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.
A simple predictive model for the thermal expansion of AlSi3 feldspars
Abstract We have investigated the thermal expansion of several synthetic feldspars, including Li-feldspar, rubicline (Rb-microcline), Rb-sanidine, and buddingtonite (NH4-feldspar). When analyzed in conjunction with earlier data on both ordered and disordered Na- and K-feldspars, it is clear that the coefficient of thermal expansion (α) decreases dramatically, and linearly, with increasing room-temperature volume. For “AlSi3” feldspars, then, chemical expansion limits thermal expansion. The relationship between α and room-temperature volume provides a useful predictive tool based simply on the volume of a feldspar at room temperature. This relationship also reveals that volumes of K-Na mixing in naturally occurring alkali feldspars decrease with increasing temperature.
Preparation of hydrogen and lithium feldspars by ion exchange
Alkali feldspars are among the most abundant rock-forming minerals. They are framework aluminosilicates with a basic stoichiometry of MAlSi3O8. The feldspar framework accommodates metal (M) ions ranging from about 1 to 1.5 Å (Na+ to Rb+) in size. Cations outside this range, such as lithium ions, have been found to enter the M site only in trace amounts, both in natural and synthetic samples. Hydrogen is also a common trace element in feldspars, but its presence seems to have considerable bearing on kinetic phenomena like oxygen diffusion and Al/Si order-disorder processes1. Neither these effects nor the structural roles of Li or H are well understood. It therefore seemed desirable to obtain the fully substituted feldspars HALSi3O8and LiAlSi3O8 for further study. In this letter I report on the low-temperature ion-exchange synthesis and lattice constants of these feldspars.