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Rubicline, a New Feldspar from San Piero in Campo, Elba, Italy

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Rubicline, a New Feldspar from San Piero in Campo, Elba, Italy AmericanMineralogist, Volume 83,pages 1335-1339,1998 Rubicline, a new feldspar from San Piero in Campo, Elba, Italy Devro K. Tnpnrsrnrrr PerR dnnNfr'FruNr C. HlwrnoRNErr.* Jur-rc PrER,2 Lu-MrN WaNcr2't and RooNBy C. EwrNcr't IDepartment of Geological Sciences,University of Manitoba, Winnipeg, Manitoba, CanadaR3T 2N2 '?Deparlment of Earth and PlanetarySciences, University of New Mexico, Albuquerque,New Mexico 87131-1116,U S A. Ansrnlcr The rubidium analogueof microcline, rubicline, (Rb,K)AlSi3O,, ideally RbAlSi3Os, 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 abundantbut small (<50 pm) rounded grains in r-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 birefringenceis low (1st order gray interferencecolors), and crys- tals 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 p,m, heterogeneouscomposition, and structural coherency of rubicline with the enveloping microcline. Rubicline is triclinic, probable spacegroup Pl, with a : 8.81(3),b : 13.01(3)c : 7.18(4)A, : 90.3(1), : " B 115.7(3),"y : 88.2(1)', V = 741 A', Z : 4, and axial ratios a:b:c of 0.6jj:l:0.57'l (calculated from electron-diffraction data). Chemical analysis by electron microprobe gave58.68 SiO,, 16.48 A1,O3,6.23K,O, ll.4t Rb,O, 0.92 Cs,O,0.12 Fe,O,,sum 99.90 wtVo and the formula (Rb.sr"K4oTcsooro)r,*,(Alonr.Feo-r)Si,oo,O*.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. INrnooucrron of structural chmactenzation. The small size, the similar- The granitic pegmatitesof Elba, Italy, contain various ity of rubicline to microcline in thin section,and the sim- rare-elementminerals and are a source of valuable gem- ilar X-ray powder-diffraction patterns probably hindered stock (Orlandi and Scortecci 1985). Pollucite and petalite earlier identification. By electron microprobe, the energy were first describedfrom Elba (Breithaupt 1846). Foitite dispersive (EDS) Rbl,ct peak is hidden under the SiKct was recently discoveredin the Filone-Rosina aplite-peg- peak, leaving only wavelengthdispersive (WDS) analysis matite dyke, which has producedmany fine specimensof and backscatteredelectron (BSE) imaging as methods of tourmaline (Pezzottaet al. 1996).During systematicstudy detecting rubicline. However, the rubicline structure was of the minerals that form part of the typical sequenceof shown to be similar to that of triclinic (Al,Si)-ordered alteration of pollucite, several occuffences of Rb-domi- microcline using high-resolution transmission electron nant feldspars were found (Teertstraet al. 1998a).These microscopy (HRTEM) on the Elba material. The new representthe first mineral(s) in which rubidium is an es- mineral and name have been approved by the Commis- sential constituent. sion on New Minerals and Mineral Names, IMA. The Rubidium-dominant feldspar is relatively widespread original sample was a single unregisteredfragment from in the core zones of complex (Li,Cs,Rb)-enrichedrare- the Ecole des Mines, Paris collection. Two halves of the element granitic pegmatitesthat contain Rb-rich potassi- type material are depositedin the R.B. FergusonMuseum um feldspars associatedwith pollucite. However, in the of Mineralogy, University of Manitoba under catalogue dozen-or-sopegmatites in which feldspars with Rb > K numbersM 6980 and M 6981. were identified, small grain size, optical similarity to mi- crocline, and structural coherency with host microcline OccunnnNcn precluded manual separationof rubicline for the purpose The Monte Capannemonzogranitic pluton is well ex- posed at the westernend of the island of Elba, Tyrrhenian * E-mail:frank-hawthorne@umanitoba ca f Presentaddress: Department of NuclearEngineering and Ra- Sea,Italy. Numerousrare-element pegmatites occur along diologicalSciences, University of Michigan,Ann Arbor, MI the easternmargin of the pluton, but pollucite is known 48109-2104,U.S.A. only from the pegmatite veins near San Piero in Campo; 0003-004x/98/I I I 2- I 335$0s.00 I 335 1336 TEERSTRA ET AL: RUBICLINE. A NEW FELDSPAR Our sample of pollucite follows the typical sequenceof alteration of pollucite from other localities (Teertstraet al. 1993): coarse (1-2 cm wide) veining by coarse- grained lepidolite and microcline (-F muscovite), fine (1- 2 mm wide) veining by fine-grained muscovite (* spod- umene), local metasomatic replacement by untwinned non-perthitic near-Orlo. adularia, followed by late anal- cimization, leaching, and argillization. Numerous small (<20-50 pm) grains of rubicline (+ quartz)occur in l-2 cm wide veins of twinned microcline (+ muscovitewith 1.4-1.7 wtToRb,O, albite Ab,-, and minor apatite with 4.1 wt%oMnO) which crosscut pollu- cite (Figs. 1A-1C). The host microcline has patchy dis- tributionsof 0.11-0.35 wtToNa,O,0.90-3.39 wt% Rb,O, and 0.10-0.31 wt%oCsrO; most microclineis P-free,but Frcunn 1. BSE imagesof rubiclinefrom Elba: (A) Large local concentrationsreach 0.35 wt%oPrOr. Average RbrO grain of rubicline(white) + quartz(black) in microcline(dark content of the microcline correspondsto approximately 5 gray);(B) Rubicline(white) including grains of quartz(black) in molTo Rbf (rubidium feldspar). Rubicline also occurs microcline(gray) associatedwith albite (black)and muscovite with albite, and locally forms thin (<5 pm wide) veinlets (mottleddark gray); (C) Rubicline(white) + quartz(black) in in microcline (Fig. 1D). Minor quantities of (Rb,K)-rich microcline(gray) in contactwith Rb-freeadularia (dark gray) at feldspar occur with metasomaticuntwinned adularia (+ margins;(D) Veinletof rubicline(white) in microcline(gray) and cookeite) that overgrows microcline and replacesmicro- smallgrains of rubicline(white) (black). associatedwith albite cline and pollucite along grain boundaries(Fig. 1C). The adularia is Na-. P-. and Rb-free with BaO < O.16 wt%o < (Rb,K)-rich these include La Speranza,Fonte del Prete, Filone della and SrO 0.16 wtTo.The adularian feldspar Speranza, and Masso Foresi (Orlandi and Scortecci is Na- and P-freewith 8.45-18.86 wtToRb,O and 0.17- (Table 1985). The sample investigated here (Ecole des Mines, L69 wt%oCs,O 1). Paris) is probably from the La Speranzadyke. In the Elba Puvsrcar. AND oPTrcAL PROPERTTES granitic pegmatites, pollucite occurs at the margins of miarolitic cavities, in association with primary micro- Rubicline is brittle, transparent,and colorless. Refrac- cline, albite, muscovite, quaftz, and elbaite; it is strongly tive indices are slightly higher than those of the host mi- corroded and locally overgrown by late-stage zeolites. crocline as determinedby Becke-line testsin thin section. Taele 1. Representativecompositions of K-Rb feldsparsfrom San Piero in Campo, Elba, ltaly sio, 63.34 64 00 58.68 ov Il 54.42 56 96 64.80 57.89 Al,o3 18.25 1812 16.48 1727 15.84 1632 1824 to oc DA 0.25 0.00 0.00 0.00 0.00 0.00 001 000 Naro 0.30 014 0.00 o02 0.00 0.00 0.00 0.00 KrO 15.60 1551 o-zJ 1035 416 to.co 508 Rbro 0.90 too 17.47 1090 19.61 21.55 000 18.86 Cs"O 0.25 022 0.92 038 137 1.10 0.02 0.78 SrO 0.00 000 0.00 003 0.09 008 o02 007 BaO 0.00 0.09 0.00 0.00 0.00 0.00 0.00 000 Sum 98.89 99 74 99.90 99 72 99 49 99.76 99.65 99.33 Si 2.980 2 999 3.001 2 996 3.032 2.990 3 006 2 991 AI 1 012 1 001 0.991 1 003 0 969 1.010 0.997 1 014 P 0.010 0.000 0 000 0.000 0.000 0.000 0.000 0 000 Na 0.027 0 013 0 000 0 002 0 000 0.000 0.000 0 000 K 0 936 o 927 0.407 0.651 0.276 0.251 0.980 0 335 Rb 0 027 0 050 o 574 0 345 0.654 0.727 0.000 0 626 US 0 005 0 004 0.020 0.008 0.030 0.025 0.000 0 017 Sr 0 000 0 000 0 000 0.001 0 003 0.002 0.000 0 002 Ba 0.000 0.002 0 000 0.000 0 000 0 000 0.000 0.000 :,M 0 995 0.996 1 000 1.007 0 963 1 005 0.980 0 980 M+ 0 995 0.998 1 000 1.008 0 966 1 007 0 980 0 982 TO; 1.O02 1.001 0 998 1.003 0 969 1.010 0.997 1 014 >T 4 002 4.001 3 999 3.999 4 001 4 000 4 004 4 008 Si/AI 2.99 3.00 300 2.95 313 296 302 2.94 /Vofe-'Atomic contents based on 8 atoms of oxygen 1 and 2 - Microclineveins in pollucite.3 : Representativecomposition of rubiclinecharacterized structurallyby HRTEM;includes 0.12 Fe,O3(0.005 apfu).4: Rubidianmicrocline associated with albite in microclinevein.5 and 6: Rubiclinein microcline.7 : Adulariareplacing pollucite. 8 : Rb-Kfeldspar in adularia. TEERSTRA ET AL.: RUBICLINE. A NEW FELDSPAR r337 Rubicline is biaxial, but it was not possible to measure 2V. Grains positively identified in thin section show low 102 o8 birefringence (lst order gray interference colors) and an roo apparentlack of twinning.
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