concentrated towards the anode, apparently in the form of drifting lattice remnants; oxygen gas is liberated. Na, K, Ca, Mn, and Mg ions concentrate towards the cathode.Relatively to basalt, the cathodic product is an alkaline and femic rock with normative nepheline and a more acid plagioclase. The anodic rock is distinctly calc-alkaline and salic, with normative quartz and a plagioclase that is more basic. The precise ‘rock-type’ developed depends on the amount of electricity passed, but the trends are distinct. In that increasing basicity and alkalinity (cathodic rocks) are accompanied by both increasing Na/K and Mg/(Fe2+ + Fe3+) ratios (which ratios decrease with decreasing basicity and alkalinity) the electrolytic series developed from a basalt magma appears to have few counterparts among natural rocks.