Rubicline, a new feldspar from San Piero in Campo, Elba, Italy
Abstract The rubidium analogue of microcline, rubicline, (Rb,K)AlSi3O8, ideally RbAlSi3O8, was found in a pollucite-bearing rare-element pegmatite at San Piero in Campo, Elba, Italy. Rubicline is the first mineral with rubidium as an essential constituent. It occurs as abundant but small (≤ 50 μm) rounded grains in 1-2 cm wide veins of rubidian microcline (± albite, muscovite, quartz, and apatite) that crosscut pollucite. Rubicline is brittle, transparent, and colorless. Refractive indices are slightly higher than those of the host microcline. The birefringence is low (1st order gray interference colors), and crystals are apparently untwinned. In thin and polished sections, cleavage passes through both the host microcline and grains of rubicline; by analogy with microcline, the cleavage is {001} perfect and {010} good. Determination of additional physical properties is hindered by an average grain size of 20 mm, heterogeneous composition, and structural coherency of rubicline with the enveloping microcline. Rubicline is triclinic, probable space group P1̅, with a = 8.81(3), b = 13.01(3) c = 7.18(4) Å, α = 90.3(1), β = 115.7(3), γ = 88.2(1)°, V = 741 Å3, Z = 4, and axial ratios a:b:c of 0.677:1:0.577 (calculated from electron-diffraction data). Chemical analysis by electron microprobe gave 58.68 SiO2, 16.48 Al2O3, 6.23 K2O, 17.47 Rb2O, 0.92 Cs2O, 0.12 Fe2O3, sum 99.90 wt% and the formula (Rb0.574K0.407Cs0.020)Σ1.001(Al0.993Fe0.005)Si3.001O8. Rubicline is, in many cases, structurally coherent with the host microcline; it formed by exsolution from a (K,Na,Rb)-enriched precursor, followed possibly by fluid-induced modification.
Isotopic Equilibrium/Disequilibrium and Diffusion Kinetics in Feldspars
The use of isotopes as tracers in diffusion kinetics measurements on feldspars is discussed with particular emphasis on oxygen and cations. The effect of feldspar composition, temperature, fugacity of water, ftigacity of oxygen, anisotropy of diffusion, and sub-sclidus phases are treated, with a view toward understanding the diffusion mechanisms and to predict diffusion coefficients where measured data are lacking. The strong dependence of oxygen diffusion on fH2O is suggested to derive from having H+ act to aid in breaking the Si-O bonds, and H2O aid in the transport through the crystal. The lack of such dependence by cations, together with their systematic behavior as a function of cation charge, and, secondarily, on ionic radius has led to a diffusion model utilizing a vacancy mechanism for most of the cations in feldspars, with Na transport being by an interstitial mechanism. The ionic porosity prediction model can predict oxygen diffusion under hydrothermal conditions, but fails for cations.
Inter-diffusion of K and Na in alkali feldspars; diffusion couple experiments
Inter-diffusion of K and Na in alkali feldspar was studied at 5 and 15 kbar confining pressure using diffusion couples annealed at 900 degrees and 1000 degrees C. Concentration profiles were measured using an electron microprobe and results were obtained for inter-diffusion in four crystallographic directions at 1000 degrees C. Inter-diffusion coefficients (D) were calculated as a function of composition from Or 10 to Or 80 . Alkali inter-diffusion is about 10 times faster normal to (001) than normal to (010), and the rate is intermediate parallel to [100] and [011]. A trace amount of water appears to have no influence on the inter-diffusion rate, and within experimental error there is no difference D between 5 and 15 kbar. There is agreement between the D values determined experimentally and those calculated from published self-diffusion coefficients. The nonideality of the alkali feldspar binary at 1000 degrees C has a significant effect on D, and this effect becomes increasingly important at lower temperature.--Modified journal abstract.