American Mineralogist, Volume 68, pages 623427, 1983

Chiavennite, CaMnBe2SisOr3(OH)2.2H2O,a new mineral from Chiavenna(Italy)

Mrnr,lla BouoI

Istituto di Mineralogia e Petrografia dell'Universitd di Bologna Piazza di Porta S. Donato l, 40127 Bologna, Italy

Wlllnvr L. GnIpprN

M iner alo gisk-Ge olo gisk M use um Universitetet i Oslo Sars' Gate 1, Oslo 5, Norway

VIrroruo Merrtotl

Via Keplero 5, 20126 Milano, ItalY

eNo ANNrseLe Motrnun Istituto di Mineralogia e Petrografia dell'Universitd di Roma Cittd Universitaria, 00185 Roma, Italy

Abstract

Chiavennite occurs as euhedral orange grains and as crusts coating beryl in the alpine pegmatites crossing the Chiavenna ultramafics in the Rhetic Alps of Italy. It is usually associatedwith bavenite and several other pegmatitic minerals. Chiavennite is optically biaxial, c:1.581(l) and 7 = 1.600(l),faintly pleochroicfrom colorlessor yellow (X) to yellow-orange (Z). The empirical formula derived from EDS microprobe analysis and separate analyses for BeO (optical spectrography)and H2O (TGA) is (Caos7Nao.05)>l 02 Mno.qz(Ber.qeAlool)>z.or(Si+.esAlo.rs)>s.oool2.63(OH)2 n'2'l6HzO or ideally CaMnBezSis Or3(OH)2'2H2O,as confirmed by the crystal structure determination.D(meas.) 2.64(l) g/cm3,D(calc.) 2.657g/cm3. The mineral is orthorhombic,P2tab, with a = 8.729(5\,b : 31.326(ll), c = 4.903(2)4, Z = 4. The most intense lines in the powder pattern (Gandolfi camera)are (as d, A, IlIs, hkt) 15.7(100)(020),4.15(30)(041), 3.93(30)(080), 3.82(30)(240), 3.28(75X201),2.903(100X251),1.944 (30X3.12.0). The genesisis relatedto late fluid activity within the pegmatite.

Introduction cross, subhorizontally, both the biotite-phengite-micro- cline augengneisses of the "lower Corbet series" (Tambd The western margin of the alpine Val Masino-Val nappe;Weber, 1966),and the Chiavennamafic/ultramafic Bregaglia(= Bergell) compositepluton in the Rhetic Alps Complex (Schmutz, 1976),a metamorphosedslab of the is crossed by swarms of pegmatites, related to the final oceaniccrust separatingthe Tambd from the Adula mass. crystallization stagesof the pluton, and most likely to the Thesepegmatites show azonal structure with grain size emplacement of the San Fedelino (: Novate) granite increasing from border to core and a concentration of body (Mottana et al., 1978).These pegmatites are late- large biotite flakes and small red garnet seamsalong the alpine in age(25-27 m.y., K-Ar on muscovite; Hanson e/ marginal zone. Although the mineral assemblageof the al., 1966\ and cross, completely undeformed, several pegmatitic zone is in general rather uniform (albite + units characterized by low pressure-high temperature quartz + microcline perthitetwhite mica) numerous ac- metamorphism up to the sillimanite zone (Wenk et a/., cessory minerals have been found, such as bavenite, 1974).The largest dikes occur on both sides of Valle di zircon, tourmaline (schorl), beryl (colorless or aquama' San Giacomo near the town of Chiavenna, Italy, and rine), arsenopyrite, bismuthinite, bismutite, gahnite, py- 0003-(M)vE3/05ffi-0621$02.00 623 624 BONDI ET AL.: CHIAVENNITE

with a calculated value of 2.657 g/cm3from unit-cell and chemical analysis data; for the theoretical composition the calculatedvalue is 2.632 glcm3. The mineral is insoluble in water, and in concentrated HCl, HNOr and HzSOa. Chiavennite is optically biaxial, but the sign and 2V could not be obtained. The refractive indices measuredat room temperaturein white light are d : 1.581(1)and 7 = 1.600(l).The pleochroicscheme, determined on the same flakesused for the measurementof indices,is: Xcolorless to pale yellow, Z yellow-orange. In thin section the mineral is colorless to yellow and orange-yellow.A sharp zoning with deep orange colors is observed near the marginal part of crystals, possibly due to increasediron content (see below). By grinding, such a zoned material produces a pale ochre powder.

X-ray crystallography The X-ray data have been obtained both by standard powder diffractometry on a slurry and by Gandolfi cam- era usinga clusterofa few smallgrains. The observedd-

Tablel X-ray powderditrraction data for chiavennite(Tanno, Chiavenna,Italy)

Fig. 1. Euhedralchiavennite crystal in vug. Maximumlength t*lr a. d)t r/r2 a tit a. d,)t rhr 0.4mm. ...G [email protected] ob 15.'l 1@b 15.663 15.'t 90 '1.432 040 7.8 n 7.83 40 pyrochlore, 140 5.84 20 5.829 5,83 11 rite, titanite, uraninite, aeschynite and lau- 011 4.85 4.844 montite.Recently one of us (V.M.) found orangecrusts of 031 4.45 5 4.438 unusual appea"rance coating beryl and usually accompa- u1 4.'t5 30 4.156 4.12 8 nied by bavenite.Later, euhedralorange crystals, up to o80 3Oc 3.916 1@ 0.4 mm in size,were 240 3,82 30 3.812 3.81 10b also found in the samelocality (Fig. 2A1 3.2@ 14 'lo l). Examination showedthis material to be a new mineral o.'lo.o 3.13 3.132 3.14 10 species here named chiavennite from the type locality 1.10.O 2.948 2.954 10 Chiavenna, frazione Tanno, provincia Sondrio, region 251 t181 2.903 1@ 2.892,2.887 2.892 Lombardy, 091 2.846 2.438 2.836 8 Italy. 261 5 The mineral has been approved by the Commissionon 191 2.7o.5 5 2.699 New Mineralsand Mineral Names,I.M.A., in September o.10.1 2.645 15 2.6h l98l , at the sametime as another submitted material from 3@,1.10,1 2.538 h 2.542,2.528 't2 Norway which proved to be indentical (Raade o32 2.394 2.387 et al., u2 2.347 10 2.340 1983,see following paper). Following the rule of priority 142 2.270 12 2.26 the name chiavennite is to be used. The type material o.'14.o,1.12.1 2,235 10 2.238.2.224 2.227 from Chiavennais depositedin the collection of Museo di 1.'t4.O 2.111 2.167 2.161 't.991 qoi Mineralogia,Universitd di Roma, under 2.14,O b I specimennum- 3.12.O 1.944 30 1.943 ber 24342. 1.10.2 1.884 5 1.885

Physical properties 2.14.1 1.833 5 1.844 352,4.'to.O 1.795 h 1 104 1 10a I tA< 't Chiavenniteis pale orange-yellowin hand specimen, o.18.'l .638 5 1.540 translucent,with a vitreousluster. Streak is white. Mohs' r indexecl on the basls of the orthorhonbic cell a = 8.729 hardnessis approximately 3. The crystals, although -- = pzagq rare b 31.326, I 4.9o3 i. (single crystal-diffracte and minute, have a flat pseudohexagonal meter, pers.com.V. Tazzoli). habit and show ? perfect "basal" candolfl cmera ('114.6 m diareter, Mn-flltered Fe-EargeE, cleavage. 120 hr) on three grains analyzed by microprobe, t The specific gravity, measured with a Westphal bal- dlffractometer paltern (N1-filtered cu-target. 1/2o29 nin-l) powder ance, is 2.64(l) g/cm3. This value compares favorably on analyzed for H2o and Beo. BONDI ET AL,: CHIAVENNITE 625

2 theta Cu/Ni

f ri 7 { { q .e,,+ \ l{ i \ Fig. 2. Diffractometer trace on powdered material (above) and intensitiesof the lines observedin the Gandolfi film on the grains Gelow). valuesand the relative intensities of the peaks, measured HzO) followed by a quick loss continuing up to 1000'C graphically on the diffractometer tracing and visually (correspondingtoa4.5-4.6wt.Voloss due mostly to OH). estimatedin the film. are listed in Table I and shown in The presenceof both OH and HzO is confirmed by the Figure 2. Due to the pronounced preferred orientation of infrared spectrum (Fig. a). chiavennite cleavage flakes, the two sets of intensities The broad bands at 3500-3400cm-r are due to ad- compaxerather poorly: in particular in the diffractometer sorbed water, and the sharp band at 1640cm-l to water pattern some reflections are enhancedas can usually be bendingvibrations. The band at 3590cm-r is attributed to seenin layer silicates. The X-ray structure determination the stretching vibrations of structural OH. Hydroxyl is of chiavennite, currently under way by Y. Tazzoli et al., confirmed also by the sharp absorption at 680 cm-l to be showed it to be orthorhombic, space group P21ab (sys- assigned to its vibrational motions. Other stretching tematicabsencesh0l: h = 2n + I and hl

Chemical composition 1 (wt. c ) (wt. $ ) Electron microprobe analyseswere carried out using a slo2 49.84 (49.39-5O,53) 52.5 Si n..r, LINK system model 860 energy-dispersive fs.oo analyzer, with o.3451 Arzo3 "'rV on-line matrix corrections by the ZAF-4 program, fitted 3,24 ( 2.96- 3.38) orvr o.oto on the ARL-EMXprobe of the Mineralogisk-Geologisk Iz.or n. d. 9,3 Be 1.e81I Museum, Oslo. The analysesare based on the following l4nO 11.10 (',|o,98-1'l.20) 12.9 Mn standards:Mn metal, Al2O3,wollastonite (for Ca and Si), CaO 9.77 ( 9.51-1O.02) l lli and jadeite (for Na). 1, Na o,29 ( O.15- O.45) O.3 Na o.os2I ", Four spot analyseson three grains (the same used for 2O Hzo 11.3 the Gandolfi powder pattern) were performed. The initial results are given in Table 2 column l. No other elements TotaI 100.'l within the sensitivity of the EDS microprobe could be detected: Fe, in particular, is absent in these grains. n.d.= not aleternlned 1. Microprobe analysls for elenents heavler than 1'l: Water was determined by TG analysis on a separate lnitial results, uncorrected for H^O and BeO. 10.68 mg aliquot of another specimen from the same 2. Preferred conblned analysis. wlth fhe previous mlqro- probe dala recalculated using the c-faclors of Albee outcrop. The total weight loss on heatingis ll.3Vo (Fie. 3) and Ray ('1970) for Beo and H-O. Be dleternlned spectro- z - and occurs in two steps: a continuous and steady loss up graphicallyr HrO by TGA. to 540-650'C (corresponding to a 6.15 wt.Vo of mostly 3. Nurilrer of aton; on the baals of nlne catlons. 626 BONDI ET AL,: CHIAVENNITE

(G. Bocchi, pers. comm.). The total BeO content is 9.3+0.2 wt.Vo;that of FeO is 1.80+0.10wt.%. Sincethe microprobe analysisof the grains usedfor the X-ray work had shown no Fe, slight chemical differencesbetween the two samples studied is to be inferred. Such a composi- tional difference is irrelevant as far as the Be content is concerned, but it points out that Fe, although not essen- tial to the chiavennite formula, is significant as a possible substitute for Mn (up to almost 20Voby weight). The observedvariations in the physical properties, and partic- ularly in the color ofchiavennite, thus find their explana- tion. TC 'i^^rr", light elements, eie. l. rrrerm"lb"h;;;", t ,n On the basis of the data obtained for the microprobe analysis was recalculated using the em- Dupont951, prog. rate 20'Clmin,"nr"*""n"time constant I ".sec). pirical a-factors of Albee and Ray (1970).The results are given in Table 2, column 2. The empirical formula, The IR spectrumof chiavennite is particularly complex calculated on the basis of a total cation content of 9 in the region from 1200to 200 cm-r. A complete assign- is: (Ca6.e7Naqos)>r.ozMno.sz(Ber.qsAlo.03)>2.0r(Sie,os ment has not been attempted, in view of the as yet 416.35)25.66O rz.er(OH)z y'2.l6}{zO. The empirical formu- insufficient structural information. However, the major la is in good agreementwith the theoreticalformula CaMn absorption bands for complex silicate ions and for beril- BezSisOn(OH)z'2H2Oinferred from the crystal structure latescan be easily recognized.The overall IR spectrumof determination (Y. Tazzoli et al., personal communica- chiavenniteresembles that of bavenite, but it is consider- tion). The small substitution of Al for Si is normal in ably more complex in detail. complex silicates, as is the substitution of Fe2* and Mg A portion of the sampleused for TG and IR determina- for Mn suggestedby the spectrographicanalysis. tions was checked by qualitative spectrographicanalysis using the Hilger E-478 quartz spectrograph. The major Conditions of formation componentswere found to be Be, Si, Al, Mn, and Ca; Mg Chiavennite is a late phase produced by the migration and Fe are presentin traces, and B, Li, and P are absent. of fluid through the pegmatite, leading to reaction with A quantitative spectrographicdetermination for Be and preexisting beryl crystals. Chiavennite forms crusts over Fe was performed on the same material, using an espe- beryl as well as occasional pseudomorphs (hexagonal cially developed method and synthetic mixes as stan- prisms consistingnow of chiavennite * bavenite).Baven- dards. Spectrographically pure reagents in the required ite, Ca4Al2Be2KOH)2SieO26l,is always present together amountswere added with graphite and SrCO3as diluents with chiavennite as the alteration product of beryl. The

^q s: q)6 E

Fig. 4. The infrared spectrum of chiavennite (Perkin-Elmer mod. 580/8 IR spectrograph,sample weight 0.7 mg, l: I l0 CsBr disk, scan time l0 min). BONDI ET AL.: CHIAVENNITE 627 fluids therefore must have been particularly rich in Ca (in to Y . Tazzoli,Istituto di Mineralogia, University of Pavia, Italy, excess of Mn) while Al and Si are probably derived for the crystal structure determination. Dr. Akira Kato, chair- directly from the decompositionof beryl. The presenceof man of the Commission on New Minerals and Mineral Names. these elementsin the fluids may be related to a late stage I.M.A., tactfully handled the case and ofiered valuable advice. or serpentinization of the ultramafic rocks into which The manuscript was reviewed by R. Mezzetti, L. Morten, G. Raade, pegmatitesare intruded. and M. Fornaseri and benefitedby the careful editing of M. Ross. Financial support was given by the Italian National The geological history and evolution of the Chiavenna Research Council, under grants CTE0.02608.05(Rome) and ultramafics is rather complex (Schmutz, 1976). They CTE0.01022.05(Bologna). started as mantle-derived peridotites intruded as a cold slab into the oceanicsequence paleo-Adula separatingthe References from the paleo-Tambd continental masses. They were then retrogradedto serpentinites,probably in the oceanic Albee, A. L. and Ray, L. (1970)Correction factors for electron- environment, but recrystallized to an anhydrous assem- probe microanalysis of silicates, oxides, carbonates, phos- blageconsisting mostly of olivine crystals with granoblas- phatesand sulphates.Analytical Chemistry, 42, l4OE-1414. tic polygonal texture ("olivinite") during the Lepontine Hanson,G. N., El Tahlawi,M. R., and Weber,W. (1966)K-Ar thermal maximum (38+3 m.y.) of the Alpine metamor- and Rb-Srages ofpegmatites in the South Central Alps. Earth phism. and Planetary ScienceLetters, l, 407413. Mottana, Annibale, Morten, Lauro, and Brunfelt, A. O. (197E) The olivinite now shows an incipient transformation to Dishibuzione delle Terre Rare nel Massiccio Val Masino-Val serpentinite (post-Lepontine), particularly along shear- Bregaglia(Alpi Centrali). Rendiconti della Societi Italiana di planes and zones that include small rodingite bands. We Mineralogia e Petrologia 34, 485497. suggestthat the migration of Ca+Mn+HzO ffuids into the Raade,G. Amli, Reidar,Mladeck, M. H., Din V. K., Larsen, pegmatites,obviously later than their emplacementat 25- A. O. and Asheim, Arne (1983) Chiavennite from syenite 27 m.y., may be simultaneouswith the formation of these pegmatitesin the Oslo Region, Norway, American Mineral- rodingites. Therefore it may indicate a very final alpine ogist,68, 62E-633. mineralization phase related to the overall uplift of the Schmutz, H. J. (1976) Der Mafitit-Ultramafitit-Komplex zwi- area. schen Chiavennaund Val Bondasca.Beikege ftir geologische Karte der Schweiz. Neue Folge. 149, The beryl pseudomorphsare composed of bavenite as l:73. Weber, Werner (1966) Ziir Geologie zwischen Chiavenna und the inner core and ofchiavennite as the outer layer. This Mesocco. Promotionsarbeit Nr. 3E0EEidgenossische Tech- suggests that the chiavennite formed even later than nische Hochschule, Ziirich, l-24E. bavenite as the last phase of the entire paragenetic Wenk, H. R., Wenk, Eduard, and Wallace, J. H. (1974)Meta- evolution. This is in full agreementwith the low tempera- morphic mineral assemblagesin pelitic rocks of the Bergell ture formation of chiavennite as implied from the high Alps. Schweizerische Mineralogische und Petrographische content of molecular water in this phase. Mitteilungen, 54, 507-554.

Acknowledgments Our bestthanks are due to G. Bocchifor the carefulspectro- Manuscript received,February 10, l9E2; graphicdetermination of BeO,to L. Poppi,for TG analysis,and acceptedfor publication, November 4, 1982.