American Mineralogist, Volume 66, pages 148-153, IggI

The structure of bayldonite:chemical analysis, differential thermal analysis,and IR spectroscopy

VerBNrrNn SuuiN DE PoRTTLLA Instituto de Geofnica, UNAM Mbxico 20. D.F.

MeNuer PoRTTLLA Quevnoo Institut o P o lit i cnic o N acional Mdxico. D.F.

AND VICToR I. STEPANoV

I MGRE, Moscow, U. S.S.R.

Abstract

Chemical analysis,differential thermal analysis,and IR spectroscopyshow that the struc- ture of bayldonite lacks water molecules and contains OH groups coordinated with Cu atoms,as well as acid arsenateanions (HoAsor)'z-. The H bondsformed by the hydroxyls of the (HOAsOr)2- groups are very short and strong,with the possibleappearance of a tunnel effect for the hydrogen atom. The formula CurpbO(HOAsOr)r(OH), is proposed.

Introduction 77o!l; a:b:c: 0.843:l:l.16l.The crystalwas opti- cally positive with la;ge 2V. The refractive indices Bayldonite is a Pb-Cu arsenatewhose structur€ were:y : 1.99-10.01,B : 1.98,a: 1.94(Guillemin, has not been established.Most samplescome from 1956).The calculatedvalue (-) 2V: 52oshows that Tsumebin SW Africa, whereit is found in the oxida- at least the value of B must be wrong. The resultsof tion zone of this deposit (Guillemin, 1956;Frondel Larsen:y : 1.99,B : 1.97,a: 1.95give -2V : 89", and Ito, 1957).Bayldonite has also been found in which could be a tolerable error. France: Verriere, Ardillats, R6he, Rebasse,Ceilhes, Bayldonite can be synthesizedat l80oC and a pH Herault (Guillemin, 1956);England: uranium mine, between5 and 8. At this temperatureand at a pH of Grampound Road, Cornwall (Guillemin, 1956);and 7, bayldonite is formed from schultenite(HPbAsOn) the USSR,Dzeskazghan (Satpaeva, 196l). This min- in the presenceof Cu(NO.)r, or from a solution of eral was describedby Church (1856)from St. Day, HrAsOo, malachite, and cerussite.When the pH is Cornwall. lowered to 34, [CuPb(AsO,XOH)]is formed Guillemin (1956)compared the chemical analysis (Guillemin, 1956). of synthetic and natural bayldonite, and gave a Cu:Pb relation of 3:l as reported previously by Mineralogical studies Church (Table l). However, the agreementbetween Bayldonite used in this study occursin the oxida- the proposedformula and the analysisis unsatisfac- tion zone of a metasomatic Pb-Zn deposit in Kay- tory, especiallythe excessof water moleculesfrom rakty in central Kazakhztan It was found in the up- the analysis. The optical properties of bayldonite per layers of barite lenses of considerable size (Larsen, 1921,p.45) showedthat this has embedded in sandstone.Bayldonite is formed by monoclinic symmetry.This wasconfirmed by Guille- pseudomorphicsubstitution of cerussite,resulting in min (1956),using X-ray methodson a samplefrom irregular accumulations in fine-grained malachite Cap Garonne. The parametersof the unit cell are:a and azurite. Concentric accumuliationsof pure bayl- : 5.03+0.02,b : 5.97+-0.05,c : 6.93+0.02A,B : donite reacha diameterof -7 cm. Occasionallybayl- 0003-oo4x / 8| / 0I 02-0I 48$02.00 SUM|N DE PORTILLA ET AL: STRUCTURE OF BAYLDONITE 149 donite is found on thick spherolite-likecrusts formed choidal fractures.The separationof Pb and Cu was in fractures in the barite. Concretions of bayl- problematic due to the sensitivity to pH. The results donite (up to 3 cm in diameter) in clay layers inter- of chemical analysisare given in Table l. bedded with sandstonewere also observed.In these concretionsconcentric color patternsoccur due to al- IR spectroscopy teration in the compactnessof the material. Bayl- The IR spectrawere recorded on a UR-20 Zeiss donite has a characteristicapple-green color with a infrared spectrophotometer(DDR). The samples yellowish hue. Dense aggregationshave an intense were prepared as emulsionsin nujol on NaCl win- oily luster and show conchoidal . The less dows. Deuteration of the sampleswas carried out in dcnse type is dull with an irregular fracture. The an autoclave at pressure -15 atm and at temper- dense bayldonite often shows cavities with small atures from 200o up to 500"C for a minimum of 48 crystals of anglesite and azurite. Both types of bayl- hours. Two sampleswere used, one from Tsumeb donite usually intergrow with , PbFe3- and another from Kayrakty. (SO)(AsO.)(OH).. Microscopicexamination of grain immersion mounts and thin sectionsof bayldonite D iffer ent ia I thermal analys is showsa microspheroliticstructure of bayldonite ag- gregates.The densetype is homogeneousand some- DTA was carried out in a Paulik Erdel rneo-58 times small inclusions of anglesitegrowing on DR analyzer.The samplesweighed 0.4 g and the rate spherolitesof bayldonite may be observed,while the of heatingwas 5oClmin. lessdense is more heterggenous. Results Chemicalanalysis The data of the chemical analysis reported in The microchemical analyseswere carried out in Table I were used to calculate two empirical for- the Institute of Geology of Mineral Deposits,Miner- mulas for bayldonite. alogy, Petrography,and Geochemistryof the Acad- (l) Bayldonite from KayraktY: emy of Sciencesof the USSR. Samples used for chemicalanalysis were apple-greenand showedcon- Cu. -Pb, *(AsOo),nr(POo)o or(OH), or'0.74Hro

Table l. Chemical analysis of bayldonite

I 2 2 3 3 4 4

Relation Relation Relation of atomic of atomic of atomic Oxides nrr-hfi+i6c weight % Weight Quantities Weight % Quantities weight ?

CUO 31.61 31.70 2.996 30.88 2.968 32.5 2.98 Pbo 30.30 32.60 1.004 30.13 1.032 32. L 1.049 As^O- 3L.20 30.09 1.968 37.76 2.LI3 JI. f, 2 z) DO 0.20 o.02L z1 tt^o+ 4.89 4.20 3.504 4.58 3.887 3.00 3 so- 1.00 J CaO 2.65 z5

100,00 99.79 100.00 99.8 q, 5.24 Lt

1.- Cu- Pbo (HASO,)^ (Calculated) 2.- Kafrakty, cenLr1I Kazakhstan, USSR 3.- St. Dayf Corni^/all (Church, 1865) Cit, after Dana's Svstem of Min., 7th ed. 4.- Synthetic Bayldonite (cuillemin, 1955), 150 SUMIN DE PORTILLA ET AL: STRT]CTURE OF BAYLDONITE

600 700 1100 1300 1500 1700 1900 2100 2300 2500 2700 2900 3100 3300 3500

o o z. 9 F o- E o !n @

Fig. I IR spectra of bayldonite (in nujol): (a) Tsumeb; (b, c) Kayrakty. * : bands of nujol

(2) Bayldonite from Cornwall: (Fig. I, Table 2) in the zr(Tr) region of the arsenate anion. This band is split into four components: 780 Cur rrPb,o.(AsOo), ,,(OH), ur.l.l lHrO cm-', 815 cm-', 840 cm-, and 870 cm-'. In the defor- For the synthetic compound we obtained the for- mation vibrations region zo(Tr), the same kind of mula: splitting appears with three maxima: 400 cm-', 440 cm-', and 475 cm-t. The z,(Ar) band is very sharp Cu, nrPb,or(AsOo), oo(OH)2 *.0.4j HrO and intense. The 1060 cm-', 1090 cm-', and ll20 From these results we conclude that the formula cm-' do not shift after deuteration (Fig. 2) and are Cu.Pb(AsOo)r.(OH), cannot be correct. The two em- probably due to zr(Tr) vibration of the phosphate an- piric formulas suggest that bayldonite contains a wa- ion which is present in bayldonite. The z,(A,) band ter molecule: Cu.Pb(AsO"),(OH),.HrO. of the phosphate anion also appears at970 cm-'as a The IR spectra of the samples of bayldonite from weakly defined shoulder. In the Me-O (Me : metal) Tsumeb and Kayrakty both show an intense band stretching vibration region only Cu-O vibrations are

Table 2. Frequencies of absorption bands in the IR spectra of bayldonite and tentative explanation of the absorption bands

?- -3 tr t- Aso4 \ or.f I -tI Cu-OH ,lOH...O 2v 5:' L ["", oo ] OE

cu-o 6oH Byaldonite from Tsumeb J. ta, I )3(r2) )4 (r2) ) /m \ \ r- r and Kayrakty ' t \r(ar) V?\rrl V/\ftl ABCD

I )U 780 400 970 1060 562 700- NNFP s OOtsO6 815 440 1090 18s 8oo 3 Freguencies, 840 475 -1 r720 cm 870 (s40) u O

( ) Numbers in parentheses represent frequencies of absorption bands of deuterated samples. SUMIN DE PORTILLA ET AL: STRI]CTURE OF BAYLDONITE l5l

r000 12C0 1,100 1600 )800 240A 2600 2800 3000 1200 3400 I600

o o z

t- a- E. o a CD

* Fig. 2. IR spectra of deuterated sample of bayldonite from Kayrakty (a, b different concentration of bayldonite in nujol) : bands of nujol. expected, due to the covalent nature of this bond. Discussion The vibrations of the Pb-O bond do not appear in The structure of bayldonite is unknown. From the predominantly the IR spectra because of its ionic chemical analysis of bayldonite from Kayrakty we character. Two sharp bands at 562 cm-' and 585 conclude that the empirical formula Cu.Pb cm-' occur in the IR spectra of bayldonite. Analo- (AsO.),(OH), given by Guillemin (1956) lacks two gous bands are present in the spectra ofolivenite and hydrogen atoms and an oxygen atom. The search for conichalcite (Sumin de Portilla, 1974), and, euchroite a possible water molecule in the structure of bayl- and liroconite (Sumin de Portilla, unpublished re- donite is important in order to establish the correct sults). These bands are related to vibrations of the chemical formula. The IR data were mainly used to Cu-O bonds. The zo(Tr) band for the phosphate an- answer this question. The splitting of the z.(T') and ion does not appear in the spectrum. A band with zo(Tr) bands in three components and the appearance very low intensity occurs at 1650 cm-' in the defor- of a z,(A,) band in the spectra of bayldonite clearly mation vibration region of water. However, this band indicate that the arsenate anion tetrahedra are does not shift after deuteration, which makes it strongly deformed and show the presence of a low highly improbable that this band should be associ- symmetry, Cr" or lower (Mzyantz, 1960). The same ated with the presence of water molecules. type of splitting is observed in the phosphate anion Several bands lie in the region of stretching vibra- bands. The z,(A,) band is probably related to the tions ofthe hydroxyl groups: an intense band occur- coordination of the arsenate anion with an atom of ring as a shoulder at ll90 cm-', a sharp and intense Cu or to the presenceofan acid arsenatein the struc- band at 2000 cm-', and two wide and intense bands ar 2600 cm-' and 3200 cm-'. Comparison of these bands with those for phosphates, arsenates, and sili- cates indicate that they are the A, B, C, and D bands of OH groups coordinated with the As atoms of the arsenate anion (Blinc and Hadzi, 1958; Hadzi, 1965; Ryskin et al.,1960;' Ryskin and Stavitskaya, 1960). The DTA and DTG curves (Fig. 3) show a double endothermic effect at 525" and 599'C. The decom- position of the mineral begins at 470"C, as can be seen from the DTG curve. Guillemin (1956) states that this decomposition occurs between 450" and 510"C. The observed weight loss in the 470' to 620'C interval is 2.75Vo,a value slightly higher than that of a single water molecule in the empirical for- mula 3CuO'PbO'As'O''2HrO. The endothermic ef- fect at 780'C is due to the melting of bayldonite. The loss from 470" to 780oC is 4.l2%o,which is overall Fig. 3. Thermogravimetric (TG), differential thermal (DTA), slightly less than the weight of water in the empirical and differential thermogravimetric (DTG) curves of bayldonite formula. from Kavraktv t52 SUMIN DE PORTILLA ET AL: STRUCTURE OF BAYLDONITE

' ture (or both). The 562 cm and 585 cm-' bands are tice. The 1650 cm-' band has a very low intensity and due to stretching vibrations of the CuO bond. The could be an overtone of the zr(Tr) band, i.e., 2v same bands appear in the spectra of other arsenates (AsOo). The spectra of anhydrous arsenates, r.e., which contain a Cu-O bond: olivenite (545 olivenite, adamite, durangite, and mimetite show this conichalcite (573 cm-') (Sumin de Portilla, 1974),"--'), eu- low-intensity band (Sumin de Portilla, 1974), which chroite (535 cm-'), and liroconite (520 cm-') (Sumin confiflns the absence of water molecules in the struc- de Portilla, unpublished results). Note that in olive- ture of bayldonite. The two stages in the weight loss nite, euchroite, and liroconite, where the vibration seen in the DTA curves may be understood if there bands of Cu-O bonds occur between 520 cm-r and are two types of Cu groups, Cu-OH and HO-AsOr, 545 cm-', the Cu atom is in octahedral coordination with different attachment energies. This agrees with whereas in conichalcite this band occurs at 575 cm-|, the results obtained from IR spectra, which suggest with Cu in tetrahedral coordination (Qurashi and the presenceofboth types ofgroups. Barnes, 1963). We therefore conclude that Cu in The 2000 cm-' band is very intense and sharp in bayldonite has a low symmetry coordination since the the IR spectra of bayldonite. Analogous bands have frequencies of these bands are very high, 565-595 not been observed in the IR spectra of other natural cm-'. [t could be a tetragonal bipyramid as in con- arsenates.Blinc and Hadzi (1958) observed sharp, in- ichalcite. tense bands in the 2000 cm-' region in the IR spectra The zo(Tr) band of the phosphate anion, which of acidic phosphates and arsenateswith very short H- should lie in the same region as the Cu-O vibration bonds, and concluded that these bands are due to a bands and does not appear in the spectrum, has tunnel effect of the hydrogen atom in hydrogen probably avery low intensity and could be hidden by bonds when the length of the hydrogen bond is less the intense Cu-O vibration bands. The low intensity than 2.44. We consider that the hydrogen bonds in of the zr(Tr) band of the phosphate anion seems to bayldonite could be short enough to present this tun- suppoft this hypothesis. nel effect and that the 2000 cm-' in the IR spectra The 6OH bands of Cu-OH probably overlap with could be due to this effect. the very intense z.(Tr) bands of the arsenate and phosphate anions. However, a 540 cm-' band ap- Conclusions pears in the spectra of deuterated bayldonite, which The IR and DTA data indicate that the structure shifts from the 700 cm-' to the 800 cm-' region (tak- of bayldonite lacks water molecules, and that there ing the coefficient of shift as 1.35). This band is prob- are OH groups attached to Cu as well as As atoms. ably due to 6OH vibrations of Cu-OH. The crystal lattice has acidic arsenate groups that Four wide intense absorption bands in the IR form very strong H-bonds, which could be accom- spectrum of bayldonite, i.e., the 3200 cm-', 2600 panied by a tunnel effect of hydrogen atoms in these cm-', 2000 cm-', and ll90 cm-' bands, indicate the bonds. Based on the chemical analyses and the IR presence of acidic (HOAsOr)'z- groups in the crystal and DTA data, we propose the formula lattice. Analogous bands are characteristic in the Cu.PbO(HOAsO,),'(OH), for bayldonite. spectra of 0 : XOH groups (X : P, As, Se or S). These bands occur in the 2800-2400 cm-' (A), 2850- ' References 1900 cm (B), 1720-1500 cm-' (C), and 1300-1000 Blinc,R. andD. Hadzi(1958) The infrared spectra of someferro- cm-' (D) regions (Blinc and Hadzi, 1958; Hadzi, electric compounds with short hydrogen bonds. Mol. Phys., I, 1965). These bands could be due to the presence of 38l-405. OH groups, and they shift after deuteration (Blinc Church, A. H. (1865) [Bayldonite]. Chem. Soc., 18,259-265. and,Hadzi, 1958). The IR spectra of acid germanates, Frondel, C. and J. Ito (1957) Geochemistry of germanium in the phosphates, silicates, and carbonates also show these oxidation zone of the Tsumeb mine, South-West Afica. Am. Mineral., 42,'143-753 four characteristic bands (Stavitskaya and Ryskin, Guillemin, C. (1956) Contribution ii la min6ralogie des arseniates, 196l; Ryskin et al., 196O1'Ryskin and Stavitskaya, phosphates et vanadates de cuivre (1, ll) Bull. Soc. fr. Miniral. 1960). The available data seem to indicate that these Cristallogr., 79,'l -95. four bands are characteristic Hadzi, D. (1965) Infrared spectra of strongly hydrogen bonded of compounds that have : OH groups bonded to electronegative atorns and systems. Pure and Applied Chem., 11,415453. Larsen, E. S. (1921) The microscopic determination of the non- linked by strong hydrogen bonds. If these bands are opaque . Bull. U.S. Geol. Sum. 679. absent acidic anions such as (HOAsOr)'z-, Mayantz, L. S. (1960) Theory and calculation of vibrations in (HOPO3)'z , and (HOSOT)'- occur in the crystal lat- molecules. Dokl. Akad. Nazft SSSR, 329-348. SUMIN DE PORTILLA ET AL: S'I'RUCTUREOF BAYLDONITE ls3

Qurashi, M. M. and W. H. Barnes(1963) The structure of con- Stavitskaya,G. I. and Ya. I. Ryskin (1961)Hydrogen bond in acid ichalcite.Can. Mineral.,7,56l-571, germanates.Opt. Spektr.,10,t43-347. Ryskin, Ya. I. and G. P, Stavitskaya(1960) Spectroscopical inves- Sumin de Portilla, V. I. (1974) Infrared spectroscopicinvestiga- tigation of H bonds in acid silicates and phosphates. Opt. tions of the structufe of somenatural arsenatesand the nature Spektr.,8,606-613. of H-bonds in their structures.Can. Mineral., 12,262-268.

of hydrated silicates.,1. Neorg. Khim., /'2,2'127-n34. Satpaeva,T. A. (1961)Mineralogical compositionof the mineral oresof th€ Dzheskazgandeposits, Great Dzeskazgan.Geologya i Manuscriptreceived, September 10, 1978; metallogenyaAtma-Ata, l'15. acceptedfor publication,May 5' 1980.