MINERALOGICAL MAGAZINE, MARCH 1983, VOL. 47, PP. 51 7 Infrared spectroscopic analysis of the olivenite- adamite series, and of phosphate substitution in olivenite R. S. W. BRAITHWAITE Chemistry Department, University of Manchester Institute of Science and Technology, Manchester, M60 1QD ABSTRACT. Infrared spectroscopy affords a rapid and Inspection of the infrared spectra of a number of easy method of estimating the position of a mineral in the natural samples suggested that infrared spectro- olivenite-adamite solid solution series, and of estimating scopy might afford a rapid and easy method for the amount of phosphate substitution in olivenites. placing the approximate position of a small sample Toman's discovery of the monoclinic symmetry of olivenites with up to approximately 20 atom ~ Zn/(Cu + in the olivenite-adamite series and for estimating Zn) has raised a problem in nomenclature. It is suggested the amount of anion substitution by phosphate. that the definition of'cuproadamite' be extended to cover Accordingly, numerous members of the olivenite- all orthorhombic members of the series containing adamite series were synthesized by the method used appreciable Cu. Studies of deuterated materials have by Guillemin (t956), Minceva-Stefanova et al. helped to solve some of the absorption band assignments (1965), and Toman (1978). In addition, the method for olivenite adamite, libethenite and related minerals. was extended to the preparation of phosphatian olivenites, libethenite, and some deuterated MEMBERS of the olivenite Cu2AsO4OH to adam- materials, required in order to distinguish vibra- ite Zn2AsO4OH solid solution series are well- tions involving O H from other vibrations (see known minerals from the oxidation zone of Cu- Experimental section). Zn-As-containingore deposits. The series is com- The infrared absorption spectra of these samples, plicated by possibilities of further substitution by and of a number of natural specimens, were other cations, e.g. Fe(II), Co(II), Mn(II), and at the measured over the 400-4000 cm- 1 range, in Nujol Cu end by anion substitution of phosphate towards mulls between KBr plates, using a Perkin-Elmer libethenite Cu2PO4OH. Thus olivenite is some- 397 grating infrared spectrophotometer, each times phosphatian, in particular the classical speci- spectrum being calibrated against polystyrene. mens from Wheal Gorland and other mines in the Examples are reproduced in figs. 1, 2, and 3. The St. Day area of Cornwall (see old analyses cited by Nujol mull technique uses less material than the Collins (1871) and Palache et al. (1951); also pressed disc method, gives better resolution, and Table I). avoids the traces of water which are almost Careful X-ray diffraction measurements by inevitable in pressed discs. Toman (1977, 1978) have shown that whereas The spectra of all the non-deuterated materials adamite is orthorhombic Pnnm, olivenite is mono- show a sharp O-H stretching absorption of clinic P21/n, the changeover in symmetry taking medium intensity near 3500 cm -1, shifted on place near 20 atom ~ Zn/(Cu+Zn). In these deuteration to near 2600 cm- 1 with isotopic shifts structures, and in libethenite, the metals occupy VoH/VoD near 1.35. The position of this band alone 5-coordinate and 6-coordinate sites, the former can be used to estimate the approximate Cu:Zn preferred by Zn(II) and the latter by Cu(II) with its ratio of a member of the series. For the Cu-rich preference for square planar coordination. Long- members a single fairly sharp absorption, weakly range ordering was detected in a solid solution, H-bonded, is observed, at 3420 cm- 1 in olivenite, replacement of Cu by Zn and vice versa taking shifting to 3490 cm -1 for the (Cu0.27Zno.73) place according to these site preferences, Zn member. For material richer in Zn the intensity of initially replacing Cu in the 5-coordinate sites. this absorption drops sharply, nearly disappearing These uncommon 5-coordinate Cu-O sites are by (Cu0.1oZno.9o), where it is just discernible at primarily responsible for the lowering of symmetry 3490 cm 1. The shift of this absorption to higher in the olivenite structure (Toman, 1978). wavenumbers as Zn replaces Cu is accompanied by ~) Copyright the Mineralogical Society 52 R.S.W. BRAITHWAITE "-'N,. P \V Yj,., ~- , ,~ ,/ d Z '12 \ f'-" m V \\ X,,V" ,,i \'. \ l v k,, 1200 11oo iooo 9oo 8oo 7oo 6oo 500 4o0 WQ.venu.l be r, cm "l 1200 11(30 ir 900 ao0 700 60O 500 400 ~Javenumber, cm -t FIGS. 1 and 2. FIG. 1 (left). Partial infrared spectra of selected synthetic members of the olivenite-adamite solid-solution series. 1. CUEASO4OH(olivenite), 2. (Cuo.ssZno.12)2AsOgOH, 3. (Cu0.6aZn0,3s)zAsO4OH,4. (Cuo.27Zno.Ta)2AsO4OH. 5. Zn2AsO4OH (adamite). FIG. 2 (right). Partial infrared spectra of synthetic olivenite, phosphatian olivenites, and libethenite. 6. CuzAsOaOH (olivenite),7, Cu2(AsO4)o.91(PO4)o.ogOH, 8. Cu2(AsO4)o.s~(PO4)o.19oa, 9. Cu2PO4OH (libethenite). * Absorptions due to Nujol. a slight sharpening of its profile, and is probably a for pure libethenite. When phosphate absorptions consequence of decreasing hydrogen bonding. Pure are present in a sample's spectrum (see below) care adamite displays a single, very sharp absorption at should therefore be taken to consider this when 3540 cm- 1, not hydrogen bonded, which weakens attempting to estimate its Cu:Zn ratio. without changing position on replacing some of the The OH group in these minerals is attached to Zn(II) by Cu(II) in the 6-coordinate sites, nearly three metal atoms in a distorted tetrahedral vanishing by (Cuo.ssZno.4s). An additional weak arrangement, the bond to hydrogen forming the absorption at 3510 cm-1 is observed in the spectra fourth arm of the tetrahedron (Hill, 1976). With this of the (Cuo.27Zno.73) to (Cuo.osZno.95) members. arrangement the hydrogen atom has two nearest These absorption changes are summarized in fig. 4. neighbour oxygen atoms with which it might form The 3400 cm- 1 band in the spectrum of adamite a hydrogen bond, one each from two separate ascribed by Hill (1976) to Zn-O-H stretching is arsenate anions, but both rather distant. very weak and broad in our spectra and is probably A strong absorption is observed in the spectrum due to traces of moisture. Our spectra show of pure olivenite at 943 cm 1 (see fig. 1), shifting on negligible absorption near 1620 cm- 1, where Hill's deuteration to 700 cm-1, and therefore due to an spectrum shows a fairly strong broad band, which O-H deformation. On substituting Zn into the must represent the H-O-H 'scissor' vibration of 5-coordinate sites the position of this band shifts to water. near 920 cm-1 for (Cuo.asZnoA/), and on further Substitution of phosphate for arsenate in oliven- substitution it becomes a shoulder on the strong ite also affects the position of the O-H stretching arsenate v3 absorption centred near 850 cm- 1, and absorption. The 3420 cm-1 absorption of pure has almost disappeared into the arsenate absorp- olivenite shifts steadily to 3430 cm-* for material tion by (Cuo.8oZno.2o), but without changing its containing 19 mole ~o phosphate, and to 3470 cm- 1 relative transmittance. The ratio of the ~o transmit- THE OLIVENITE-ADAMITE SERIES 53 ~ ^ Zn /(Cu + Zn), atom % ' 355o~ zo 20 30 40 50 60 70 8o 90 loo 5540 - - 1 353O h' ' ~ <h d 351o I x/2 , V~/ J I y ~ 349C m I ," \ ~, 3470 ' /'~ i o , f ~1~). I \ I ' 5460 'r 5440 W ' ' 3430 I 3420 , 12o0 llCO iooo 9o0 800 700 600 5oo 4oo 1 0~9 0.8 0.7 0:6 0.5 0.4 0.3 0.2 0.1 0 W~vemur~Qr) ~m "I n in (CUnZnl_n)2AsO40H FIG. 3. Partial infrared spectra of deuterated materials. FIG. 4. Position of O-H stretch in the infrared spectra of 10. Zn2AsO4OD (adamite). cf. fig. 1, spectrum 5. members of the olivenite adamite solid-solution series. 11. Cu2AsO4OD (olivenite). cf. fig. 1, spectrum 1; fig. 2, spectrum 6. 12. Cu2(AsO4)o.s(PO4)o.2OD (phosphatian olivenite), cf. fig. 2, spectrum 8. 13. Cu2PO4OD (libethen- ite). cf. fig. 2, spectrum 9. * Absorptions due to Nujol. however, reveals it at 720 cm- x. Using the isotopic + O-D absorption, not present in parent mineral. shift of 1.34 for D-libethenite, determined from Position ofO H absorption present in parent mineral. VOHstretch/rOD stretch, the 720 cm-10-D band must be derived from an O-H band near 975 cm-1. This absorption band is also present in the infrared spectra of many other basic copper tance difference between the minima near 1000 arsenates and phosphates (e.g. Moenke, 1962, 1966; cm- 1 and 910 cm- 1, and the absorption maximum Sumin de Portilla, 1974; references cited by Ross, near 940 cm-1 and the minimum near 910 cm-1, 1974). can be used as a fairly accurate measure of Another O-H deformation is hidden under the composition, given reasonable transmittance values, arsenate v 3 band in olivenite and is revealed by as can be seen from fig. 5. The presence of a distinct deuteration as an O-D band at 608 cm-1. Using absorption maximum near 940 cm- 1 in this series the isotopic shift of 1.35 calculated from Yon stretch/ is thus a coincidental indication that the sample I~ODstretch, the corresponding O-H band should be concerned is an olivenite, of monoclinic symmetry. near 820 cm-1. This might correspond with the Another indication of the symmetry change is the O-H deformation observed in the spectrum of disappearance of the 480-500 cm -1 band (see libethenite, with no arsenate absorption to conceal below) as Zn/(Cu + Zn) exceeds about 20 atom %.