Cana dian M ineralo gis t, Vol. lN pp.262-2f8 (1974) INFRARED SPECTROSCOPICINVEgTIGATION OF THE STRUCTUREOF SOME NATURAT ARSENATESAND THE NATURE OF H-BONDS IN THEIR STRUCTURES V. I. SUMIN or PORTILLA Conseio de RecrtrsosNafurales no Renouabla Nihos Heroesg Dr. Nauarro,M*ico D.F., Madqw ABsrrAcr to t 2 cm-1 in the 400-2000cm-1 region and The IR-spectra of the arsenates durangite, mime- to i 5 cm-l in the 2000-3800 cm-1 region. tite, conichalcitg austinitg olivenite and adamite cor- Deuteration of the samples was carried out by roborate the character of the symmetry of the arsenate means ol a hermetic oven (Nasedkin 1962). Of groups in thce minerals. The changes in the sym- the minerals studied, the only one completely metry of the point group of the anion (AsOn;s- It deuteratedwas conichalcite and the substitution the result of a coordinative bond with metal atoms. took about 20 hours. The hydroxyl group in conichalcitg austinite, olivenite All the minerals investigatedare of the isolated and adamite is linked through H-bonds and coordi- nated with metal atoms. The frequencies of stretching tetrahedron type. Two, druangite and mimetite, vibrations of the hydroxyl group indicate that there have almost regular tetrahedrons and lack hy- are H-bonds with variable strengths between the OH droxyl groups; the others (conichalcitg austinite, groups and the oxygen in the O-AsO3 groups. The olivenite, and adamite) possesshydroxyl groups strength of the H-bond falls in the following order : and their tetrahedral (AsOr) a- structure is some- conichalcite > austinit€ > olivenite ) adamite. what deformed. In the minerals investigated, A band at 740 crn-t in the spectra of Zn-bearing three kinds of interactions are e)rpected: 1) minerals such as adamite and austinite indicates thar donor-acceptor,2) H-bond and 3) electrostatic. the oxygen group partly atom of tlre arsenate is The strongest type of interaction is probably the ptotonated by the hydrogen atom of the OH group. This confirms that there is interaction betwen the the donor-acceptor one, because most of the anion and the cation (coordination) and that OAs metallic cations are capable of forming strong (OH) groups may exist in the structure. The absence complexes and because the anions are strong of bands in the 1500-1700cm-r region mnfirms the electrodonors.These interactions result in changes absence of water molecules in t"heseminerals. in the structure and in t}le chemical bonds and The following structural formulae are proposed : consequently the IR-spectra of these minerals conichalcite, Ca(CuOtI) (AsOa) ; austinite, Ca(Zn are different from the spectra of free arsenates OH)(AsO4) ; olivenite, Cu(CuOlI)(AsOa) ; and and free OH groups. adamitg Zn(ZnOH) (AsOa). According to Mayanlz (1960) and Siebert (1953, 1954, 1966), the tetrahe&al qrmmetry, Imroouc:row 7a, of the arsenateion is indicated by four ab- Durangite, NaAl(AsOa)F, mimetitg Pbs sorption bands of which only two appear in the (fuO4)icl, adamite, Znz(AsO*)OH, olivenite, IR-spectra and have the theoretical values Cu2(AsOa)OH, conichalcite, CaCu(AsOa)OH vr(F2) : 887 cm-1 and ,tE(F) :463 cm-1. and austinite, CaZn(AsOa)OH have scarcely The four bands appear in Raman spectra (Gupta been studied by IR-spectroscopy. Adler (1961) 1948). When coordination of the arsenate ion and Moenke (1962) include the lR-spectra of with metals occurs, as in donor-acceptor bonds, some of these minerals, but they do not discuss then the sJrmmetry of the arsenate group falls them in detail and the spectra are used mainly from C,(Cg,) to C"o (Fig. 1). A redistribution for mineral identification. As r-rav data are available for this group of minerali, their IR- spectra, stmctures, and the nature of their chemi- cal bonds may be correlated and analyzed. 1?1 A1l samples that were used were identified by Ys optical methods and by r-ray analysis. The IR.- 4,& ^" 'Jr(At) 'le (El rlr( Fe) 'J.( F2l spectra registered were with an lR-spectrophoto. Js(XY) dc(Ym Jd(xyl 8a(yxyf meter (Zeiss, GDR) in the 400-3800 cm-l re- Flg. I gion. The samples were prepard. as emulsions Frc. 1. Forms of vibration for the (Asor;s- ,uo.- in nuiol. The spectra remrded were reproducible hedral group. 262 Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/12/4/262/4004688/262_12_4_cm.pdf by guest on 30 September 2021 INFPARED SPECTROSCOPIC NWESTIGATION OF NAT1IRAL ARSENATES 263 of the electronic cloud octurs which raises the hedron, giving this mineral a Go sfrnmetry. sfength of some of the fu-O bonds, and de. This agreeswith the correlation scheme (Table creases the strength of other bonds. This also 1) and with the x-ray data. The arrangement results in the formation of new metal-orygen in the correlation table enables one to find the bonds. The lR-spectra permit us to follorv these symmetry of the ligand in the structure of the changes and to interpret their nature. Alt the minerals (Nakamoto 1966). The interatomic symmetry bands vr(Ar) and vr(E) are present. distancesare Pb-Cl : 3.16A and Pb-O: 2.564, As the degeneracy of the vg(Fz) and va(Fz) which are bigger than the sums of the ionic radii bands disappears,these split into two or tlree (2.654 and 2.164), and this fact indicates that componentsand the band of valency vibrations the Pb-O bond is mainly ionic. The ionic nature of the Me-O (Me : metal) bond appears.Other of the Pb-O and Pb-Cl bands is also confirmed bands related to the metal-arsenatesystem also by the absence of absorption bands in the 500- appear.The coordination of the hydroxyl groups 600 cm-1 region where the bands of stretching causes sharp changes in the IR-spectra. New vibrations of the Me-O bonds usually appear. bands related to deformation vibrations of the The IR-spectra of mimetite from Tsumeb, Me-OH bands appear in the 600-1200 cm-1 which has only traces of phosphate, show four region and bands related to stretching vibrations bands of medium intensity in the region of appear in the (Youxnevitch 300-900cm-l region valency vibrations of the phosphate group, that 1963,1970). is in the ,ta(Fz) region. These bands dr€ vga: 960 cm-l, lrab: 990 cm-1, lrac: 1010 cm-1 MrMsrrrE and a weak band vr(Ar) : 925 cm-r (Table 2). Mimetite, Pbo(AsOa)gCl, is an analogue of In the region of deformational vibrations, the apatite and is hexagonal,bipyramidal, 6/m, The v+(Fz) band is split in tlree bands: ! : 545 unit cell has lwo Pbo(AsOa)aClunits (Hendri&s Ctrl*1, i46 : 555 cm-r and Vac: 570 cm-l. 1932; Machatski 1950). The As forms a regular This kind of splitting indicates that the site- tetrahedron with four oxygen atoms, in which sfmmetry of the (POa)u- group in the correla- the As-O distance is 1.94. The tetrahedra are tion scheme must correspond to Cro symmetry, interlinked by Pb atoms. The Pb qtoms in the that is, to a symmetry lower than that of the octahedraresemble empty cylinders on the walls (AsOa)3- groups. of which are the Pb atoms, parallel to the c The IR-spectra of mimetite from Cumberland axis the Cl atoms are ; distributed directlv on show an intense band va(Fz) due to the phos- the c axis. Pb forms a continuous column in phate anion. A weak band at 1320 cm-1 is which each Pb atom coordinates g with oxygens. probably due to an overtone 2 va(POn)3- and These columns are linked together by the (AsO!)f- &trahedra. We studied two samplesof mimetite, one from tumeb, S.W. Afoica, with 1.93% PzOs and an- other from Cumberland, England, with lI/6 PzOs. The lR-spectra of both samples (Fig. 2) show a slight splitting in the region of stretching vibrations of the AsOa groups,that is, the vs(Fz) band. This splitting gives rise to two bands at 790 cm-1 and 820 cm-1. The vr(Ar) band appearsas a weak shoulder at 735 cm-1. In the region of deformation vibrations of the arsenate ion there is a weak splitting of the va(Fz) band into two bands at 415 cm-1 and 430 cm-1. From the lR-spectral data we conclude that there is a slight deformation of the (fuOa)s- tetra- TABLEI. CORRELATIONTABLE FOR GROUPS OF LOCALSYMI'IETRY IN TETMHEDMLANl0Ns (Nakamoto 1966) ta Frc. 2. Infrared spectra of mimetite and durangite: cz"ilil;'$ectfa (a) mimetite from Cumberlan4 England: (c) durangite from Durango, Mexico. Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/12/4/262/4004688/262_12_4_cm.pdf by guest on 30 September 2021 2M THE CANADIAN MINERATOGIST the 1650cm-1 band may be related to the over- cies of the fu-O vibrations are 100 cm-1 higher tone 2 v(AsOn;s-. than in mimetite (Table 2). The lR-spectra of durangite are considerably more complex than the spectra of mimetite (Fig. 2). In the region Dunalrcrrr of valency frequencie vs(F2), a very weak split- Durangite, NaAl(AsOa)R is a rare mineral. ting is observed and it gives the bands rlso: Its structure (Kokkoros 1938) is monoclinig 2/m" 860 cm-l and vga: 915 cm-1. In the v.(Ar) spacegroup Czn, Z: 4. The coordination num- region there is a shoulder at 790 cm-1. In the ber for Na is 7 and for A1, 6.