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.
Refinement of the crystal structure of a synthetic non-stoichiometric Rb-feldspar
The crystal structure of hydrothermally synthesized Rb-feldspar (monoclinic, space group C2/m, a = 8.839(2) AÊ , b = 13.035(2) AÊ , c = 7.175(2) AÊ , b = 116.11(1)8, V = 742.3(3) AÊ , Z = 4) has been refined to a final R of 0.0574 for 692 independent X-ray reflections. Microprobe analyses of the Rb-feldspar suggest deviation from stoichiometry, with excess Si and Al, resulting in a unit formula of Rb0.811&0.127Al1.059Si3.003O8. Infrared (IR) spectra indicate the structural occupancy of large H2O content, which implies that the &Si4O8 substitution favours the structural incorporation of the H2O molecule at the M-site. The mean T O distances are 1.632 AÊ for T1 and 1.645 AÊ for T2, revealing highly disordered (Al,Si) distribution with Al/Si = 0.245/0.755 (T1 site) and 0.255/0.745 (T2 site). There are two geochemical implications from this refinement: (1) identification of both rubicline triclinic with (Al,Si) ordered distribution and synthetic monoclinic RbAlSi3O8 with (Al,Si) disordered distribution implies that Rb cannot be one of factors disrupting the (Al,Si) ordered and disordered distributions in feldspars; and (2) natural and synthetic feldspars capable of accommodating the large cations tend to incorporate &Si4O8, excess Al and H2O components in their crystal structures.
Time and Heritage
As a historian studying what can be considered a form of intellectual history, I have slowly come to adopt Michel de Certeau’s observation made at the end of the 1980s that ‘objectifying the past, for the last three centuries, has undoubtedly left unconsidered time within a discipline that has continued to use it as a taxonomic instrument’. To a certain degree, time has become commonplace for the historian who has preserved or instrumentalized it. It is not considered because it is inconceivable, but because we do not think of it or, more simply, we do not think about it. As a historian attempting to pay attention to the time I’m living in, I have thus, like many others, noticed the swift development of the category of the present until it has become obvious that the present is omnipresent. This is what I refer to here as ‘presentism’.