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.
Thermodynamic mixing properties of Rb-K feldspars
Abstract Hydrofluoric acid solution calorimetric measurements conducted at 50 °C on a series of Al-Si ordered Rb-K feldspars reveal that the enthalpies Rb-K mixing (Hex) for this series are positive, with maximum values skewed toward Rb-rich compositions. However, the magnitudes of Hex are greatly reduced relative to those for Na-K feldspars, as are the positive volumes of Rb-K mixing observed for this series. Both observations can be explained by the substitution of two ions that are relatively similar in size. The difficulty in synthesizing Rb-rich sanidine via Rb-exchange of K-sanidine, contrasted with the ability to synthesize rubicline from K-microcline by such exchange, together with a vanishingly small ΔV between ordered rubicline and disordered Rb-sanidine suggest that the volume of Rb-feldspar is close to the limit to which the feldspar structure can be expanded at 1 bar.
Calorimetric investigation of Na-K mixing and polymorphism in the alkali feldspars
Heat of Solution measurements of the enthalpies of Na-K mixing, Al-Si ordering, and fusion for crj-stalline solutions and end-member phases in the system Na(AlSi308)—K(AlSi308) were obtained with a vacuum-jaicketed, isoperibolic calorimeter using 20.1 percent hydrofluoric acid at temperatures ranging from 40 to 60°C. Heat of Solution data for artificially prepared microcline (—low-albite) crystalline solutions at 49.7°C are represented by Hso\ = —149.408 + 5.230 1V2 — 5.928 A^AV — 8.457 A'jA^^ kcal/mole based on 39 measurements of 14 different bulk compositions, where A'i and N2, denote the mole fractions of NaAlSisOs and KAlSiaOg respectively. The thermochemical uncertainty of the above least-squares formulation is ± 0.032 kcal/ mole. Unit-cell Parameters determined by least-squares refinement of diffraction data indicate that the excess volumes of Na-K mixing of these microcline solutions are positive over the entire composition ränge. Combining the calorimetric and crystallographic data leads to the following expression for the Variation of molar enthalpy of mixing with pressure: Bex = (5928 + 0.110 P ) NiNi^ + (8457 + 0.110 P ) AVVi^ cal/mole . The sign and magnitude of Hex determined by calorimetry agree closely with JieiL calculated from a third-order Margules formulation of high-temperature two-phase equilibrium data in the critical region.