American Mineralogist, Volume 77, pages 1266-1274, 1992

The light-induced alteration of realgar to pararealgar

D. L. DoucLASS. Cmcnl.Nc SnrNcDepartment of Materials Science and Engineering, University of California at Los Angeles, Los Angeles,

California 90024-l 595, U.S.A.

Gn WmcDepartment of Materials Science and Engineering, Ohio State University, Columbus, Ohio 43202, U.S.A.

Ansrru,cr

Realgar from various locations, high-purity synthetic realgar, and synthetic realgar with

2 rnolo/o Sb were subjected to unfiltered sunlight, filtered sunlight, and filtered light from

a quanz-rungsten-halogen lamp for various times. Both single-crystal and powder samples

were used. All of the realgar transformed to pararealgar at wavelengths between about 500

and 670 nm, but no transformation occurred at wavelengths greater than about 670 nm.

The reaction rate decreased at wavelengths above 560 nm and was very slow at wave-

lengths greater than 610 nm. The high-temperature polymorph,0, also transformed to

pararealgar. The alteration to pararealgar did not proceed directly; an intermediate phase,

1, of unknown crystal structure but having the same stoichiometry as realgar, formed first.

The time to form 1 increased with increasing wavelength at a constant flux of photons.

There was iro measurable difference in stoichiometry between realgar and pararealgar;

both were slightly hyperstoichiometric. Pararealgar forms on the surface as a thin layer or

nodules and then fissures at some critical thickness, causing the well-known degradation

ofrealgar exposed to light.The reaction is reversible at elevated temperatures. In all cases, even below the a-B

realgar transformation temperature, l always formed initially, followed by the formation

of p. The a phase then formed from the p phase at a rate dependent upon temperature.The time to transform B (formed during the reverse reaction) was about I d at 220'C and

2 d at 17 5 "C, whereas transformation of normal 0, cooled from above the transformation

temperature to either l7 5 or 220'C, takes several months.It is proposed that light breaks As-As bonds, which are weaker than As-S bonds, and

that the covalently bonded cage molecules form a new crystal structure in which free As

is intercalated. The behavior is compared to the photo-decomposition of orpiment, which

has been studied extensivelY.

INrnooucrroN nir, 1970), but there remains much confusion in the lit-erature. However, some significant observations made by

The alteration of realgar (AsS) to a friable yellow-or- Cahoon (1965) showed that a new phase formed duringange powder is well known and is a phenomenon that has the alteration of realgar from Pampa Larga, Chile. Hallc4used chagrin among most mineral collectors and mu- (1966) identified the alteration product as 7-AsS, whichseum curators. It is stated in most handbooks and min- formed by exposure to infrared radiation in a vacuumeralogy books that the alteration products are orpiment below 130'C. The 7-AsS formed from both natural and(AsrSr) and arsenolite (As,Or) (Battey, 1972; Sinkankas, synthetic realgar. Roberts et al. (1980) identified a new1966; VandersandKerr, 1967; Mottanaetal.,lgT8).The polymorph of AsS, pararealgar, from two locations information of AsrS, in ambient air is not possible, how- British Columbia, Mount Washington on Vancouver Is-ever, because this compound is less stable thermodynam- land and the Gray Rock property, Lillooet district. Rob-ically than realgar and thus has a higher equilibrium dis- erts et al. pointed out that pararealgar was the alterationsociation pressure ofS. The partial pressure ofS, even in product of realgar and that orpiment was not produced.

industrial environments, is much less than that required It is most likely that Hall's 7-AsS is actually pararealgar.

to form orpiment. Pararealgar appears to be an equilibrium phase in theMuch has been reported on the crystal structure of re- presence of light, but it is not included in the phase dia-

algar, the phase transformation, and stability of the poly- gram determined by Blachnik et al. (1980). The diagrammorphs (Clark, 1970; Ito etal., 1952; Lu and Donohue, in Figure I shows the existence of three sulfides, AsoSr,1944; Mullen and Nowacki, 1972 Porter and Sheldrick AsS, and AsrSr, all of which melt congruently. Eutectics1972:Robertsetal.. 1980:Roland.l972:streetandMu- occurbetweenAsoSrandAsandbetweenAsSandrealgar.

0003-oo4x/ 92/ | | | 2-r 266502.00 t266

Likewise, a eutectic exists between realgar and orpiment.No range of composition is given for realgar, which isshown as a line compound on the diagram. The transfor-mation temperature is given as 255 "C, although Roland(1972) reported that the temperature was compositiondependent, i.e., 239 "C in the presence of As, 252 "C forstoichiometric AsS, and 263 "C in the presence of AsrS,and S vapor. These results led Roland to suggest thatthere might be a slight range of stoichiometry of realgarwhich he stated is "certainly much less than 5ol0."

A spectacular specimen of realgar (crystals about 5-10mm in size) on a gray quartz and white calcite matrix,from the Getchell mine, Nevada, in the collection of oneof the authors (D.L.D.) altered significantly in artificialroom light with some direct sunlight (about I h/d in afew months). X-ray diffraction of the yellow-orange al-teration product clearly showed that pararealgar hadformed. The apparent light-induced transformation, thefact that pararealgar was not shown as an equilibriumphase on the phase diagram, and the fact that not allrealgar samples are prone to alteration (Clark, 1970)prompted research concerning the transformation in aneffort to understand the process. Prior to presentation ofresults obtained in this study, it is worthwhile to discusssome previous work on crystal structures and to pointout some of the inconsistencies among the variousstudies.

B,tcxcnouNo

The low-temperature structure is usually designated asrealgar (or p-AsS) and has a monoclinic structure (Ito eta l . , 1952 ) , w i t h a : 9 .27 , b : 13 .50 , c :6 .56 A . and 6: 106'37'. Mullen and Nowacki (1972) also found amonocl in ic s t ructure wi th a :9-325, b: 13.57 1, c :6.587 A, and 0 : 106o24', values very similar to those ofIto et al. The high-temperature form is usually designatedas a-AsS, although Street and Munir (1970) used a-AsSfor the low-temperature structure and B-AsS for the high-temperature form. Porter and Sheldrick (1972) also usedthe terminology of Street and Munir. Care must be takento ascertain whether a given reference used a-AsS for thehigh-temperature or for the low-temperature modifica-tion; much confusion exists over terminology. In dis-cussing the experimental results of the present work, wewill refer to the low-temperature form as a and the high-temperature form as 6.

The high-temperature structure was reported by Porterand Sheldrick (197 2) to be monoclinic with a : 9.957 , b: 9.335, c : 8.889 A, and A : f i229'. Roland (1972)prepared the high-temperature phase by reacting AsoS,with S vapor using materials of 99.999o/o purity. Rolandalso found a monoclinic structure with a : 9 .92, b : 9 .48,c: 8.91 A, and B: 101"50' . Street and Munir f l970)synthesized AsS from material of 99.99990/o purity anddetermined the high-temperature phase to be orthorhom-bic, with a: 9.95, b: 13.49, and c : 7.07 A. Their low-temperature form was found to be monoclinic with latticeparameters virtually identical to those of natural realgar

1267

& 5 0

Mol Vo

Fig. 1. As-S phase diagram (Blachnik et al., 1980).

from Humboldt County in Nevada. Kutoglu (1976) re-acted pure S and As at 500-600 iC and quickly cooledthe product to room temperature. The orange-yellowphase was monoc l i n i c w i t h a : l l . l 93 , b :9 .994 , c :7 .153 A . and 6 :9248 ' .

There seems to be good agreement for the low-temper-ature phase with respect to its structure (monoclinic), lat-tice parameters, and the value of 0. However, this is notthe case for the high-temperature phase. Porter andSheldrick (1972) and Roland (1972) reported that rhe high-temperature phase is monoclinic. The lattice parametersand values ofp determined by these researchers are rea-sonably similar. However, Street and Munir (1970) foundan orthorhombic structure, and Kutoglu (1976) found amonoclinic structure whose parameters differed consid-erably from those of Porter and Sheldrick(l972) and Ro-land (1972).

It should be noted, however, that Street and Munir's dvalues and intensities are very similar to those of bothClark (1970), see below, and Roland (1972), even thoughtheir structure analysis gave different results.

Roberts et al. (1980) determined that the structure ofpararealgar is monocl in ic ,wi th a:9.929, b:9.691, c: 8.503 A, and B : 97"4'. It is interesting to note thatthe new polymorph, pararealgar, has a structure whoselattice parameters are fairly close to those of the high-temperalure polymorph.

Clark (1970) studied realgar from Mina Alacr6n, Pam-pa Larga, Chile, and noted that specimens which hadbeen exposed to sunlight for at least 20 yr, as well asspecimens removed directly from the underground mine(no light), had identical X-ray patterns, and there was noevidence of alteration on the samples that had been ex-posed to sunlight. The patterns were identical to those ofHall (1966) for synthetic a-AsS, the low-temperature form.No changes were observed in Clark's samples after grind-

DOUGLASS ET AL.: ALTERATION OF REALGAR TO PARAREALGAR

Qo

F ,|t)0

6 0 7 0 8 0 9 0 s

S

t268 DOUGLASS ET AL.: ALTERATION OF REALGAR TO PARAREALGAR

Tlgle 1. Summary of experiments for light-induced alteration of realgar

Filter First appear-ance of Comolete alteration

pararealgar to pararealgarRadiation Type 7" TransmissionFirst appear-

ance of 1

SunlightLampSunlightSunlightSunlightSunlightSunlightl-ampLampLampSunlightSunlightLampSunlightSunlightLampLampLamp

nonenone

>450-nm pass>495-nm pass>530-nm pass>560-nm pass>570-nm pass>61o-nm pass<500<547

4 5 0 + 54 7 0 + 3505 + 205 1 5 + 5532 + 10540 + 155 5 5 + 25 7 5 + 2

1 .25 h42

-3-3

I1 2

- 190none al 272none at 30none at 32

-6none at 30

<93I

20

0.9 h< 1 8<8<6

220

-60-277none at272

30none at 32

20none at 30

<9330

7

6-12 h26I7

307

> 1557

a + P a t 1 4 72

15427

> 154> 154none at 1 54

100100

30 at 560-95 at 59550 at 570-95 at 620

0 at 500. 25 at 480. 73 at 400

426

835359634724

ing under toluene and subsequent exposure to sunlightfor over 2 yr. Exposure to infrared radiation also failedto cause alteration. Samples examined from the MinaAlacrdn dump showed no alteration from long-time ex-posure to light. Surprisingly, material removed from thecenters of those samples gave X-ray patterns of the high-temperature polymorph, the patterns being identical tothose previously given by Cahoon (1965), Hall (1966),and Roland (1972). Sunlight exposure of these samplesfor 2yr failed to cause alteration. Although Cahoon (1965)reportedly observed alteration of realgar from PampaLarya in Chile, Clark's work suggests that the presence ofthe high-temperature phase may have a relationship tothe alteration process. As stated earlier, a comparison ofthe lattice parameters and so forth of the high-tempera-ture form as determined by Porter and Sheldrick (1972)and by Roland (1972) to the new polymorph, pararealgar,shows that very little difference exists between the twostructures.

ExpnmprrNTAL PRocEDURES

Both natural realgar and synthetic compounds wereutilized. The natural realgar was in the form of irregularpieces about 0.5-l cm in size. These included materialfrom the Getchell mine in Nevada, supplied by AnthonyR. Kampf, Natural History Museum of Los AngelesCounty; from Sacramb (Nagyag), Romania, supplied byCarl A. Francis, Harvard University Mineralogical Mu-seum (catalog no. l3l 525); and from Hunan Province,China, supplied by Chris Wright, Hot Springs, Arkansas.The natural realgar samples were mounted in lucite andground and polished to give a flat, mirrorlike finish.

The synthetic samples were prepared from As and S of99.99990/o purity. Stoichiometric amounts of each ele-ment were weighed and placed in a vitreous silica am-poule. The ampoule was evacuated and sealed under vac-uum. The ampoules were placed in a furnace held at 280"C (above the transformation temperature but below themelting point) and at 240'C (ust below the transfor-

mation temperature). The samples were maintained atthe desired temperatures for 24 hr, cooled to room tem-perature, and subjected to X-ray diffraction (XRD), usinga G.E. diffractometer with filtered CuKa radiation. Otherampoules were heated above the melting point and thencooled. In all cases, the high-temperature structure (des-ignated as 0) was retained at room temperature. Trans-formation of the P phase to the low-temperature a phaseoccurred after annealing for several weeks at 240'C. Thehigh-temperature phase was remarkably stable at roomtemperature, as previously noted by Street and Munir(1970). The synthetic realgar formed as rather small par-ticles; thus they were ground further prior to investiga-tion. All of the synthetic materials were tested as pow-ders.

The bulk natural samples were exposed to light in theirmounts. The powder samples of either synthetic materialor of pulverized natural crystals were placed on a double-sided cellophane tape and attached to a plastic plate forlight exposure and subsequent X-ray diffraction. The bulksamples were used for the observation of a given area inthe SEM, after light exposure for various periods, to fol-low the transformation microscopically.

Realgar from all sources was exposed initially to unfil-tered sunlight. In order to determine if the alteration wasenhanced by various wavelengths, a series of filters wasused. A UV-IR pass filter was used initially. This filterblocked out all of the visible spectrum and passed bothUV and IR. No transformation occurred after two weeks.This result clearly showed that wavelengths in the visiblespectrum were required. Accordingly, long-wavelength-pass f i l ters of455,495, 510, 530, 560, 570, and 610 nmwere used. In addition, two short-wavelength-pass filtersof 500 and 547 nm were used. Several narrow-band-passfilters in the range of interest were employed also. Theircharacteristics are described with the results ofthese ex-periments in Table 1.

It was of considerable interest to determine if realgaris stoichiometric or if it exhibits a range of composition,

TrEu 2. XRD data for the x phase

d(A) tl t;

5.815.745.054.774.183.963.303.243.193.012.87

'Obtained from plot of 20 vs. /, G.E. diffractometer

and if a range exists, if it is related to the transformation.High-purity materials were used to prepare AsoSr, AsS,and AsrS.. An evacuated capsule was prepared in whichAsoS, was placed at one end and AsS placed at the otherend. A similar experiment was performed with AsS andAsrSr. Long-time experiments of two months were per-formed at 240'C so that equilibrium could be attained.If AsS exists over a range of composition, the capsulewith AsoS, should produce AsS,_", and the capsule withAsrS. should produce AsS,*". After exposure the sampleswere analyzed by XRD and electron microprobe analysis(EMPA) using a Cameca Camebax electron microprobeoperated at 15 kV and 12 mA, with pure As and pure Sas standards. Natural realgar and pararealgar producedby the transformation were analyzed also by EMpA andXRD.

Photomicrographs were made in a Cambridge Stereo-scan scanning electron microscope (SEM). It was possibleto obtain good images in some cases without coating thesamples. In other cases, the samples charged, and poorimages were obtained. It was decided not to coat the sam-ples with C during the experiment, because the coatingwould absorb visible radiation. Coating was performedonly after an experiment had been finished.

Optical transmission was measured with an Hp 8452Adiode array spectrophotometer. Measurements were madeon single crystals that had been mounted in resin andpolished. The polished mount was sliced to produce awafer 0.1 cm thick. The cut side was polished, and thesamples were then subjected to measurement.

The resistance of the Alacr6n (Chile) realgar to alter-ation, (Clark, 1970), raised the question of whether thepresence of the high-temperature B phase in that materialmight have had an influence. Also, the fact that Sb iscommonly detected in most realgars suggested that per-haps impurities might affect the alteration. Synthetic re-algars were prepared with pure As, S, and Sb (99.99990/opurity). AsS and AsS containing 2 molo/o Sb were pre-pared by melting in evacuated quartz ampoules.

Rnsur_rsWavelength effects

All realgar altered to pararealgar in unfiltered sunlight.XRD patterns show peaks for both realgar (designated as

t269

7 0 h

I

' " eie. 2." enec: o,

"**r*. ;-. ; r.-; r*r-*1"***-0"*filter) showing transformation to x, to the transition phase, andultimately to pararealgar.

a hereafter) and pararealgar (P). No reaction occurredduring exposure for at least one month in the laboratoryunder fluorescent lighting.

It was of interest to determine if ultraviolet (UV), vis-ible, or infrared (IR) radiation caused the alteration. Ac-cordingly, a filter was used that passed both UV and IRbut screened the visible radiation. Single-crystal samplesof Getchell realgar were exposed for 85 h. XRD showedonly a peaks with no evidence of alteration.

Once both UV and IR were ruled out, a series of long-wavelength-pass filters was used. No alteration was ob-served by exposure for 70 h to light having wavelengthsgreater than 6 l0 nm.

A 570-nm-pass filter was used next, the results of thistest giving a most unexpected result. Starting at 2 h ex-posure, some low-intensity XRD peaks were observedthat could not be indexed as a, B, or P. The unknownpeaks increased in intensity with increasing exposure time,whereas the a peaks decreased in intensity. Both the un-known peaks, corresponding to an unknown phase (here-after referred to as 1), and a peaks coexisted through 36h exposure. At 58 h, ll peaks existed, all of which wereunidentifiable. The change in the XRD patterns as a func-tion of exposure time is shown in Figure 2. XRD datafor the 1 phase are summarized in Table 2. The XRDpattern for this phase is typical of that of a layered struc-ture, as noted by line broadening and asymmetrical peaks(Wagner, personal communication, l99l). There appear

DOUGLASS ET AL.: ALTERATION OF REALGAR TO PARAREALGAR

2 52e

2 52e

10060451 5

c

251 025't0

6055

2 3 h

x

1

3 6 h

x

1 2 h

I "n lx

t270

Fig. 3. SEM micrograph ofGetchell realgar exposed 18 h tounfiltered sunlight. The flakelike sheets on the surface are para-realgar in the process of spalling from the realgar substrate.

to be some peaks masked by the more intense peaks; thusthe pattern probably has many more peaks not ngted inthe 1able.

Continued exposure of this sample with the 570-nmfilter resulted eventually in the formation of pararealgar,

which coexisted with the 1 phase at 78 h. A further in-

crease in the exposure showed that the intensity ofthe 1peaks decreased, whereas the intensity ofthe pararealgarpeaks increased, showing that pararealgar formed at theexpense ofthe 1 phase.

A similar set of experiments to the above was per-

formed with the 560-nm filter. No changes were detectedduring 4-h exposure. A small amount of 1 was detectedat 8 h, and at 16 h the sample had been completely trans-formed to y. Pararealgar appeared after 20 h, and both 1and pararealgar existed at 30 h. The behavior of realgarexposed under the 560-nm filter is basically the same asthat exposed to the 570-nm filter. However, there was a

Fig. 4. SEM micrograph of single-crystal Romanian realgarexposed to sunlight with a long-wavelength-pass filter of 570 nm.The light glanular material is pararealgar.

DOUGLASS ET AL.: ALTERATION OF REALGAR TO PARAREALGAR

Fig. 5. SEM micrographs of single-crystal Chinese realgar ex-posed 38 hours to unfiltered sunlight. (a) Sheets of pararealgarpartially detached from the realgar substrate, (b) the fine struc-ture ofpararealgar on the sheets'

marked difference in the rate of alteration. Complete al-

teration occurred in 8 h with a 455-nm filter and in 30 h

with a 530-nm filter, both the 1 phase and pararealgar

coexisted at 70 h with a 560-nm filter, only the 1 phase

formed during 70 h with a 570-nm filter, and' finally, no

alteration at all was observed after 70-h exposure with a

610-nm fi lter.

SEM micrographs of alterations

Pararealgar appears to form on the realgar surface as a

thin layer that initially adheres to the surface. It is not

known if the two phases are epitaxially related. The para-

realgar is apparently less dense (more voluminous) than

a realgar, and eventually a limiting thickness is reached

at which the adherent paratealgar fissures and spalls. An

example of this behavior is shown in Figure 3 of a single

crystal of Getchell realgat.The 1 phase is seen in Figure 4 on a single crystal of

Romanian realgar (HR2), which was exposed for 58 hwith the 570-nm filter. The patatealgar is the light phase

that charged in the SEM. This is indicative of a lower

TABLE 3. Electron microprobe analyses

RealgalAs (at%) S (at%)

127 |

o

oo

I

0

RVEIAGM

Fig. 6. Absorbance spectnrm of single-crystal Romanian re-algar. sample I mm thick.

Effect of crystal structure and inpurities

XRD of high-purity synthetic realgar with and withoutSb showed that both materials consisted of 1000/o B atroom temperature. Exposure to unfiltered sunlight for l0h caused a nearly complete alteration to pararealgar. Bothof the exposed samples contained a small amount of theX phase.

It afpears that it makes no difference whether the start-ing material is the monoclinic a or the monoclinic andorthorhombic 6 phase. The behavior was identical. It alsoappeared that the presence ofSb had no effect. It shouldbe noted that the Sb was in solid solution, since no evi-dence of SbrS, was detected. EDX analysis of the realgarcontaining Sb readily detected the Sb.

Optical transmission

A typical absorbance spectrum of a single crystal of arealgar from Romania, which was about 0. I cm in thick-ness, is shown in Figure 6. The absorbance was essentiallyl00o/o at wavelengths below 565 nm and decreased rap-idly with increasing wavelength above this value, show-ing a sigmoidal curve.

Reversibility of the transformation

Samples of l00o/o pararealgar formed by aiteration ofpulverized natural realgar from all three sites (China, Ro-maina, and Getchell), were sealed in quartz ampoulesunder vacuum and heated at various temperatures. Sam-ples heated to 300 "C, in the B-phase field, transformedto B in 5 min. Samples held at 420 "C, above the meltingpoint of realgar, also formed B in 5 min. In every casethe color changed from yellow-orange to bright red.

Experiments performed at various temperatures withinthe a-phase region yielded some unexpected results. First,heating to 220 "C produced B in 5 min even though thelowest reported a-B transformation temperature is 239"C. A Getchell sample was held for two months at 220'C and was transformed to equilibrium a phase duringthis period. Treatment at 17 5 "C showed no reaction in5 min, but the unknown X phase formed in 50 min. Lon-ger times at l7 5 "C produced B. Reaction at 190 "C for Id resulted in a mixture of all four phases, P, y, g, and a.

DOUGLASS ET AL.: ALTERATION OF REALGAR To PARAREALGAR

LAG2

LARl

DG1

DG2

HR3

wc2

DG1

47.6547.7147.8347.9948.0347.9247.6647.7747.9647.2147.2147.4248.4548.4948.2447.6146.5847.12

Pararealgar46.9048.0347.2647.4647.4148.62

s l .9251.8951.7251.9951.9452.0452.2852.1 I52.0152.4352.4652.2451.5551 4951.7652.3653.41s2.88

53.1051.9752.7452.5452.5951.38

ivotej D : Douglass collection, H : Harvard, W : Wright's Rock Shop,LA : National History Museum of Los Angeles, R : Romania, C : Chini,G : Getchell.

electrical conductivity than that ofeither realgar or oftheX phase.

Another single crystal, Chinese realgar (WCl), formeda sheet of the 1 phase that partially spalled and that ex_hibited a series of parallel fractures, as noted in Figure5a. A higher magnification of the sheets (Fig. 5b) showsmany small particles, which are pararealgar. pararealgarnucleates on the x phase, which is a precursor that ap-pears to be necessary for pararealgar formation.

Compositional variations

EMPA was used to determine the composition and todiscern if nonstoichiometry existed. High-purity As andhigh-purity FeS, were used as standards. Three areas wereanalyzed from numerous crystals. The S content was foundto always be greater than 50 at0l0. The accuracy of themeasurements is +0.50/0, and, since the S content wasapproximately 52 ato/o, it was concluded that a realgar isindeed S rich. Data are summarized in Table 3.

The composition of pararealgar could not be deter_mined accurately by wavelength-dispersive (WDX)EPMA. This is due to the flocculent, porous nature ofthis phase. Accurate EPMA requires a fully dense phasewith a planar surface. EDX was therefore employed, al-though it may be inherently less accurate than WDX.Nevertheless, the crystals previously analyzed, by WDXwere used as standards for EDX. Within the accuracy ofthe technique there was no compositional difference be-tween realgar and pararealgar. They are both slightly hy_perstoichiometric. (Hyperstoichiometry can result fromeither As vacancies or interstitial S.)

t272

TlaLe 4. Summary of reverse reaction, P -. a

DOUGLASS ET AL.: ALTERATION OF REALGAR TO PARAREALGAR

Phases formed at various times-

3 d2 d1 d3 ht h10 m inr fc)x ' Ax , A

1 a R

190220230300420

P + 1 ( t r )x 'A

'too% p

x , 9 , oA ' x ' olo0/o a

a, x (tr)

1OO% p

100% I

A/ote.'tr: trace.. Listed in order of abundance.

A summary of results obtained during the reverse reac-tion is given in Table 4.

Irradiation using a Tungsten-quartz-halogen lamp

The vagaries of weather prompted the use of artificiallight. This provided a constant source of irradiation 24h/d as opposed to the variations in sunlight intensity withtime ofthe day, day ofthe year, partial cloud cover, etc.

The lamp used was a QL-500-WL manufactured by theRegent Lighting Corporation. A photocell indicated thatthe power density at the sample surface was 53 mw/cm2at a distance of30 cm (about 65-700/o ofnatural sunlight)and 28 milcm2 at a distance of 45 cm. Initial experi-ments were performed with the light at a distance of 30cm from the samples. Temperature measurements showedthat the sample temperature reached 60 'C. The some-what elevated temperature eventually led to some AsrO,formation after long times. Complete oxidation occurredin 1200 h. In order to reduce the temperature, the sam-ples were placed 45 cm from the light. The temperaturereached 40'C.

Results obtained were basically the same as those ob-tained in sunlight. The times required for initiation andcompletion of alteration were slightly longer with the lampthan with sunlight because of the reduced intensity ofradiation. The most significant observation was that long-time exposures with the 610-nm long-wavelength-passfilter caused alteration, and that the 610-nm wavelengthwas not a cutoff value below which alteration occurredand above which it did not. For example, 1 and a coex-isted after 190 h, and pararealgar started to form after277 h.

DrscussroN

The alteration ofrealgar to pararealgar is dependent onthe wavelength of the light to which the realgar is ex-posed, as determined from a number of experiments em-ploying various types of fllters. It appears that no alter-ation occurs at wavelengths shorter than about 500 nm.The exact wavelength value below which there is no re-action is difficult to determine because filters do not havestep-function characteristics. It is clear, however, thatvisible light is required, because no alteration was ob-served with a filter that passed both UV and IR radiationbut blocked out all ofthe visible. Realgar from all sources

behaved virtually identically. There were no discernibledifferences.

The upper wavelength limit for alteration was not de-termined. Transformation did occur with a 610-nm long-wavelength-pass filter after very long times compared withshorter wavelengths. This behavior is attributed to theabsorption and transmission characteristics of realgar.There is zero transmission up to about 575 nm, at whichvalue transmission increases rapidly with wavelength,following a sigmoidal relationship. This means thatwavelengths greater than 610 nm are basically passing

through the crystal without being absorbed, and little en-ergy is being transferred to break bonds.

The fact that alteration takes so much longer with lon-ger wavelengths, which are mostly transmitted throughthe crystal, strongly suggests that a certain number ofphotons of light is necessary to cause the reaction to takeplace. Another experiment that supports this hypothesisis the one in which a narrow-band filter of 575 nm wasused. This filter was very sharp, passing 575 + 2 nm witha total transmission of only 24o/o. Thus, the number ofphotons passing through this filter was extremely small,and no alteration was observed after 154 h.

A kinetics effect (described below) was clearly apparentin the range of 560-6 l0 nm. Long-wavelength-pass filtersof530, 560, 570, and 610 nm (Table 5) showed that fora given time, 30 min, complete alteration to pararealgaroccurred with the 530-nm filter, but no alteration hadbeen observed with the 610-nm filter. Since the filterspass all wavelengths greater than the indicated value, thereis apparently a higher efficiency of light in the range of530-560 nm for alteration than there is above 560 nm.All wavelengths are absorbed in the range of 530-560nm; thus the energy of the incident light is important inaddition to the number of photons absorbed.

The alteration process involved formation of an inter-mediate phase, 1, for which the X-ray diffraction patterncould not be identified in the JCPDS file. The phase

seemed to be the precursor to pararealgar, but its presencewas difrcult to detect in experiments involving shorterwavelengths of 500-530 nm because the reaction was somuch faster than at wavelengths greater than 560 nm.The unknown phase appears to have a layered type ofstructure based on the shape of peaks in the X-ray pat-tern. It was not known initially if either 1 or pararealgar

TABLE 5. Effect of wavelength on realgar alteration

Phase(s) observed afterFilter- 30 min exposure 7o absorption

>530 nm>560 nm>570 nm>610 nm

1273

Fig. 7. Structure ofd realgar, [001] projection, showing thelocation of the cage molecules Mullen and Nowacki (1972). Thecage molecules, referred to as bisphenoids, are covalently bond-ed. The cages are held together by weak Van der Waal's forces.The 0 phase has similar cage molecules, but the molecules arearranged differently compared with the a phase.

terial taken from the mine did not alter under direct sun-light. The unique aspect to this realgar was the presenceof the high-temperature phase, P, which is not usuallypresent. Perhaps the closeness of the structures of bothpararealgar and B precludes the formation ofpararealgar.It would be of great interest to study this material if somewere available.

Another perplexing aspect is what the light is actuallydoing to the realgar structure. Let us examine the struc-ture in more detail. Realgar is most properly representedby the formula AsoSo. This is a cage stnrcture (sometimesreferred to as a cradle structure) in which As atoms forma tetrahedron (also referred to as a bisphenoid) and asquare of S atoms. The cage molecules are covalentlybonded. Each As atom is bonded to three nearest neigh-bors, two S and an As atom. The crystal structures ofboth a realgar and B realgar form by joining of the tet-rahedra by weak Van der Waals forces, as seen in Figure7, a unit-cell projection on [001] of a (Mullen and No-wacki, 1972). Dimensions of the cage structure (Ito et al.,1952) are as follows: As-S bonds : 2.24 A, As-As bonds: 2.59 A, As-S-As angle : l0lo5', S-As-S angJe : 928',and S-As-As angle : 97'8'. Yang et al. (1987) studied thetemperature dependence of extended X-ray absorplionfine structure spectra and concluded that the As-S bondsare about 300/o stronger than the As-As bonds.

Keneman et al. (1978) researched the behavior of or-piment and realgar when exposed to light and thermalcycling in order to understand the optical-storage effectofthese compounds. They propose that photodecompo-sition causes the following reaction in orpiment: AsrS, -yAs + As, uSr, which results in the formation of clustersof free As. The reaction was reversible upon heating.However, their work was on thin vapor-deposited films,and it appears from their remarks that equilibrium didnot exist in the films. Unexposed films reportedly con-sisted of AsrSr, AsrSr, and S.

DOUGLASS ET AL.: ALTERATIONOF REALGAR TO PARAREALGAR

100% pararealgarx + pararealgarlOOo/" y1O0o/" q

100100-67- 1 0

' All long-wavelength-pass filters

were truly equilibrium phases. Consequently, a series ofexperiments was performed which involved heating ofthe pararealgar to various temperatures while it was sealedin evacuated ampoules to prevent either oxidation or lossof S.

The reverse reaction, P - a, is more complex than theforward reaction, and it too yielded a most surprisingresult. Heating of pararealgar in the B region, e.g., 300'C,or above the melting point, e.g., 420 "C, both producedvery rapid transformation of pararealgar Io B. The p wasretained upon cooling to room temperature. As men-tioned earlier, transformation ofB to a occurs only afterlong times at temperatures close to the transformationtemperature, as determined by Roland (1972) and con-firmed by us. However, the reverse reaction in the a-phaseregion resulted in the formation of y at 175 "C in 50 minand B at 220 and 230 "C in 5 min. The formation of 6 attemperatures markedly below the a-B transformationtemperature is most unusual, but it may be related to thel phase, which appears to be an intermediate precursorbetween pararealgar and B during the reverse transfor-mation. The B formed during the reverse reaction at 220'C transformed to d after only I d, compared with a timeof several weeks for normal 0 (0 not involved in the re-verse reaction). Obviously, the temperature of the furnacewas well below the a-B transformation temperature, andthe fact that p formed prior to a cannot be attributed toan extraneously high temperature.

The above experiments strongly suggest that both 1and pararealgar are equilibrium phases. Transition phasesoccur commonly in many solid-state transformalions; aclassical example is the age hardening of Al-Cu alloys(Shewmon, 1969). Quenching of the high-temperaturesolid solution (supersaturated a) to an intermediate tem-perature below the solvus causes formation of Guinier-Preston zones, followed by formation of d", d', and even-tually equilibrium d in a solid solution of a that has theequilibrium composition given by the Al-Cu phase dia-gram. Reheating does not involve any of the transitionphases. They are metastable, and their formation occursbecause each one provides a small decrease in free energy,eventually leading to equilibrium.

All realgar that was tested readily altered to parareal-gar. The samples included material from the Getchell minein Nevada, from China, and from Romania. The mostdifficult thing to explain is why the realgar from PampaLarga, Chile (Clark, 1970), did not transform after 20 yrof exposure to sunlight in the mine dump, and why ma-

.=14t!

1274

Orpiment is also a molecular structure in which cova-lently bonded units join by Van der Waals forces to formthe overall crystal structure. Ito et al. (1952) noted thatthe realgar and orpiment structures bear little similaritybut that practically the same bond distances and anglesexist in each. Because the bonding of the cage units issimilar (even though the crystal structures of realgar andorpiment differ) and As-As bonds are weaker than As-Sbonds, one can speculate that light creates free As in re-algar that remains in the structure (thus explaining thesame stoichiometries of realgar and pararealgar), but theunits then rearranged to form the pararealgar crystalstructure. This behavior can be viewed as intercalation,which is well known in layered structures. A good case inpoint (Chen and Dorrglass, 1990; Wang et a1., l99l) in-volves intercalation of cations such as Nir* and Fe2* intothe layered structure of MoS, during sulfidation of Ni-Mo and Fe-Mo alloys. The intercalated ions fit into oc-tahedral interstices between the covalently bonded sheetsof MoSr, which are held together by weak Van der Waalsforces. However, Al3* is much smaller than the site of theoctahedral hole, and destabilization of MoS, occurs,causing the formation of Alo rMorSo, an inverse spinel.Thus, it is possible that free As forms by interaction oflight with realgar, and the free As destabilizes the realgarstructure, even though the basic molecular cage units re-main unchanged. This process, as in the case of orpiment,is reversible upon heating.

AcrNowlnocMENTS

Although this research was unfunded, there are many who offered gen-erous support. First, thanks are given to those who supplied samples.These people include Carl A. Francis, Mineralogical Museum, HarvardUniyersity; Peter C. Keller and Anthony R. Kampf, Natural History Mu-seum of Los Angeles County; and Chris Wright, Wrighfs Rock Shop, HotSprings, Arkansas. Second, numerous lengthy telephone conversations wereheld with a number of individuals early in the program, who providedmuch useful information on background, etc. These include Peter Kellerand Anthony Kamp{ Natural History Museum of Los Angeles County;Gail Dunning, Sunnyvale, California; Carl A. Francis, Harvard Univer-sity; Taras Bryndzia, University of Chicago; Alan Clark, Queen's Uni-versity, Ontario, Canada; and George Rossman, California Institute ofTechnology. Finally, the comments from C.N.J. Wagner, UCLA, con-cerning the X-ray patterns and suggestions from Jeffrey Zinc, IJCLA,about photochemistry were most helpful.

Figure I is reprinted here with the permission of Barth Verlagsgesell-schaft; Figure 7 is reprinted with the permission of R OldenbourghVerlag

RrrnnrNcns crrEDBattey, M.H. (1972) Mineralogy for students, 323 p. Oliver and Boyd,

Edinburgh.

DOUGLASS ET AL.: ALTERATION OF REALGAR TO PARAREALGAR

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Mnm;scrrr"r REcETVED Ooosrn 15, l99l

MeNuscnrrr AccErrED Jr:w 19,1992