Tun AUERTcAN MINERALocrsrJOURNAL OF THE MINERALOGICAL SOCIETY OF AMERICA

Vol.48 JULY-AUGUST, 1963 Nos. 7 and 8

PROUSTITE-PYRARGYRITE SOLID SOLUTIONS1PnrBsrr,Bv TourutN, 3o, U. S. Geological Surtey, Washington 25, D. C.

AssrRA.cr

Proustite (AgaAsSr) and pyrargyrite (AggSbSr) form a complete series of solid solutionsat temperatures at least as low as 300o C. Evidence from natural occurrences suggests amiscibility gap at lower temperatures, but demonstration of such a gap has not been madein the field or in the laboratory.

As would be expected from the crystal structures, the c-axis length of the solid solutionsis nearly constant, while the o-axis length is a function of the composition, increasing withincreasing Sb content. Because of the resulting change in axial ratio, difiraction-angle sepa-ration between certain pairs of peaks in the difiraction pattern may be used directly as anindication of composition.

Study of specimens of natural ruby silvers has not revealed any examples of equilibriumtwo-ruby-silver assemblages. A specimen of pyrargyrite from Casapalca, Peru, has unit-cell dimensions corresponding to a composition of about 35 formula per cent Ag3AsS3,about twice the arsenic content of any pyrargyrite heretofore reported.

InrnouucrroN

The extent of solid solution between the ruby silver minerals, proustiteand pyrargyrite, has not previously been established satisfactori ly.Miers (1888), in a very detailed study of the ruby silvers, concludedthat the two minerals are "two species which may be always distin-guished." He presents analyses of pyrargyrite containing up to 2.60per cent As (17.2 formula per cent Ag3AsS) and of proustite with asmuch as 3.74 per cent Sb (16.2 formula per cent AgaSbSa), but statesthat the crystallographic elements of the minerals are not affected bycomposi t ion (Miers, 1888, p.99; Miers and Pr ior , 1887, p. 200) .

Jaeger and van Klooster (1912) investigated the melting relations inthe system Ag3AsSe-AgeSbSa by the method of cooling arrests and con-cluded from their thermal data and from microscopic examination oftheir charges that a complete series of solid solutions existed betweenproustite and pyrargyrite.

The system does not seem to have been studied since the introductionoI *-ray diffraction methods, and the present study was undertaken inorder to determine the extent of solid solution and to investigate the

I Publication authorized by the Director, U. S. Geological Survey.

PRIESTLEY TOULMIN, 3D

possibil i ty of using the compositions of coexisting ruby silvers as ageothermometer.

Existing analyses, as well as spectrographic analyses made in con-nection with the present study, indicate that the ruby silver mineralsare very nearly binary. Therefore the compositions of coexisting proustiteand pyrargyrite, if this equil ibrium assemblage is possible, should bedetermined by temperature and pressure alone.

ExpnnruBNrAL METHoDS

Charges were sealed in evacuated sil ica-glass tubes and heated innichrome-wound muffie furnaces. Temperatures were measured potentio-metrically by chromel-alumel thermocouples. The temperature uncer-tainty does not exceed 5o C.

X-ray difiraction studies were carried out on a Norelco diffractionspectrometerl precise cell-edge measurements were made with theNorelco counting-rate computer, using glass-slide smear mounts withNaCl as an internal standard. Ni-fi l tered Cu radiation was used, thewavelenglhs being taken as 1.54050 A ior CuKar, 1.54434 A for CuKar,and 1.5418 A for CuKa. For NaCl , o was taken as 5.6399 A at roomt-e mpera ture (26 ! 3' C.) by calibra tion against metall ic sil icon, a : 5.43062A (Pa r r i sh , 1953 , p . 14 ) .

All materials were made up from the pure elements. Spectrographicanalyses of the elements used show less than 0.01 per cent impurit iesin the silver, arsenic, and antimony, and less than 0.02 per cent in thesulfur. From the elements, crystall ine AgzS and SbzSa and vitreous As2S3were prepared by pyrosynthesis, and these in turn were fused togetherand crystallized to pure proustite and pyrargyrite. Intermediate rubysilvers were then made from the pure end-members. All syntheses werecarried out in sealed, evacuated, sil ica-glass tubes.

ExrsreNcB AND PRopERTTES oF Sorro Sor-uuoNs BETwEENPnousrrrB AND PYRARGYRTTE

X-ray diffraction patterns of charges of composition AgrAsSa andAgrSbSa, quenched after having been annealed near 450" C. for severaldays, correspond to those of natural proustite and pyrargyrite (Table 1).The pattern for proustite agrees with that given by Wernick et al. (1958).

After similar heat treatment, charges of intermediate composition givediffraction patterns intermediate between those of the end members.Since the character of the pattern changes continuously as a function ofcomposition, it is concluded that a continuous solid solution seriesexists, at 450o C., between Ag3AsSr and AgaSbSr. Microscopic examina-tion confi.rms the homogeneity of charges of intermediate composition.

P RO U S T I T E- P Y RA RGY RI T E

Teslr 1. Powonn PlrrrnNs or Svnrnrerc Pnousrrru mvo PlrlncrrnrrrNi-filtered Cu radiation. Patterns prepared on Norelco difiractometer

Pyrargyrite Proustite

d""r", A d . r " , A r / \ o

110012211202300t222201131313123211040424to2230242322143301 4 1

5024135 1 1134A a a

4M152600125333431324520342315006t6l0541162446t2440235523514306621532226434262710425

5 . 5 2 63.9673.3413 .2233 .1902 . 7 U2.7632 . 5 7 22.5392 .2672.1292 .1252.O982.0892.0021 .9831.961r.8671 .8+2r . 77 rr . 7 5 31 . 6 9 61 . 6 8 71 .6841 . 6 7 r1 . 6 1 1I .5991.59 .51 . 5 7 11 . 5 5 61 .5481 l i l

I . . ) . J . t

1.4801 . 4 5 71 4531 . M O1 . 4 3 81.4051 . 3 9 21 .3841.3821 .3651 .3561 .3501 . 3 2 21.3121 .305t .2861 . 2 7 61 . 2 7 01.2681 . 2 5 5

3 .953 . 3 43 . 2 23 . 1 82 . 7 7 9

2.5662 . 5 3 22.260

2 . 1 2 1

2 .085

1 . 9 5 71.8641 .838

1 . 7 5 11 . 7 0 4

1.682r .6691 . 6 0 9

1 . 5 9 3

1 . 5 2 9

1 .453

0 . 20 80 . 90 . 71 . 0

0 .970 . 90 . 3

o . 2

0 . 1

0 . 50 . 30 . 1

o . 20 . 1

0 . 3o . 20 . 2

0 . 1

0 . 1

0 . 1

hkl d*t"., A dou" , A

1100122 t L202300122220113I J I

31210432104241022302423221433024r5024r31345 1 142240115260012533332443r520342006.t r.)0541611 1 62M6r2440t ? (

52351430662r532226434

5.4083.9433 . 2 7 93 . 1 8 63 . 1 2 22 . 7 4 52 . 7 0 42 .5552.4892.2302 . l l 72 .0862.0622.044t . 9 7 71 . 9 7 21 . 9 2 61 .8521.803r . 7 3 51 . 7 2 01 . 6 7 0 \r .667 I1 . 6 5 21.6391 .5931 .5691 . 5 6 11 . 5 6 11 . 5 3 11 . 5 2 8I . J I O

1 .5001 . 4 5 21 . 4 5 01 . M 51.4191 . 4 1 01.400I . J / J

t . 3571 .352t .3521 .332\! #o-rI . J I J

1.2851 . 2 7 91 . 2 7 7| . 2 5 7

o .020 .02

5 . 4 1 0 . 23 . 9 4 0 . 23 . 2 8 0 . 93 . 1 9 1 . 03 1 2 0 . 42 . 7 M 0 . 8

2 . 5 5 7 0 . 72 . 4 9 0 0 . 72 . 2 3 0 0 . 22 . r t l 0 . 0 2

1 . 9 7 1 0 . 1| .925 0 .31 . 8 5 4 0 . 11 . 8 0 0 0 . 0 7| . 7 3 4 0 . 0 91 . 7 2 0 0 . 0 7

1 . 6 6 6 0 . 2

1 . 6 3 8 0 . 11 . 5 9 3 0 . 0 9

1 . 5 6 3 0 . 0 9

1.448

0.04

0 .07

0 .02

728 PRIESTLEY TOULMIN, 3D

In order to refine the unit-cell edges of the ruby silvers, the six strongestl ines in the diffraction pattern were chosen for measurement. These sixl ines ( (21* l ) , (20*2) , (30*0) , (12*2) , (11*3) , and (13*1) ) and the (111) ,(200), and (222) Iines of the NaCl internal standard were determined bystep-scanning over the intensity peaks at angular increments (20) of0.01o, using a Norelco counting-rate computer. The positions of the ruby

silver l ines were corrected by comparison with the NaCl l ines. The re-

sulting six values for each sample were then used to calculate a and c

Tlelr 2. Uxrt-crr,l Panaunrnns ol SvNr:nenc Rulv Srr-vnns

Stated uncertainties for a and c are standard errors of estimate derived from least-

squares treatment of diffraction data for six lines. Stated uncertainties for Yuc and cfo

rn'ere calculated on the assumption that the uncertainties in o and c are independent of

each other.

Formulaper centAgaAsSr

Vuc, A3 c/a

0 . 0 07 . 5 0

1.5 .001 7 . 9 32 6 . 9 436. 884 6 . J . )

56 6264.707 5 . 8 88+.269 2 . 7 6

100 .00

11 .0520+0 .0015tr.0329+0 002411 .0198+0 .003611 .0110+0 .001310. 9840 + 0.002610.9632+0.003110 .9351+0 .001210.9247 + 0.001510. 8914 + 0. 001410. 8701 + 0.004110.8466+0 003110.8434 + 0.008310. 8160 + 0.0010

8.7177 +O 00168.7247 +0.00308 . 71 70 + 0.00458.7197 +0.00178.7234+O.00338.7218+0.00408.7320+0 00158 .7119 + 0 .00198.7149 + 0.00188 .7103+0 .00548.7039+0.00408.6932+O.01078.6948+0.0013

9 2 2 . 1 8 + O . 3 09 1 9 . 7 2 + 0 . 5 1916.74+O 769 1 5 . 5 6 + 0 . 2 89 1 1 . 4 7 + 0 5 5907 .85 + 0 .669 0 3 . 3 2 + 0 . 2 59 0 0 . 4 5 + 0 . 3 28 9 5 . 2 9 + 0 . 3 08 9 1 . 3 1 + 0 . 8 7886.81 + 0 .658 8 5 . 1 9 + 1 7 4880.90+0 13

0. 7888 I 0. 00020.7908+0.00030.7910+0.000s0.7919+0 00020.7942 +0.00040.7956+0 00040.7977 +0.00020.7974+0.00020.8002+0.00020.8013+0.00060.8025+0.00040.8017 +0.00120.8006 + 0.0001

by the method of ieast squares. The results are given in Table 2 and areplotted in Fig. 1.

It wii l be noted that the length of the c-axis remains nearly constantfrom one end of the series to the other, whereas the length of the o-axischanges rapidly with composition. The explanation of these relations is

readily found in the ruby silver structure (Harker,l936;Hocatt, 1937),

i>>

Frc. 1. Unit cell parameters as functions of ruby-silver composition. Lengths of vertical

bars represent twice the standard error of estimate for each point. The thin lines are meant

only to represent reasonable fits to the sets of data points through which they are drawn.

il.o4i

lr.02

il.oo

to.98

to.96

ro.94

to.92

to.90

to.88

ro.86

1o.84

ro.82

8.?2 I

8.70

e20 lE

9to

900

890

880

o.800

o.?90

40 50 60 70formulo par canl

AgrSbS. lO 20 30

730 PRIESTLEY TOULMIN, 3D

which consists of two sets of spiral chains parallel to the c-axis. Eachchain is built of alternating Ag and S atoms, and the chains of each setare interconnected by As (Sb) atoms, which are the apices of f lat pyram-idal AsSa (SbS3) groups. Each S atom is part of a different Ag-S chain.Substituting a larger Sb atom for a smaller As atom tends to flatten

Ag.SbS. lO 20 30 40 50 60 70 80 90 Ag3AsSgtornulo par canl

Irrc. 2. Angular separation of the (13*1) and (11*3) difiraction peaks for CuKar

- radiation as a function of ruby-silver composition'

sti l l further the pyramidal groups, thereby increasing the separation of

the chains from one another. The repeat distance parallel to the chains,

on the other hand, is controlled principally by the bond angles and

lengths of the Ag-S chain and so is much less affected by the Sb/Assubstitution. The slight decrease in the length of the c-axis at composi-tions more As-rich than about 1: 1 seems to be real and presumably

reflects deformation of the Ag-S bonds by the small As atoms.Because of the anisotropy of change of dimension with composition,

the axial ratio, cfa, changes rapidly, and different lines in the diffractionpattern change position at unequal rates. For this reason, the angular

nI

=lf)

(DN

PROASTITE-PYRARGYRITE 731

separation of certain pairs of diffraction lines can be used directly as ameasure of composition in the proustite-pyrargyrite series. The pair,(11*3) and (13*1), have proved especially useful, and a plot of angularseparation versus composition (Fig. 2) permits approximate determina-tion of the composition of a ruby silver. For more precise results, thecell-edge lengths may be determined from the d-spacings of several l inesand compared with the data in Fig. 1 or Table 2.

The unit-cell volume is seen to be a l inear function of mol fraction,within limits of error of measurement. An earlier incorrect statement tothe contrary (Toulmin, 1958) resulted from fortuitous agreement ofpreliminary data with a calculated model.

PuasB Rpre:rrorqs

Solaus. The evidence suggesting a miscibility gap in the proustite-pyrargyrite system consists of 1) the supposed restriction of natural rubysilvers to compositions within 17 formula per cent of the end members,and 2) the supposed coexistence of two ruby silvers in nature. Particu-larly remarkable is the description of an intimate intergrowth of proust-ite and pyrargyrite from the San Antonio mine, Casapalca, Peru(Orcel andPlaza, 1928, p.221 and Pl. VI A). The writer has not beenable to obtain a specimen of the material illustrated by Orcel andPlaza,but has examined a specimen of pyrargyrite from the same mine,kindly furnished by the late Professor H. E. McKinstry of HarvardUniversity. This material appears homogeneous in polished section underthe microscope. Its *-ray diffraction pattern is that of a single-phaseruby silver, except for a broadening and incipient splitting of the (30*0)peak. No other peak of the pattern is split, though some other peaks(e.g., (13*1)) are more sensitive than (30*0) to variations in the ratio

1s/Sb. If (30x0) is neglected, the pattern yields a value for a ol10.966A, indicating a composition of about 35 formula per cent AgBAsSB, abouttwice the maximum arsenic content previously reported for pyrargyrite.Semiquantitative spectrographic analysis (Table 3) corroborates the highAs content.

Examination of selected ruby-silver specimens in the collection of theU. S. National Museum has not brought to l ight any hand specimenscontaining more than a single ruby silver. Difierent specimens from thesame mine or district commonly have difierent ruby silvers, however.Table 3 shows the results of semiquantitative spectrographic analysesof a number of natural ruby silvers. The content of elements other thanAg, As, Sb, and S is low except in sample t699I, which contained a l itt lecalcite. Elements characteristic of gangue minerals constitute a largepart of the "impurity" in all the specimens. These analyses thus support

732 PRIESTLEY TOULMIN, 3D

the inference that natural ruby silvers do not contain in solid solutionIarge amounts of elements other than those represented by the compo-nents Ag3AsSr and AgaSbSa. A consequence of this is that the r-ray datafor the synthetic ruby silvers may safely be applied to the determination

Te,lr,n 3. SnltrquaNrrr,trrvn Sprctnocrupnrc AxalvsBs ol Narun.Lt, Runv Srr-vpns

Figures are in weight per cent and reported to the nearest number in the series 7, 3,

1.5,0.7,0.3, 0.15, etc. (geometr ic group midpoints) . Compar isons of th is type of semi-

quantitative data with results of quantitative methods indicate an empirical probability

of about 60 per cent that the analytical value will fall within the assigned group. Helen

Worthing, analyst.

Ag

SbBeCrCuGeMoNiPbSnZnG1

> 1 00> 1 00.00300 .030.00300.0030.007000 0 1

>100> 1 00 000150.00030 .030.0300.0070 .070.00070.030 . 1

> 1 07 .>1000.00030 0 30000.000150.000700 .03

> 1 0> 1 00 . 70.000150.00030.0030000000.007

> 1 0>107 .00.00030.00300.00070.0030 .01500.03I

> 1 0> 1 0>1000.00030 .07000.0070 .0150.000700 . 3

A. Pyrargyrite, Colquechaca, Bolivia. U. S Natl. Museum, spec. no. C-779 Sample was a

crystal from a vug.B. Same specimen as A Sample was from drusy coating.C. Pyrargyrite,LaLtz Mine, Guanajuato, Mexico. USNM no. R-1073.

D. Proustite, Chanarcillo, Chile, USNM no. R-1078.E. Proustite, La Suz (sic) Mine, Guanajuato, Mexico. USNM no. 16991'

F. Pyrargyrite, San Antonio Mine, Casapalca, Peru. Specimen furnished by the late

Prof. H. E. McKinstry, Harvard University.The following elements rvere looked for but not detected: Na, K' P, Au, Bi, Cd,

Ce, Co, Cs, Dy, Er, Eu, Ga, Ge, Hf, Hg, Ho, In, Ir, La, Li, Lu, Nb, Nd, Os, Pd, Pr, Pt,

Rb, Re, Rh, Ru, Sc, Sm, Ta, Tb, Te, Th, Tl, Tm, U, V, Y, Yb,Zr.

1 G is the total of the foliowing elements: Si, Al, Fe, Mg, Ca, 'Ii, Mn, B, Ba, Sr, W.

of natural ruby silvers. Table 4 shows the values of A20s.p11.3 found fora number of natural ruby silvers and the compositions implied by them.

The experimental results of the present study clearly confirm the con-clusion of Jaeger and van Klooster (1912) that there is complete misci-bil i ty between proustite and pyrargyrite at solidus temperatures. Experi-ments in search of a miscibil i ty gap at Iower temperatures have been

P ROU S T I T E-P Y RA RGY RI T E

made by heating both a homogeneous solid solution and a mechanicalmixture of end members at 450o, 400o, 350o, 300o and 135o C. for periodsup to 17 months. Both the solid solution and the mixture were approxi-mately 50 formula per cent AgrAsSr. The mechanical mixture homog-

Teer-r 4. ANcuur Snpena.r:roNs ol rnn Dmlnacrror Prnrs (13*1) eNn (11x3)ol Narunlr- Rusv Srr.vnns lon CuKa Rlornrror, AND THE

Cowosrrrons Dnnrvro Trrnnnlnou

733

Specimen

Pyrargyrite, Colquechaca, Bolivia, U. S. Natl. Mus., spec. no. C779Pyrargyrite, Colquechaca, Bolivia, USNM no. R-8051Pyrargyrite, Soracaya Mine, Bolivia, USNM no. C4996Pyrargyrite, La Luz Mine, Guanajuato, Mexico, USNM no. R1073Pyrargyrite, Little Emma Mine, Lump Gulch District, Jefferson

County, Montana. USNM no. 11151Pyrargyrite, Montana, USNM no. 11926Pyrargyrite, Lander County, Nevada, USNM no. 13451Proustite, LaSuz[si.cl Mine, Guanajuato, Mexico. USNM no. 16991Proustite, Chafiarcillo, Chile, USNM no. R1078

L20

0.47"0 .47 "0 .4900.495'

0 .480o .47 5o0. 5450 .9450 .95

Formulaper centAg:AsSr

I2I

4

2r+139798

enized in one month at 450o and 400o C., and in 4 months at 350o C.After 14 months at 300o C., the solid solution showed no signs of exsolu-tion, and the mixture had largely homogenized. Seventeen months at135' C. produced litt le reaction in a mechanical mixture of pureproustite and pyrargyrite.

In this connection, it is interesting to note that homogenization of themixture seems to be progressing in an unexpected fashion. The chargealter 4 months at 300o consisted of two phases in unequal amounts, theless abundant having a composition of nearly pure pyrargyrite (about1.5 formula per cent proustite component), and the more abundanthaving a composition of about 64 formula per cent proustite component.The overall composition of the batch is 50.0 formula per cent proustitecomponent. After 14 months, still less pyrargyrite was present, and themajor phase contained about 55 formula per cent AgaAsSr. The inferenceseems inescapable that the pyrargyrite phase is dissolving directly inthe proustite phase with practically no concomitant solution in the otherdirection. Thus the larger, heavier atoms of Sb seem to be more mobilethan the smaller, Iighter atoms of As. No satisfactory explanation forthis phenomenon has yet come to mind.

Melting. Jaeger and van Klooster (19L2) report sharp melting tempera-

734 PRIESTLEV TOULMIN, 3D

tures for all compositions in the system AgaSbSrAgrAsSa. Their datasuggested a melting diagram of a binary system with complete miscibil-ity in l iquid and solid states and a minimum temperature of crystall iza-tion at an intermediate composition. They were unable, however, todetect any separation between liquidus and solidus, although such aseparation must exist in such a system at all compositions except thoseof the end members and the minimum point.

Attempts to check Jaeger and van Klooster's diagram by the methodof heating and cooling arrests had to be abandoned because of the severedifficulties presented by undercooling of the melt. The experiments wereperformed by heating and cooling the tube in an electric furnace andrecording the emf and a chromel-alumel thermocouple whose bead wasplaced in a "dimple" in the sealed, evacuated sil ica-glass tube containingthe charge. The emf was recorded. every 5 seconds on a variable-range,variable-span recording potentiometer so adjusted that the entire chartwidth (11inches) represented 10 mv. In a typical experiment, a chargeconsisting of pure proustite was heated unti l a very slight break in thecurve of temperature vs. t ime indicated melting between 490o C. and497" C. The charge was kept well above this temperature for severalhours and then cooled slowly. Crystall ization commenced only when thetemperature had fallen to 418o C.; the latent heat released raised thetemperature to 430o C. in about 15 seconds, where it remained for about15 more seconds before dropping very rapidly to the temperature of thefurnace. The supercooling could be reproduced quantitatively. Evenvigorous agitation at 440" C. did not initiate crystallization of the melt,which was surprisingly nonviscous.

Jaeger and van Klooster (1912, pp. 267-268) obtained a crystalliza-tion temperature by the method of cooling arrests that agrees well withthe temperature of melting found in the present study for proustite. Noverified explanation for the apparent absence of supercooling in theirexperiments can be given, but the supercooling in the present work maybe related to the high degree of purity of the reagents or to the fact thatthe present experiments were carried out in vacuo rather than under anitrogen atmosphere, as was done by Jaeger and van Klooster.

Because of the very l imited geologic interest of melting relations inthis system, further investigation along these lines was temporarilyabandoned.

Inaersi.ons. The rare minerals, pyrostilpnite and xanthoconite, arenaturally occurring dimorphs of pyrargyrite and proustite and have beensynthesized in aqueous and other media (Weil and Hocart, 1953; B6land,1946; Peacock, 1947). They did not form in the experiments reported

PROUSTITE-PYRARGYRITE 735

in this paper. If these minerals are truly stable at all, their stabilityfield(s) must l ie at low temperature, for ruby silver solid solutions arestable at the solidus in the system, Ag3SbSr-AgaAsS3. It is, of course,possible that other elements enter and stabil ize pyrosti lpnite andxanthoconite; existing analyses do not support such a view, but the dataare old and few.

CoNcrusroNs

The following conclusions may be drawn from the present study:1) The ruby silvers form a complete solid solution series from solidus

temperatures down at least to 300o C.2) The composition of synthetic ruby silvers can be determined with

an accuracy of 2-3 per cent by r-ray measurements, if o is found byleast-squares analysis of the six strongest l ines. Measurement of theangular separation of (13*1) and (11*3) yields the composition with anaccuracy of about 5 per cent.

3) Since natural ruby silvers are apparently nearly binary solid solu-tions of the components Ag3AsS3 and Ag3Sb53, and since the end-memberminerals, proustite and pyrargyrite, give r-ray diffraction patternsidentical to those of the corresponding synthetic phases, the same r-raymethods should be applicable to the determination of the composition ofnatural ruby silver solid solutions.

4) The binary nature of the series also implies that if two ruby silverscan in fact coexist stably, their compositions wil l be fixed by the tempera-ture and pressure alone. The temperature at which such an assemblagecould form, however, would have to be considerably less than 300o C.,and no conclusive evidence for it seems to be available.

AcrNowr,BocMENTS

I am glateful to my colleagues Paul B. Barton, Jr., E-an Zen, andPhil ip M. Bethke for helpful and stimulating discussions. Mrs. ShirleyK. I{osburg provided valuable assistance in making and measuringmany difiraction charts. Barbara L. Hammond helped with some of ther-ray work, and D. E. Appleman devised the computer program andcarried out the calculations on the Burroughs 220 digital computer. p. B.Barton, Jr., E. H. Roseboom, Jr., and D. E. Appleman read the manu-script and suggested notable improvements.

Rtlnn-nNcns

B6r.a.No, RnNf (1946) Synthesis of the silver sulpho-minerals in alkali sulphide solutions.Ph.D. thesis, Univ. of Toronto.

Hanren, Davro (1936) The application of the three-dimensional patterson method and

736 PRIESTLEY TOULMIN, 3D

the crystal structures of proustite, AgrAsSa, and pyrargyrite, Ag:SbSa. Jour. Ckem.

Physics,4,381-390.Hoc.tnr, Re.vuoNn (1937) Sch6ma structural de la proustite et de la pyraryyrite. Acail.

S ci. (P aris) C om ptes r end.us, 29\ 68-7 0.

Jercrn, F. M. ,u.to H. S. vns Kr-oosrnn (1912) Studien iiber natiirliche und ktinstliche

Sulf oantimonite und Sulf oarse rrite. Z eit. anor g. C hemie, 7 8, 245-268.

Mrnns, H. S. (1888) Contributions to the study of pyrargyrite and proustite. Mineral.

Mog.8,37-102.- AllD G. T. Pnron (1887) On a specimen of proustite containing antimony. Mineral.

Mag.7,196-200.Oncrr-, J. ,lNo G. R. Pr,rzt (1928) Etude microscopique de quelques minerais m6talliques

du P6rou. S oc. Jr anq. miner aI. BuIl., 51, 213-246.

P.tnnrsn, Wrr.r.reu (1933) X-ray reflection angle tables for several standards. Philips Lab.,

Res. Lab. Tech. Rept.28.Pnecocr, M. A. (1947) Artificial proustite and xanthoconite: Unio. Toronto Stud'ies, Geotr-

s er., 51, C ontr. C ana.d. M iner atro gy, 1946, 85-87.

Tour-urr, Pnrcsrlnv 3d (1958) Preliminary report on the system Ag3SbSr-AgaAsSi

(abs). GeoI. Soc. Am. 8u1L 69,7769.

Wmr-, RrN6 ero Rlvuono Hoc.mr (1953) Recherches expdrimentales sur Ia formation

des minerais d'argent. C. R. Congris soc, sattanl'es, Toulouse, sect, sci" 183-188.

WnnNrcr, J. H., S. Grr-r,Bn .um K. E. BoNsoN (1958) Synthetic proustite, A*AsSs AnlJ.

Chem.30,303.

Manuscri.pt receited, Ianuary 14, 1963; occepled' Jor publication, February 16, 1963.