AmericanMineralogist, Volume 83,pages 1335-1339,1998

Rubicline, a new 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 , 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 (+ , 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 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 (lst order gray interference colors) and an roo apparentlack of twinning. In thin and polished sections, ? ^ o6 cleavage passes through both the host microcline and oe8 J or grains of rubicline (Fig. 1A); by analogy with microcline, : the cleavageis {001} perfect and {010} good. Determi- 096 o2 nation of additional physical propertiesis hindered by an oo average grain size of 20 pm, compositional variability 0 94 096 096 100 102 104 0o o2 04 05 06 and structural coherency of exsolved rubicline and host M-cotion cho19. (pru) J04 microcline. 2)B D

Cnnurclr, coMposrrloN a.\ .: The Rb-K feldsparswere analyzedusing WDS analysis roo \ t ooJ E-'q. on a CAMECA SX-50 electron microprobe operating . 3 at + 2ga o 002 15 kV and 20 nA with a beam diameter of 5 pm. Data .':'; were reduced using the PAP procedure of Pouchou and 296 o 0t (1) t Pichoir (1985).Major elementswere measuredusing san- a': 2912 idine from Eifel, Germany (KKcr, AlKa, and SiKct) and 094 096 o9A 100 102 104 o2 04 06 08 Bonovdl.nt M calioi! (opru) synthetic RblnSi,O,, glass (Rblct). The composition of the had been optimized to agreementwith ideal Frcuno 2. Elementvariations in K-Rb feidsparsfrom Elba: feldspar stoichiometry according to the procedure of (A) (A1+Fe-P)vs M-cationcharge with a 1:1 trendline; (B) Teertstra et al. (1998b). The accuracy of the results is (Si+2P)vs. the sumof monovalentM cations;(C) K vs. Rb, : withtn 2Vo,and the precision is approximately l%o (4o). with a lineindicating lM 1; (D) Cs vs.Rb. Symbols: micro- clinevein in pollucite(n) andassociated rubicline (l); latead- Minor elementswere measuredusing albite (NaKa), pol- ularia (o) and associated(Rb,K)-feldspar (O) The arrowsmark lucite (CsLa), fayalite (FeKct), (BatB), barite SrTiO. trends for (1) the plagioclaseJikesubstitutions and (2) the (Srfa), (PKcr). and VP,O, The elements Ca, Mg, f; Pb, nSioO,substitution Mn, Ga, and Ti were sought but not detected. Feldsparformulae were calculatedon the basis of eight atoms of oxygen per formula unit (apfu). To compare and IZ : 4.001 apfu (no. 5 in Table 1). Extreme values compositionsof P-free feldspar with feldspar locally con- for RbrO in the rubicline correspond to 72.7 mol%o taining P, the framework charge was calculated using RbAlSi3Os (Fig. 2C). In the Rb-richest samples, maxi- TO; : (Al + Fe:*-P) and Si was adjustedusing Si + 2P mum substitution of Cs is 0.04 apfu, i.e., 4 molVo of a according to the (AlP)Si_, berlinite substitution. Minor hypothetical cesium feldspar component (Fig. 2D). concentrationsof P are found in the microcline, but P is absent from rubicline, adularia, and associatedadularian TEM aNar,Ysrs oF THE,srRUcruRE Rb,K-rich feldspar (Table 1). Attempts to separaterubicline yielded a microcline * In the BSE images of Figure L, gray levels of K-Rb feldspar with Rb > K + quartz mixture. X-ray powder feldspar increasewith mole percentrubidium feldspar.In- diffraction of this mixture did not show the presenceof dividual grains of microcline-hosted rubicline display a phasesother than feldspar and quartz, thereby establish- slight compositional heterogeneity (Figs. 1A and 1C). ing the absenceof any potential non-feldspathicstructure Clusters of grains have a slightly greater compositional that could be attributed to a (Rb,K)AlSi,O, compound. range than individual grains. Overall, RbrO concentra- As the cell parameters of the vein microcline [a : tions exsolved blebs of Rb-enrichedto Rb-dominant feld- 8.597(1),b : 12.963(6),c : 7.210(5)A, a = 90.62(13), spar throughout microcline range from -10 to -22 wt%o, B : 115.99(2),t :87.87(13)', v:121.8(1) Arl are correspondingto -30 to -7O mol% Rbf. close to those of ideal maximum microcline, an exsolved Individual values for M-cation charge [M * : mono- feldspar phase coherent with this structure can also be valent cations + 2x(divalent cations)l tend to be slightly expectedto have a high degree of (Al,Si) order. lower than calculated values of framework charge (TO; After identification of areasrelatively abundantin ru- : Al + Fe-P), suggestingthat minor (<1 atvo) concen- bicline by BSE imaging, several 3 mm diameter disks trations of light-elementM-cations may be presentlocally were drilled from the thin section for TEM examination. (Fig. 2A). Values of (Si + 2P) cluster near 3.0 apfu for Petropoxy betweenthe sample and glasswas removed by microcline, adularia, and adularianK-Rb feldspar; for ru- -24 h oxidation in a Technics Plasma Excitor using ox- bicline, the data lie along the trend for the nSi*O, sub- ygen gas. Slotted copper disks were then glued to the stitution (Fig. 2B). High SilAl ratios correspondto low sample using Superglue. Samples were thinned to elec- M-cation sums and equally low (Al-P); an extreme value tron transparencyusing a Gatan Duo-Mill ion mill with with -3Vo nSi*O8 has Si : 3.032 (or 3.000 + 0.032) argon gas. Identification of Rb-rich areasrelative to the apfu, Al : 0.969 apfu, M* = 0.966 (or 1-0.034) pfu, milled hole was possible only by multiple cycles of mill- 1338 TEERSTRA ET AL.: RUBICLINE, A NEW FELDSPAR

Teele 2. Rubidiumfeldspar d-spacings (A) -d-"*"4""hkt

5.82 5.814 0 'I 1 5.77 5.775 0 'I 1 4.62 4 607 0 2 1 'I 3.88 3 855 0 361 3.616 0 1 'I J.OU 3 596 1 359 3.588 0 1 2.94 2 938 1 4 'I 2.65 2.653 1 2.63 2.633 4 1 'I 261 2.607 0 zco z coz t 1 * d-values were measured from indexed electron-diffractionDatterns The differences between d..." and 4d" are quite large, reflectingthe in- trinsically lower accuracy and precision of electron diffraction relativeto Frcunn 3. TEM micrographs:(A and B) the interfacebe- conventionaldiff ractometry. tweenrubicline and microcline; (C andD) high-resolution,,lat- tice" image and SAED pattem of rubicline and microcline, respectively. an exsolution origin for an (A1,Si)-ordered rubidium feldspar. ing and location using BSE imaging on a scanningelec- Onrcrn oF RUBTcLTNE tron microscope (SEM). Once rubicline had been inter- Rubicline, ideally RbAlSi3Os, is the first mineral to sected by margins of the ion-milled area, samples were contain essentialrubidium and is a new member of the carbon-coatedand examined on a JEOL 2000FX scan- feldspar group, forming a solid-solution series with mi- rung transmission electron microscope equipped with a crocline. It occurs in pollucite-bearing rare-elementgra- Noran 5500 EDS system and a JEOL 2010 HRTEM nitic pegmatitesthat attain a high degree of alkali-metal equipped with a Link ISIS EDS system. fractionation. The primary potassium feldspar was prob- The peaks for SiKa (at 1.740 keV) and RbZct (at 1.694 ably (Al,Si)-disordered and monoclinic, and had consid- keV) overlap in EDS analysis, and the RbKa peak (at erably higher concentrationsof Na, Rb, and P than cur- 13.375 kev) was used for analysis of Rb and identifica- rently occur in the host microcline. In K-rich feldspar,the tion of Rb-rich areas.Natural compositionally homoge- monoclinic --> triclinic transition results from (Al,Si)-or- neous triclinic rubidian microcline from Kola Peninsula, dering rather than framework collapse, and precedes Russia [of composition -(Ko?.Rbnro)AlSi.Or;Teertstra et exsolution of albite (Brown and Parsons 1989). Textural al. 19971was used as a standardfor quantitative analysis. features suggest that rubicline exsolution postdated that The alkali content of the structurally characterizedrubic- of albite, probably due to the slower rate of diffusion of line from Elba, measuredusing EDS analysis,is approx- Rb compared to that of (Giletti imately RbounKo.,apfu. Na 1994). The rnicrocline-rubicline exsolution does not texturally A brighrfield image (Fig. 3A) shows a fairly sharp resemble the lamellar form typical of solid-state reac- interface between rubicline and microcline. Hish-resolu- tions. The most probable reason for the irregular shape tion imagesof the interfacebetween rubicline ulnd ^i.ro- of rubicline, and its diffuse transition into the K-dominant cline indicate continuity of the structure (Fig. 3B), and host, is the very small dimensionaland angulardifference individual diffraction patterns of rubicline and adjacent between the unit cells of end-membermicrocline and ru- microcline show that both have the samecrystallographic bicline (e.g., McMillan et al. 1980). The differences are, orientation (Figs. 3C and 3D). However, there is an in- of course, still smaller between intermediatemembers of creasein d-spacingsfrom the Rb-poor microcline to the the (K-Rb) series that constitute most of the exsolved Rb > K areasof rubicline (Table 2). Diffraction patterns were indexed using cell parametersof synthetic triclinic rubidium feldspar prepared by ion-exchange of micro- Taale 3. Comparisonof celldimensions for syntheticrubicline cline (McMillan et al. 1980). The cell parametersof ru- (Rbouul(oon)and rubicline(RbourKoo,Csoor) bicline determinedfrom refinementof electrondiffraction data are of much lower accuracyand precision compared Synthetic. Naturalt to the cell parametersdetermined using conventional X- a (A) 8 7s8(3) 8 81(3) ray powder diffraction (McMillan er al. 1980) for rubid- b 12.967(3) 13.01(3) 7.243(3) 7.18(4) ium feldspar of similar composition (Table 3). Neverthe- (') e0 56(1) " s0 3(1) less, within the accuracy of the method, the TEM cell p 116 08(2) 115 7(3) "v parametersare reasonably close to those expected for a 87.93(1) 88 2(1) triclinic rubidium feldspar. The coherency of structure . McMillanet al. (1980) parameters from well-ordered microcline to rubicline also supports t This work.The cell agreewithin 2 standarddeviations TEERSTRA ET AL: RUBICLINE. A NEW FELDSPAR r339 pairs of feldspars.The exsolution amounts to small-scale partmentof Earth and PlanetarySciences, University of New Mexico We migration of alkalis in an aluminosilcateframework that thank Ecole des Mines, Paris, for donation of pollucite and feldspar specrmens suffers only very minor distortion and preservesperfect three-dimensional coherence between the exsolved RnpnnnNcnscrrED phases.It is, however, conceivable that the present-day (1846) textures reflect a degree of fluid-induced Breithaupt,J F.A X NeueMineralien 4 and 5 KastorundPollux modification, PoggendorffsAnnaien, 99, 1391 1400 which may also be suggestedby the silica-excesscom- Brown, WL and Parsons,I (1989) Alkaii feldspars:Ordering rates,phase position of some of the rubicline and by its local asso- transformationsand behaviourdigrams for igneousrocks Mineralogical ciation with quartz. Magazine,53,25 42 (1994) The processesof (Al,Si)-ordering and exsolution are Giletti, B J. Isotopic equilibrium/disequilibrium and diffusion ki- netics in feldspars In I Parsons,Ed, Feldsparsand Their Reactions, catalyzedby HrO; however, a nSioO, substitution,which p. 351-382 NATO ASI SeriesC, Mathematicaland PhysicalSciences, may allow partial occupancy of the feldspar M-site by Reidel.Boston H,O, has been recently identified and shown to be wide- McMillan, PE, Brown, W.L, and Openshaw,RE (1980) The unit-cell spread in potassium feldspar from the hydrous environ- parametersof an ordered K-Rb alkali feldspar series American Min- ment of granitic pegmatites (Teertstraet eralogist, 65, 458-464. al. 1998a). At Orlandi, P and Scortecci, PB (1985) Minerals of the Elba pegmatites Elba, substitution of nSi"O, in microcline does not ex- Mineralogical Record, 16, 353-363 ceed the "detection limit" of I mol%o;however, up to 4 Pezzotta,F, Hawthorne, FC , Cooper, M A , and Teertstra,D K (1996) molTo nSioO8 occurs in rubicline. The primary feldspar Fibrous foitite from San Piero in Campo, Elba, Italy CanadianMin- may have been hydrous and contained more Si than is eralogist,34,741-144 Pouchou, J L and Pichoir, F (1985) PAP (phi-rho-Z) procedurefor im- now present in microcline. Typically, the nSioO, com- proved quantitative microanalysis In J T. Armstrong Ed , Microbeam ponent occurs in all generations of feldspar at a given Analysis, p 104 106 San FranciscoPress, Califomia. locality, or is absentfrom all of them. The Elba feldspars Teenstra,D K., Lahti, S I, Alviola. R , and Cern)t.P (1993) Pollucite and do not follow this pattern; quartz may have exsolved dur- its alterationin Finnish pegmatites Geological Survey of Finland, Bul- ing the structural microcline-forming phase letin 368, 1-39 transition. Teertstra,D K, Cernj, P, and Hawthorne,FC- (1997) Rubidium-rich feld- Rubicline may have formed by diffusion of Rb* and nu- sparsin a granitic pegmatitefrom the Kola Peninsula,Russia Canadian cleation in the nSioOr-rich microcline surroundinggrains Mineralogist,35, 12'77-1281. of quartz. The occurrenceof rubicline surroundinggrains Teefistra,DK, Cernj, P, and Hawthorne,FC (1998a)Rubidium feld- of quartz is also unique to this locality. sparsin granitic pegmatites CanadianMineralogist, in press. Teertstra,D K., Hawthorne,FC.. and Cern!. P (1998b)Identification of AcrNowr-BocMENTs normal and anomalouscomposition of mineralsby electron-microprobe analysis: K-rich feldspar as a case study Canadian Mineralogist, 36, We thank W.L Brown and H Kroll for their commentson this manu- 81 96 script This work was suppoted by NSERC Operating,Major Installation, Equipment, and InfrastructureGrants to PC and EC H. D K T. was sup- M,qNuscnrprRECETvED Dncevaen 6. 1997 poted by a University of Manitoba Duff Roblin Fellowship The TEM MaNuscnrpr ACCEPTEDJuNs 22, 1998 work was done in the Electron Microbeam Analvsis Facilitv in the De- Prpen uaNoren gv NeNcv L Ross