Native Silver and Its New Structural Modifications

Native silver and its new structural modifications

M.I. Novgorodovo, A.I. Gorshkov, and A.V. Mokhov

Native silver is distributed in ores of de- an antimonian variety of native silver, crystal- posits of various genetic types, particularly lizing in the hexagonal system - allargentum gold-silver deposits. Examples of the latter of Ramdohr ( 1962). The structure of allar- are known in Central Asia and in Northeast gentum, containing 8-15% Sb and observed USSR, within the Carpathian-Balkan region, together with dyscrasite as a product of the in the western part of the American Cordillera, decomposition of a solid solution of silver and and in Japan. Judging from the available geo- antimony, can be determined, according to logical information, gold-silver deposits belong Ramdohr, as a solid solution of silver in hexa- to shallow formations and are closely associated gonal closest packing with statistical distribution with products of volcanic activity, located in of antimony, Allargentum is generally accepted volcanic rocks of similar age of the andesite- as a separate phase of the system AgSb. This dacite series, often within the vent facies of is not certain, however, and it is possible to paleovolcanic structures of central type, or in assume that antimony is a stabilizer of the hexa- direct proximity to subvolcanic dikes, conclud- gonal structure characteristic of native silver. ing the volcanic cycle of development of the It thus becomes evident that careful studies region. Ore deposits of this type differ in are required of the structure of native silver, complex mineralogical composition, lengthy starting with the assumption that it can deviate single-stage process of formation. of the min- from the face -centered cubic motif of the dis- eralogical association, often by telescoping tribution of atoms. and coincidence within the ores of various types of mineralization. The mineralogy of gold- silver ores of Northeast USSR has been de- METHODS OF WORK scribed in papers by N. A. Shilo, M. S. Sakha- rova, V.I. Goncharov, A.S. Siderov, I.S. Structural studies were carried out by the Rayevskaya, N.Z. Sav, A.N. Nekrasova, and X-ray ( Debye) and microdiffraction methods. others. Native silver, however, one of the Electron-microscope studies were made on the principal economic minerals of these deposits, JEM-100s apparatus at the Institute of Geology has been studied insufficiently. of Ore Deposits, Petrography, Mineralogy and Geochemistry of the USSR Academy of Sciences; Despite the similar crystal structures of some of the photographs were taken by L. M. gold, silver, and copper, the similar sizes of Ol'shanskiy on the megavolt electron microscope their atoms with similar structure of their JEM-1000 in the Baykov Institute of Metallurgy. electronic shells, complete miscibility in the Samples for study by transmission electron series Au-Ag and Cu-Ag is not attained. A microscope were prepared by means of ion break of solubility in the high-silver part of the thinning by P. P. Perstnev in the Institute of system Au-Ag, predicted by Vernadskiy ( 1904), Crystallography of the USSR Academy of Sciences. was recently found by mineralogical (Petrov- Thinning of the platy deposits of native silver

Downloaded by [University of Arizona] at 14:32 26 October 2011 skaya et al., 1978) and experimental (Sakharova was carried out by means of the ion beam in an et al., 1976) work. The causes of the limited atmosphere of argon in the Edwards JBJ-200 ( 1-2%) solubility of gold in silver are still not apparatus. The angle between the ion beam entirely clear. Also unclear are the causes of and the specimen surface was ZOO, the impressed the extremely low mutual solubility of silver voltage 5 kV, the current of each beam 40 fia, and copper, attaining in all (for 200') 0.06% speed of thinning 5 microns/hour. In the final Cu on the one hand, and 0.83% Ag on the other stages of the thinning, after the first appearance (Kornilov et al., 1966). Native silver, as of perforations in the sample, the surface was well as native gold and copper, has always been polished with speed of thinning 0.5 microns/hour the standard face-centered cubic cell with space and with a change of the angle between the beam group Fm3m. Along with this there is known and the surface of the sample to 7".

The composition of the native silver was TransIated by Michael Fleischer from Samorodnoye estimated by means of a microprobe attachment serebro i yego novyye strukturnyye modifikatsii, to the electron microscope JEM-100C of the Vses. AIineralog. Obshch. Zapiski. 1979, vyp. 5, "Kevex-Rayst* (energy dispersive spectrom- P. 552-563. eter). aantitative analysis was made by A. I.

Internat. Geology Rev., V. 23, no. 4 IGR is gt registered with the Copyright Clearance Center, Inc. 485 IKTEKSATIOSAL GEOLOGY REVIEW

In addition to native silver, often with inclusions of kustelite and electrum, they contain argentite, pyrargyrite , a late generation of pyrite , chalcopyrite, galena, tetrahedrite-tennantite, and many rare sulfosalts of Ag and Sb, sometimes Bi ( sternbergite, polybasite , andorite, matild- ite, and aramayoite) . Fine disseminations of native silver, with accompanying silver-containing minerals, are present both in ore veins of upper horizons with collomorphic-scalloped structure of quartz-adularia-chlorite aggregates, and in massive coarse-grained quartz-feldspar- rhodonite veins of lower horizons.

Constantly associated with native silver is a late generation of pyrite, chalcopyrite, galena, and tetrahedrite-tennantite, distinguished by high contents of admixed silver and often form- ing close, subgraphic, regular intergrowths with deposits of silver. These minerals evidently determine a narrow paragenetic association of native silver. Judging from the character of its interrelations with the accompanyin,0 argentite and silver sulfosalts, the latter belong to a somewhat later formation. Signs of corro- sion and replacement of native silver by sulfur compounds are particularly clearly observed in its intergrowths with argentite. Relicts of native silver amid argentite occur in nearly all of the samples. Even where native silver forms isolated deposits in inclusions in the mineral silicate matrix, within each grain are found very small inclusions of argentite of unusual forms, suggesting their origin as the result of the process of sulfidization of native silver. These inclusions, a few tenths of a micron in size, have a regular globular and radiating- fibrous structure; individual fibers of argentite are a few hundredths of an angstrom wide (fig. 1). According to analysis on the "Kevex-Rayil microanalyzer, a small amount of copper is present in the argentite.

The close association between native silver and argentite thus bears a donor-acceptor character, and in the terminology of Petrovskaya

Downloaded by [University of Arizona] at 14:32 26 October 2011 and co-authors ( 1978) the second considered mineralassociation of native silver can be referred to the orthogenetic type a variety of FIGURE 1. Globular deposits of finely acicular argen- - tite in monocrystalline (a) and polycrystalline (b) areas narrow mineralogical assemblages of components of platy native silver. Electron microscope photographs not in mutual equilibrium. of thinned preparations. AIagnificntion: a - x 40,000; b - x 100,000. MORPHOLOGY OF DEPOSITS OF NATIVE SILVER

Most deposits of native silver are small Tsepin by the X-ray spectrographic method on (hundredths and a few tenths of a millimeter) , the MS 46 microanalyzer of the "Cameca" irregular clotted or flattened forms. In unusual firm. cases we see large (1-1.5 cm) dendritic intergrowths of platy crystals of native silver, flat- MINERAL ASSOCIATIONS OF NATIVE SILVER tened on the octahedron. The dendritic intergrowths are located in cavities of exfoliation In gold-silver deposits of the trpe considered, along fine banded collomorphic-scalloped ag- the silver -containing mineral associations were' gregates of quartz-adularia-chlorite composi- formed in the late stage of mineral formation. tion

486 IGR 81/4 81.1. NOVGORODOVA, 11.1. GORSHKOV, AXD A.V. MOKHOV

TABLE 1. Chemical composition of native silver according to X-ray spcctrographic analyses.

I I I Sample Ag I Au I Sb cu Fe I Cr I Bi DV -261 98.79 I 0.33 I 0.37 I 0.23 - 99.72 99.12 0.16 0.32 0.18 - 99.78 DV-133 99.93 0.04 0.04 0.18 - 100.19 DV -260 99.69 0.11 0.24 0.26 - - 10.30 DV -300 98.90 - - - 0.50 0.01 99.43 98.21 - - - 0.42 0.01 98.63 Note. Analyst - A. I. Tsepin.

Deposits of native silver are single grains, the large dendrites of this mineral contain ad- or rarely consist of intergrowths of two to mixtures of Cu, Fe, Cr, and Ni. Jktermina- three grains. Rare rectangular zones of growth, tions of the composition of the admixtures by oriented along [110] in the plane of the octa- means of the energy dispersive spectrometer hedron, alternate toward the periphery of the showed that there are no essential differences grain with fine scalloped-curved converging between their amounts in areas of single-crystal zones. The native silver from the rhodonite- and finely dispersed polycrystalline structure quartz veins has fibrous structure. Some of the deposits of native silver. grains of native silver from the quartz-adularia- chlorite ore have a completely concentric-zoned The studied samples of native silver contain structure of collomorphic type. Relicts of small amounts of Au (table 1) ; inclusions of such structure have also been found by etching kustelite and electrum noted within their de- platy crystals from dendritic intergrowths. posits were described in detail by Petrovskaya Areas with a finely dispersed phase preserved et al. ( 1978). within a crystal of native silver were clearly noted on the electron-microscope photographs obtained with thinned samples (fig. 2): The dis- MICRODIFFRACTION AND X-RAY STUDY persed particles measure around 200 A, their form is irregularly rounded, sometimes hexa- Microdiffraction patterns, obtained with gonal. Ring-shaped reflections on their char- thinned platy crystals of silver, for the most acteristic diffraction patterns indicate a rela- part reflected their structural inhomogeneity. tively high degree of recrystallization. Micro- Distinctly different were the ring-like reflec- X-ray-spectrographic analysis by means of energy tions from areas of polycrystalline structure dispersive spectrometer showed that among the (on calculation they correspo!d to a face- predominant mass of particles of pure silver, centered cell with a0 = 4.08 A, characteristic some contain Cu, Fe, Cr, and Ni, and have of silver) and the punctuate reflections from ring-shaped reflections not referable to silver. single-crystal areas (fig. 2). The character The formation of these ultra-fine polymineralic of the pnctuate reflections permits one to be-

Downloaded by [University of Arizona] at 14:32 26 October 2011 intergrowths is possible only in a highly viscous lieve that the single-crystal areas have a two- medium with a delayed course of the diffusion phase structure. Besides the intense reflections process. It is not excluded that the finely dis- corresponding tq the face -centered cubic cell persed silver has a primarily gel nature and with a0 = 4.08 A, reflections are present con- that its large dendrites containing relicts of the tradicting it. It is known that effects of bom- finely dispersed phase, like the grains with barding crystals with electron beams of high concentric-zoned and fibrous structure, were energy are very small and cannot lead to dis- formed during later processes of recrystalliza- placement of atoms from their equilibrium tion. position; they lead only to the redistribution of point defects ( Hirsch et al., 1968). In this case there is observed on the diffraction pattern CHEMICAL COMPOSITION OF NATIVE a hexagonal net of weaker reflections of the SILVER same diffraction pattern that can be referred to the basal plane of a hexa onal phase with The native silver we investigated contains parameter ag = bo = 2.9 f The evidence small amounts of admixtures ( table 1). The that these reflections belong to an independent small, irregular forms of the deposits, asso- phase and are not determined by tetrahedral ciated with sulfides of Cu, Fe, and Pb, and defects of packing in the plane ( lll), known with silver-containing sulfosalts, contain admix- for face-centered cubic lattices, is based on tures of Sb and Bi, rarely Cu and Fe, whereas analysis of the extinctions and intensities of the

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FIGURE 2. Structural inhomogeneity of platy deposits of native silver (a) in areas of polycrystalline structure (b). Electron-microscopc photographs with microdiffraction chart of thinned preparations. hIagnification a - x 28.000; b - x 1,000,000.

reflections. The appearance of additional re- apparently is explained by their interlayering flections on the diffraction patterns in this case parallel to the studied plane ( 111). The rela- cannot be explained by defects of packing, be- tion of the intensities of the reflections of the cause thereby changes would be suffered only two phases indicates a significant predominance by those reflections with (h + k + 1) = 3N + 1, of the cubic modification. where N is a whole number, whereas reflections of the type ( 111) , the broadening of which can Attempts at separate visualization of th? be explained, for example, by the appearance two structural varieties of silver were made by close to the center of the hexagon of additional obtaining their images in reflected light, char- reflections, are not changed. On most phuto- acteristic for each phase. Figure 3 shows graphs, a net of individual cubic and hexagonal dark images of the same area, obtained re- phases of native silver is not obtained, which spectively in the light of the intense reflection

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(220) of the face-cenEred cubic cell and the the X-ray pattern of sample DV-260 contains weaker reflection ( 1010) of the hexagonal cell. more lines referred to the hexagonal phase in- On the photographs are distinctly differentiated dicates, evidently, a larger amount of it in areas referred to the different structural modi- this sample than in sample DV-300. fications of silver. From the measured intelplanar spacings, In the thinned preparations some fragments the parameters of the unit cells of the cubic consisting mainly of the hexagonal phase of and hexagonal modifications were calculated silver were found (fig. 4). This is indicated (table 2). Silver with admixtures of Cu, Fe, by the fact that the reflections on the diffraction Cr, and Ni is characterized by soornewhat larger pattern are more homogeneous in intensity unit cell parameter (a0 = 4.084 A) , than silver than those of the two-phase area (fig. 3), so containing Sb and Bi (a0 = 4.077 A) ; the par- that the reflections close to the center of the ameters of heXagOM1 cells also differ somewhat hexagon of reflections in Figure 4 are even (table 2). The latter are notably larger than some intense reflections, which may character- the theoretical, calculated for ideal hexagonal ize the face-centered cubic lattice. Calculation closest packing. The volume of the cub'c unit of the diffraction pattern obtained from plates cell is 67-68 As, of the hexagonal 75. 183; inclined to the electron beam perpitted us to the X-ray densities are, respectively, 10.5 establish the parameter co - 10 A, for hexa- and 9.5. The specific gravity, measured for gonal silver. Thus, the hexagonal phase of platy crystals of native silver (sample DV-300), silver is described by a four-layer structural 10.35, corresponds to a mixture of the two type . ..%BAC.. . (4p) with parameters a = modifications. b = 2.9A, co - 10A. The space group P63mc is established from Besides the four-layer hexagonal modifica- the extinction rule; multiples of two for plams tion of native silver, predominant among the of type (0001) an! (hhm), tyo and three for hexagonal phases, we find occasional mono- planes of type ( hhOl) and (hohl) ( table 2). crystalline areas with a hexagonal phase characterized .by the same magnitudes of a0 and Table 2 gives the X-ray powder patterns bo = 2.9 A, but onlyahalf the magnitude of the of native silver in comparison with that of the parameter co = 4.8 A, which does not contra- hexagonal E -phase of the system Ag-Ab - the dict the assumption of the presence also of a synthetic analogue of allargentum. Silver con- two-layer modification of type .. . ABAB.. . ( 2H). taining less than 8-10% Sb retains its cubic structure, in which the unit cell dimensions The presence of hexagonal phases is con- are larger than those of pure cubic silver (table firmed by the X-ray patterns obtained for platy 2). Dimensiolis of the hexagonal unit cell of crystals of native silver studied by the micro- the E -phase correspond to a two-layer packing, diffraction method ( sample DV-300), and also model ,2H with parameters a0 = 2.92 & c0 = of its massive deposits (sample DV-260) (table 4.774 A, c/a = 1. 632. The interplanar spacings 2). The samples are differentiated by their of the structure of the E -phase, which can also assembly of minor elements: Cu, Fe, Cr, Ni be called hexagonal antimonian silver in two- in DV-300; Sb, Bi, Au in DV-260; the contents layer packing, is close to that calculated for of admixtures are indicated in Table 1. pure hexagonal silver in four-layer packing; the indices of the latter differ by doubled magnitudes of 1 ( table 2). Characteristic for the The X-ray patterns of native silver are two-layer packing are the reflections 2.38

Downloaded by [University of Arizona] at 14:32 26 October 2011 compared with the interplanar spacings calculated ( 0002), agreeing in magnitude of d/n with the for ideal hexagonal closest packing; the param- reflection ( 111) of cubic silver, and 1.20 (0004). eters of these ideal structures were calculated, The latter is present on the X-ray pattern of starting fromothe magnitudes of a0 {or cubic native silver sample DV-260, which permits silver, 4.08 A, and are a0 = 2.88 A, co = one to assume the presence in this sample, be- 9.62 (for 4-layer packing of type ABAC) . In sides the four-layer hexagonal modification, the sample DV-300, in which the hexagonal also 0.f the two-layer one with parameters a0 = phase has been satisfactorily established by 2.93 A, co = 4.79 A, c/a = 1. 63. The presence microdiffraction analysis, the reflections 1.917, of silver 2H in sample DV-300 has been estab- 1.634, and 1.008 were not indexed in the cubic lished by microdiffraction. symmetry; the second sample gives a still larger number of reflections not characteristic of cubic silver (table 2). These reflections Thus, our studies permit us to establish are indexed in the hexagonal symmetry; the new hexagonal modifications for native silver. measured interplanar spacings are close to the Because these structural modifications have calculated. Lines of the powder patterns be- different sequences of layers in densest packing, longing to hexagonal phases have weak intensities, they must be referred to polytypes of native which agrees with the data of microdiffraction silver. In correspondence with the recom- on the absolute predominance of the cubic modi- mendations (Some problems . . ., 1977), it fication in the studied samples. The fact that is proposed to designate these structural

490 IGR 81/4 TABLE 2. Results of calculation of X-ray patterns -of native silver. Antimoniar Ag-LH Supergene Sample Sample silver (6% Ag-4H (Sb fWJ 0m Ag DV-300 DV-260 (ASTM. Sb) (Somac I: (Chebotarel znd Clark, 2-1145, I952 et al., 1976 ------1966) din I din d In 1 d/n hkil din - - - - -__ - - -

- 5 2.721 - - - - 2.88 2.602 6 2.58E 2.50 too 2.53 1010 2.56 - 1 2.443 2.425 - - - - 2.360 10 2.34: - 60 2.38 0002 2.35 - 1 2.241 2.236 100 2.24 1011 2.26 2.039 8 2.036 2.00 - - - 2.04 1.917 1 1.875 1.767 40 I.74 1012 - 1.634 1 1.668 1.666 - - - 1.659 1.591 6 1.587 - - - - 1.596 - 1 1.526 1.561 - - - - 1.444 10 1.443 1.443 80 I.46 1120 1.450 1.360 2 1.358 1.386 80 I.35 1013 1.366 1.303 2 1.300 ------2 1.286 1.270 60 1.25 1122 - 1.231 10 1.229 1.240 60 1.23 2021 1.238 - 1 1.213 1.212 - - - - - 1 1.1 99 - 20 1.20 DO04 - 1.181 9 1.178 1.170 - - - 1.184 1.128 1 1.1 60 1.118 20 1.12 2022 - 1.033 4 1.033 - 20 1.08 1074 1.036 1.0285 4 1.021 - - - - 1.023 1.008 2 0.990 1.oo 40 3.99 3223 1.010 - - - - 40 3.94 213 - - - - - 40 3.93 1124 -

Unit cell parameters Ag-3C 4.084 4.077 LO92 Downloaded by [University of Arizona] at 14:32 26 October 2011 68.12 67.76 - 10.51 10.57 -

2.94 2.93 - 10.11 10.18 - 1.72 1.73 - 75.15 75.15 - 9.53 9.53 -I - - Ag-211 2.94 2.93 - 4.80 4.79 - 1.63 1.63 - - 35.40 - - 10.11 - - Note. Conditions of photograph: RKIJ camera (3R = 114 mm), diameter of rubber spherc 0.2 mm, unfiltered Fe radiation.

IGR 81/4 49 1 I STE K SAT I 0 S A L G EO LO G Y H EV I EU'

FIGURE 4. Electron microscope image of the hexagonal phase of silver (Ag-411) and the microdiffraction pattern of thc plane (10-10) [sic]. AIagnificntion x 32.000.

modifications as silver-2H (two-layer hexagonal The distribution of the hexagonal modifica- packing) and silver-4H (four-layer hexagonal tions of silver is apparently not restricted to packing), in distinction from cubic silver with ores of the deposits discussed. Thus, Chebo- face-centered cell (silver 3C). Regular inter- tarev et al. ( 1976) give an X-ray powder pat- growths of different polytypes in the same crys- tern of supergene silver from a gold deposit tals of silver were formed according to the of Uzbekistan, in which some of the interplanar rule of parallel planes (111) of-the cubic modi- spacings ( 2.26, 1.659, 1.010, table 2) refer fication and the basal plane ( 1010) of the hexa- to a possible hexagonal phase mixed with the gonal modification. It is evident that the forma- cubic. tion of different polytypic modifications occurred under the same physicochemical conditions of

Downloaded by [University of Arizona] at 14:32 26 October 2011 mineral formation; their origin, most probably, PHYSICAL PROPER TIES is connected with growth phenomena. The poly- typy of native silver was aided by slow crystal- We studied the dispersion of the reflectance lization from a primary gel material; it is not and the variation of hardness by microimpres- excluded that the hexagonal modifications are sion for the native silver. The reflectivity was metastable formations of an early stage of measured by L. N. Vyal'SoV on the "PIORV7 growth. This may be especially indicated by apparatus (Uitz 3b objective with effective the sharp decrease of the frequency of occur- aperture 0.1, direction of vibration of polarizer rence of the silver polytypes in the order 3C- perpendicular to the plane of incidence). Stan- 4H-2H. Errors in the sequence of the super- dard-pyrite. The relative error of measure- position of layers originating during the growth ment is +2%. The grains of native silver are of silver-4H ( .. . ABAC. .. ) and silver -2H isotropic; birefringence and anisotropy not ( .. . ABA3.. .) were probably stabilized by observed in air, which is explained by the abso- admixtures, mainly Sb, Bi, Cu, and Fe. At lute predominance of the cubic modification in the same time, one cannot state that the admix- the samples. The reflectance is notably lower tures are the principal factor in the origin of than that given in handbooks (Bezsmertnaya et the non-cubic polytypes of silver; varieties are al. , 1973) ; the character of the dispersion of known containing the same admixtures in far reflectance (R, %) in the visible region of the greater amounts and keeping the cubic structure spectrum (wave length, nm) is analogous to (Minerals, 1960). thatknmn for native silver (fig. 5) : 67.0 - 440;

492 IGR 81/4 31.1. KOVGOHODOVA, '1.1. GORSIIKO\', AX11 ti.\'. MOKIIOV

3. The polytypes of silver were formed during the recrystallization of finely dispersed 95 c -2 deposits of this mineral, having a primary gel 590 //- --2 nature. It is possible that these hexagonal polytypes are metastable formations, the origin of which is determined by the slow speed of growth of the crystalline individuals. Admix- tures, present in the silver in amounts of tenths of a percent, play the role of stabilizers of 70 errors in the sequence of superposition of the 65 layers of the crystal structure.

500 600 7U9 4. Silver-containing mineral associations A, nm were formed in the late stages of the process of ore deposition in gold-silver ores at small FIGURE 5. Curves of dispcrsion depth. We candistinguishanarrow paragenesis of reflectance of nativc silver. of native silver with late generation of pyrite, 1 - platy deposit of mixture of chalcopyrite, galena, tetrahedrite -tennantite , cubic and hexagonal modifications; and an orthogenetic association of this mineral 2 - after Bczsmertnaya et al. with argentite and complex silver sulfosalts. ( 19 66).

RE FERE NCE S

69.4 - 460; 71.5 - 480; 73.6 - 500; 75.0 - ASTM, 1952, X-RAY DIFFRACTION DATA 520; 76.5 - 540; 77.8 - 560; 78.8 - 580; CARDS, 2-1145, Philadelphia. 79.6 - 600; 80.2 - 260; 80.9 - 640; 81.5 - 660; 82.0 - 680; 82.5 - 700; 83.0 - 720; Bezsmertnaya, M.S., Chvileva, T.N., and 83.5 - 740. Agroskin, L. S. , 1973, DETERMINATION OF ORE MINERALS IN POLISHED SEC- The hardness of native silver was measured TIONS FROM REFLECTION SPECTRA on a PMT-3 microsclerometer, standardized AND HARDNESS: Izd-vo Nedra. with NaC1. According to 50 measurements on 10 samples, the ma&tudes of hardness vary Chebotarev, G.M., Volkov, Yu.A., and Tro- within the range 51-59, average 53.5 f 5 kg/mm2. nenok, N.V., 1976, NATIVE SILVER The form of indentations is isotropic. FROM MARDZHANBULAKA (WESTERN UZBEKISTAN) : Uzbek. Otdel. Vses. Mineral Obshch. Zapiski, vyp. 29. CONCLUSIONS Hirsch, P.B., Howie, A., Nicholson, R., 1. In gold-silver ores of a deposit in North- Pashley, D., and Whelm, M., 1968. east USSR new hexagonal modifications were ELECTRON MICROSCOPY OF THIN found in native silver, occurring in regular CRYSTALS: Izd-vo Mir. [Translation intergrowth with cubic silver. The new modifi; of ibid., 1966, Plenum Press, New York. cations are polytyges Ag-2H (a = 2.93 f 0.01 A, co = 4.79 f 0.01 A, c/a = 1.63, d. %-ray = Kornilov, 1.1. , hlatveyeva, N. hl. , Pryakhina, 10.11) and Ag-4H (a0 = 2.93 f 0.01 A, co =

Downloaded by [University of Arizona] at 14:32 26 October 2011 L.I;, and Polyakova, R.S., 1966, ME- 10.11 f- 0.04 A, 1/2 c/a = 1.72, d. X-ray = TALLIC PROPERTIES OF ELEMENTS OF 9.53). The unit cell parameter of cubic silver THE PERIODIC SYSTEhl: Izd-vo Nauka. ( Ag-3C) with admix@res of Cu, Fe, Cr, and Ni is 4.084 f 0.005 A, that wjth admixture of The measured MINERALS, REFERENCE WORK, V. 1, 1960: Sb, Bi, Au is 4.077 f 0.005 A. Izd-vo Nauka. specific gravity of a mixture of cubic and hexagonal modifications of native silver is 10.35. The amount of the polytypes of silver in their Petrovskaya, N.V., Novgorcdova, M.I., Fro- natural mixtures decreases sharply in the order lova, K.Ye., and Tsepin, A.I., 1978, 3C-4H-2H. The rule of intergrowth is 111 KUSTELITE AND PROBLEMS OF DIS- (3C) lOiO(4H, 2H). CONTINUITY IN THE SERIES Au-Ag: AN SSSR Izvestiya, ser. geol., no. 3. Characteristic for the hexagonal silver are the following X-ray lines: 2.241, 1.668, 1.286, 1.213, 1.199, 1.160, 1.00. Sakharova, M.S., Batrakova, Yu.A., and Ryakhovskaya, S. K. , 1976, EFFECT OF 2. The natural polytypes of native silver ANIONIC COMPOSITION OF SOLUTIONS are possibly the cause of the low solubility in ON COPRECIPITATION OF GOLD AND silver of the structurally similar analogues of SILVER IN SULFIDES: Geokhimiya, its cubic modification-gold and copper. no. 6.

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Somanohi, S., and Clark, L., 1966, THE OC- 1977: Vses. hfineralog. Obshch. Zapiski, CURRENCE OF AN AgfiSb PHASE AT WP..- 6. COBALT, ONTARIO: Canadian hlineral., v. 8, pt. 5. Ramdohr, P., 1962, ORE hTINERALS AND THEIR INTERGROWTH: Izd-vo Inostr. Lit.

SOME PROBLEhlS OF MINERALOGICAL- Vernadskiy, V.I., 1904, DESCRIPTIVE hm- CRYSTALLOGRAPHIC NOMENCLATURE, ERALOGY: v. 1, St. Petersburg. Downloaded by [University of Arizona] at 14:32 26 October 2011

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