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.
IDENTIFICATION OF NORMAL AND ANOMALOUS COMPOSITIONS OF MINERALS BY ELECTRON-MICROPROBE ANALYSIS : K-RICH FELDSPAR AS A CASE STUDY
The precision of electron-microprobe (EMP) measurements at a 4sigma level of confidence is about 1%. By using internally consistent standards with compositions compatible with stoichiometries of the structural formulae, minerals with a generally similar matrix may be analyzed with < or =2% accuracy. Systematic analytical error of a standard may be greatly reduced by examination of the results of analysis of closely related unknown minerals. If a standard is compositionally and structurally well characterized, the accuracy of analysis of samples of near-identical composition may then approach 1% absolute. The bulk of the results, which give the expected stoichiometry of the structure, may be classified as normal. Anomalous measurements, which do not conform to ideal stoichiometry of feldspar, were used to identify analytical overlap with hematite micro-inclusions, light-element substitution, and a compositional trend toward []Si 4 O 8 in adularia from granitic pegmatites.
Regional strategies for the minerals industries of the states of the SADCC
The SADCC has been in existence for eleven years but little progress has been made in regional coordination of the minerals sector even though this sector is the most important exporter of the region. There is however scope for minerals development at the regional level.