American Mineralogist, Volume 73, pages487499, 1988

Experimental investigationof the effect of oxygen fugacity on ferric-ferrous ratios and unit-cell parametersof four natural clinoamphiboles

Cpr,r,q.A. Cr-own,* Ronnnr K. Popp Department of Geology, Texas A&M University, CollegeStation, Texas 77843, U.S.A' SrBvnN J. FnIrz Department of Earth and Atmospheric Science,Purdue University, West Lafayette,Indiana 47907, U.S.A.

Ansrucr To investigate the relation between oxygen fugacity, ferric-ferrous ratio (R), and unit- cell parameters,four natural clinoamphiboles (grunerite, tschermakitic hornblende, mag- nesio-hornblende,and a riebeckite-arfvedsonitesolid solution) were reacted at 650'C, I kbar at oxygen fugacitiesdefined by several solid oxygen-bufferassemblages. In order to produce more highly oxidized samples, heating in air at 700 'C was also carried out. Variation in R is accomplishedmainly by the oxidation-dehydrogenationequilibrium Fe2* + OH : Fe3* * 02 + t/z}{r, but the results suggestthat other mechanismsmay also be involved. All four amphiboles exhibited systematically higher ferric-ferrous ratios with increasing6, of equilibration. Equilibrium R values were achieved relatively rapidly and could be readily restored to onginal values by treatment at the appropriate buffer. In some cases,a metastableequi- librium of ferric-ferrous ratio was achieved before the amphibole decomposedto other Fe3*-bearingphases. Of the four amphiboles, grunerite is apparently the least able to accommodate Fe3* within its crystal structure and decomposesat relatively higher oxygen fugacities. The value of a sin B decreasessystematically uniformly as the Fe3*content of tscher- makitic hornblende, magnesio-hornblende,and riebeckite increases,reflecting increasing Fe3*in the octahedralcation sites.The variation in a sin B ofgrunerite is significantly less than for the other three amphiboles. The variation in b for the two hornblendessuggests that Fe3*produced by oxidation is not strongly ordered into the M(2) site.

INrnolucrroN gen fugacity stability have been carried out on a wide variety compositions representing the major Amphiboles are the most chemically complex of the of bulk groups (e.g.,Gilbert et al., 1982). major rock-forming mineral groups. This complexity chemical of amphiboles results of the existing experimental studies have arisesfrom the wide variation of unique structural sites The in chemical compositions and phys- available for cations. Fe is a major component in many shown that changes properties as a function ofoxygen fugacity and naturally occurring amphibole solid solutions, but the ical occur of Fe in an amphibole resultsini- factors that control the proportions of Fe3* and Fe2* in temperature.Oxidation formation of oxy-amphibole component any given amphibole are still very poorly understood tially from the quantitatively. by a dehydrogenationreaction ofthe type Two types of experimentshave been carried out in at- Fe2** OH- : Fe3*+ Ot + V2H2, (l) tempts to produce variation in the ferric-ferrous ratios in amphiboles: heating of amphiboles in air, and hydro- in which O' replacesOH- in the O(3) anion site of the thermal synthesisand/or treatment of amphibolesat oxy- amphibole. This process,first suggestedby the results of gen fugacitiesdefined by the standard solid oxygen buff- Barnes(1930), has also been proposed to be operative in ers. The earlier air-heating studies were carried out mainly other hydrous minerals such as the micas (e.9., Wones, on abestiform amphiboles, but more recently, nonasbes- 1963;Vedder and Wilkins, 1969).Most studiesin syn- tiform varieties have been so-treated (see Hawthorne, thetic systemsare complicated by the fact that 1000/oyields 198I , 1983, for reviews).Hydrothermal studiesto delim- of amphibole are not achieved. Therefore, changesin it physical properties and the pressure-temperature-oxy- either the proportions or compositions of the nonamphi- bole phasesmay causevariations in amphibole cornpo- * Present address: Department of Geological Sciences,The sitions. That is, changesobserved in physical properties University of Texasat Austin, Austin, Texas78712, U.S.A. may result from changesin amphibole bulk composition 0003-004x/88/0506-0487$02.00 487 488 CLOWE ET AL.: Fe3+/Fe'z+AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES

TABLE1. Compositionsof startingamphiboles rolite grade amphibolite (sample 73-318, Spear, 1982); a mag- nesio-hornblendefrom Strickland quarry, Portland, Vermont, Tschermakitic Magnesio- for which petrologic information has not been published (Na- Grunerite hornblende hornblende Riebeckite tional Museum of Natural History specimen A6169); and a rie- sio, 49.19 43.44 47 05 51.39 beckite-arfvedsonitesolid solution, hereafter referred to as rie- Tio, 0.03 0.60 0.31 0.78 from a granitic pegmatite(Scofield and Gilbert, 1982). Al,o3 0.51 13.82 10.24 0.80 beckite, FeO 44.30 12.53 12.54 21.67 Fe,O" N.D. 3.70 1.05 11.85 Separationtechniques MnO 0.66 0.25 0.74 0.71 MgO 3.21 9.80 12.24 0.25 In order to ensureclose to 100o/opurity in the starting mate- CaO O.32 11.57 11.93 0.15 rials, each amphibole sample was subjectedto a variety of sep- Na,O 0 04 1.45 1.13 8.87 aration techniques.All sampleswere disaggregated,washed in K,O N.D. 0.51 0.59 1.54 magnetic and heavy-liquid sepa- F 0.17 0.13 0.72 1.97 acid, subjectedto isodynamic cl o.12 N.D. N.D. N.D. ration techniques,and hand picked to produce 99-1000/opurity. Sum 98.55 97.80 98.54 99.98 Additional details ofthe separation techniques are given in Clowe si 7.94 6.41 6.87 800 (1987).The final grain sizesofsamples are reportedin Table 1. 'vAl 0.06 1.59 1.13 With the exceptionof a small proportion of crushedgrains, there vrAl 0.04 0.83 0.64 0.15 was no changein amphibole gmin size during runs. Ti 0.01 0.07 0.04 0.09 Fe,* 5.99 1.54 1.54 2.83 Fe3* 0.42 o.12 1.40 Phase identification and characterization Mn 009 0.04 009 0.09 Electron-microprobeanalysis of the starting materials and se- Mg 0.78 2.16 2.67 0.06 Ca 0.06 183 1.87 0.03 lected run products was carried out using the nnI- instrument in Na 0.10 0.41 0.32 2.68 the Department of GeologicalSciences at Virginia Tech, Blacks- K 0.09 0.11 0.30 burg, Virginia. The analytical proceduresand operating condi- cl 0.01 tions were essentiallyidentical to those describedby Solbergand F 0.09 0.06 0.33 0.98 o 23.00 23.00 23.00 23 00 Speer(l 982). Fe3+/Fe2+ratios were determined using a single-dissolution Grainsize (mm) Max 1.0 x 0.025 0.19 x 0.025 0.5 x 0.1 0.15 x 0.05 techniqueapplicable to small samplesizes (Fritz and Popp, 1985). Min 0.1 x 0.025 0.075 x 0.075 0.1 x 0.2 0.1 x 0.02 Sample aliquots ranging from 0.95 to 11.96 mg were digested with HF and H'SO. in the presenceof o-phenanthroline, after Nofe; N.D. : not detected. Chemicalcompositions (in wt% oxides and as atoms per 23 oxygens) and grain sizes of amphibolesused in experi- which Na-citrate and boric acid solutions were added. The so- ments. Chemicalcompositions were determinedby electron microprobe. lutions were then analyzedcolorimetrically for FeO. Total Fe as Ferric-ferrousratio determinedby wet-chemicalanalysis. Amphibole com- FerO, was determined by atornic absorption spectrophotometry positions were normalized to 23 oxygens because of unknown oxy-am- of the sampleswith the quartz blank used ohibolecontent. after further dilution in the FeO determination. Duplicate solutions were made for each sample in the colorimetric analysis for FeO, and each of those solutionswas diluted and analyzedtwice for FerO,. Ferric- as oxygenfugacity varies, rather than solely from changes ferrous ratios, reported as molar Fe3*/(Fe3*+ Fe'?*)and abbre- in the ferric-ferrous ratio. For most synthetic hydrother- viated R, are consideredto be preciseto +0.01 (Fritz and Popp, mal studies, precise chemical compositions and, more 1985). importantly, ferric-ferrous ratios of the amphiboles have A complication arisesin electron-microprobe analysesofphases not been determined routinely. that contain oxy-amphibole component becausetotal Fe is gen- reported as FeO and the formulas are normalized to 23 This study reports the results of experimentsdesigned erally oxygens.For an end-member grunerite, for example, this pro- to elucidate further ferric-ferrous equilibrium in amphi- cedureis equivalent to expressingthe chemical formula in terms boles.Four natural amphiboleswere annealed over a range of oxideswith one molecule of water presentin the stucture,i.e., ofoxygen fugacitiesat constanttemperature and pressure : to determine the variation in Fe3* content and in unit- Fe'SirO',(OH), TFeO'8SiOz'H:O. cell parameters.Natural amphiboleswere chosenin order The formulas are normalized to 23 oxygensbecause the electron to avoid the complication of less than 1000/oyields in microprobe cannot analyzefor H, and therefore, the amount of synthetic systemsand also to provide crystals large enough oxygen necessaryto form "hydrogen oxide" is not included in for single-crystalstructure refinements. Results that de- the normalized formula. In the caseof a totally dehydrogenated scribe the variation ofoptical properties as a function of grunerite,it is necessaryto normalize microprobe analysesto 24 23 oxygens,i.e., ferric-ferrous ratio and the results of crystal-structurere- rather than finements have been presentedelsewhere (Phillips et al., Fel"Fel-SirO.o: 5FeO FerOr'8SiOr. 1986;Cloweand Popp, 1987;Phillipset al., 1988). Thus, normalization of the formula of an amphibole containing a significant oxy-component to 23 oxygensresults in a total ox- ExpnnrvrnurAl METHoDS ide weight percentthat is too low by a factor that representsthe Four natural clinoamphibolesrepresenting the three major additional oxygenpresent as FerO. rather than as FeO. To cor- amphibolechemical groups (iron-magnesium, calcic, and sodic rect an analysisbased on 23 oxygensto the proper number of amphiboles)were selected for the study.These amphiboles in- oxygens,(1) the total elementalweight percentof Fe is multiplied cludea gruneritefrom metamorphosediron formation(sample by the ferric-ferrousratio (R) to determine the absoluteelemen- 1,Klein, 1964);a tschermakitichornblende from a kyanite-stau- tal weight percent ofFe in each valence state, (2) the elemental CLOWE ET AL.: Fe3+/Fe2+AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES 489

TneLe2. Compositionsof amphibolerun products .I1A AOE3AG AEO3AG AOE24 AOE 11A AEO AOE14 AOE19 AOE32 AOE32 AOE31 sio, 49.26 49.91 43.70 44.12 47.59 51.01 50.05 Tio, 0.o2 0.05 0.59 0.66 0.29 0.94 0.87 Al,o3 0.46 o.14 13.64 13.18 8.23 1.05 0.71 FeO 36.99 44.06 11.03 12.33 12.82 14.37 23.92 FerO" 7.82 N.D. 5.50 3.86 N.A. 20.30 9.33 MnO 0.64 0.67 0.28 o.32 0.79 o.70 0.70 Mgo 3.21 3.22 10.00 10.33 12.81 o.22 0.21 CaO 0.33 0.32 11.61 11.73 11.86 0.15 0.18 Naro 0.07 0.06 1.40 1.24 0.88 8.87 8.64 KrO N.D. 0.01 0.51 0.47 0.47 1.46 1.35 F 0.17 0.08 0.18 o.47 0.62 2.00 1.98 cl 0.11 0.11 N.D. N.D. 0.01 N.D. N.D. Sum 99 08 98.63 98.44 98.31 96.37 101.08 97.94 Si 782 7.98 8.04 6.40 6.43 6.46 7.09 7.78 7.94 8.O2 IVAI 0.10 0.02 1.60 1.57 1.54 0.91 0.19 0.06 urAl 0.07 0.03 0.76 0.80 075 0.54 0.14 0.14 Ti 0.01 0.01 0.01 0.08 0.03 0.11 0.11 0.11 0.07 0.07 't.87 Fe2* 4.91 5.01 5.94 1.36 1.36 1.51 1.59 1.83 3.21 Fe3* 0.94 0.96 0.61 0.61 0.43 2.34 2.39 1.13 Mn 009 0.09 0.09 0.04 0.04 0.04 0.10 0.09 0.09 0.10 Mg 0.76 0.78 0.78 2.18 2.19 225 2.85 0.05 0.05 0.05 Ca 0.06 0.06 0.06 1.82 1.83 184 1.89 0.03 0.03 0.03 Na o.02 0.o2 0.02 0.41 0.41 0.35 0.25 2.62 2.68 2.68 K 0.09 0.09 0.09 0.09 0.29 0.30 o.27 cl 0.03 0.03 0.03 0.01 F 0.09 0.09 0.04 0.09 0.09 0.03 0.62 1.00 1.00 0.97 o 2300 23.48 23.00 23.00 23.10 23.00 23.00 23.00 23.49 23.00 Note: N.D. : not detected; N.A : not available.Chemical compositions (in wt% oxides and in atoms per formula unit normalizedto given number of oxygens)of selectedrun products (see Table 3 for details).Samples with > 10% oxy-amphibolecomponent have been normalizedto the appropriate number of oyxgens per formula unit (see text for discussion).

weight percentsare conyerted to oxide weight percentsby mul- erature is excellent. Previous analysis for the magnesio-horn- tiplication by the proper conversion factor, (3) the oxide weight blende has not been reported. The analysisofriebeckite-arfved- percentsare converted to moles, and (4) the formula is normal- sonite reported in Scofield and Gilbert (1982) contains 0.3 Li ized to the required number of oxygens (23 plus one-half the per formula unit, which is not analyzedby the electron mrcro- number of Fe3* ions present as oxy-amphibole component per probe and, therefore,is not included in the table. The formulas formula unit). The effectof the correction for grunerite is shown in Table t have been corrected to 23 oxygens,because the per- in the two columns labeledAOE 34,G in Table 2. The grunerite, cent oxy-component is unknown. As discussedbelow, the pres- which originally containedno Fe3*,was oxidized in air, but there ence of Fe3* in an amphibole does not necessarilyimply the was no evidence for decomposition to secondaryphases. Thus, presenceof oxy-component. its chemical composition should remain constant exceptfor loss Phaseidentification and unit-cell parameterswere determined of H and oxidation of a portion of the Fe2*.The corrected for- using a Philips X-ray diffractorneter with Ni-filtered CuK. ra- mula of the oxidized grunerite based on 23.48 oxygensis vir- diation. Peak locations used to calculate unit-cell parameters tually identical to that ofthe unoxidized starting material (Table were collectedfrom four scans(two oscillations)collected at 0.5"/ 1), whereasthe formula normalized to 23.00 oxygenscontains min using synthetic calcium fluoride as an internal standard, overall lower atomic amounts. It is important to note that this except in the case of riebeckite, for which synthetic MgAlrO4 method of correctingchemical formulas presumesknowledge of spinel was used in order to avoid significant overlap of peaks. the amount of Fe3+present as oxy-amphibole component. Be- Centersofpeaks were located at % peak height. Reflectionswere causeFe3+ can enter the amphibole structure by processesother indexed in comparisonto the calculatedpowder patternsin Borg than the formation of oxy-amphibole component (see Discus- and Smith (1969). Unit-cell parameters, reported in Table 3, sion), the amount of oxy-amphibole component is not necessar- were obtained by least-squaresrefinement (Evans et al.,1963), ily the same as the number of ferric irons per formula unit. For weighting all peaks equally. example, in the case of the tschermakitic hornblende starting material, which originally containedFe3* (Table 1), the chemical formula was based on 23 oxygensbecause the amount of oxy- Apparatus amphibole was unknown. However, when the tschermakitic Hydrothermal nrns were carried out in horizontally mounted hornblende was oxidized without decomposing, the chemical Rene-4l pressurevessels (e.g., Ernst, 1968, Fig. 6) using either formula could be correctedto the increasednumber ofoxygens Ar or methaneas the pressuremedium. Pressureswere generally that representsthe increasein Fe3*as oxy-amphibole component maintained within + 10 bars, with a maximum variation of 30 (columnslabeled AOE l1A, Table 3). bars recorded. Temperature gradients within the vesselswere Chemical analysesof the amphiboles used in the experiments calibrated previously at elevatedtemperature and pressure,and are given in Table 1. Chemical zoning and significantimpurities are 3'or lessover a 2.54-cm capsule.Temperature variations in both within and between grains were not observed.Agreement most runs were + 1 'C, with a maximum variation of +4 oC. betweenthese analyses and those reported previously in the lit- The l-atm nrns were made in a Lindbere mufle furnace that ^^o ^^^ - oi^^ ^^-o o6o -e 6'Q O (L Ugg 99U giit'b' o(ooo @Fo or N@ oo$ soo o{FF 5d'=V 6';6' ;d'E'tr it q 6n56 666 ntioi\i\ oo-o No@ tE QQ q?\ o s cidiq qqa? iiaa u?qqq u?u?1 9'1| 99 ototo ooo) ooroo, orooo, ooo, oo, o)o, o o o) 6 E tl t& Ora I'i o+E c rS ^^N O ^^^ O^^- ^r (')e ho$ N (ooN d)to)6 io-it@N @NoF Nr F(O N (D+O F N@$ FOFN Nio+ +otN d)r r$ @ .> $COO N N6@ N\fSN Ootfi) *SgF Fo F.!r @ ONO F @F(o @OO)$ o+o< Nsro oro oo - 63, qooq \ oq\oq \o?o?q o666 0000 00 Al@ N or FFo FF $\f$b 66nb i.66o $6 (Ofi) q) o66o oooo oo oo o o: 6E iltl ^^at- io- ao-io-^ 6-^^^ all @ !Q i-'{'+ N ;..1 i- ;.o'-O -OOO OUUS9 9Sl =g =

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6ObO06o v> ON**N$$@$ O*@tCO

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Tschermakitic-Hornblende Enor = 10.01 Ferric-Ferrous Ratio (R) 777) 0.10 0.20 0.30 0.40 0.50 ll .rll ,tl o s.m. GM4d.€

GM 8d

AtoGM 4d . rl FMQ 12d .-{

NNO 4d '{ NNO 8d O* MH 4d O- MH 8d O-

GMtoMHsdf A700"C30m

Fig. 1. Resultsofexperiments on tschermakitic hornblende. Circles representferric-ferrous ratio ofunaltered starting material. Tips of arrows indicate ferric-ferrous ratio in run products; precision of measurementis +0.01. Boxes representruns in which starting materials were products of previous experiments. Abbreviations: s.m. : unaltered starting material; GM : graphite- methane;FMQ:fayalite-magnetite-quartz; NNO:nickel-nickeloxide;MH:magnetite-hematite; A:air;d:days; m: mtnutes.

In the products of the air and NNO runs, some gru- and 8 d (R:0.22), and at the FMQ buffer for 12 d (R : nerite grains were observed to exhibit small, widely 0.18) resulted in little changein ferric-ferrous ratio rela- spaced,dark striations perpendicular to the long axis of tive to the starting material. A small amount of quartz the crystal when viewed optically before crushing.Optical (<30/0)was detected optically in the 8-d NNO run, but examination was unable to reveal whether the features the chemicalcomposition of the amphibole(AOE l4,Ta- were fractures or inclusions, but electron-microprobe ble 2) did not changesignificantly. Whether the presence analysis,including dot maps, indicated no compositional of quartz indicates an original quartz impurity or actual variations. This phenomenon was also observed in rie- decomposition is unclear. In an experimental investiga- beckite treated at the graphite-methanebuffer. tion at pressuresabove 7 kbar, Oba (1978) observed Tschermakitic hornblende. The tschermakitic horn- qvartz, clinopyroxene, and garnet in the dehydration as- blende exhibited continuous variation in ferric-ferrous semblageof tschermakite.However, data on the stability ratio as a function of increasing oxygen fugacity. Run relations in the temperature-pressurerange of this study results are shown graphically in Figure 1. Air oxidation are not available. for 0.5 h produced an increase in R from 0.21 in the Treatment of tschermakitic hornblende at the CCH. untreated sample to 0.47, which representsoxidation of bufer produced the somewhat reduced ferric-ferrous ra- 0.52 Fe per formula unit. The 4- and 8-d runs at the fo, tios of 0. 16 after 4 d, and 0. 15 after 8 d. of the MH buffer resulted in statistically identical R val- Treatment of the air-oxidized sampleat the CCH4buff- ues of 0.30 and 0.31, respectively,with no evidencefor er for 4 d reducedR from 0.47 to 0. 14, which agreeswell decomposition.The composition of the amphibole in the with the values of 0. 15 and 0. 16 obtained using unoxi- 8-d run did not change,except for R, after correction for dized starting material. Re-annealingthe amphibole from the increasedoxy-amphibole component (AOE I lA, Ta- a CCH. run at the MH buffer for 4 d resulted in a value ble 2). of R : 0.30, which is identical with that obtained by Samplestreated at the NNO buffer for 4 d (R : 0.19) starting with the original amphibole. In contrast, treat- CLOWE ET AL.: Fe3+/Fe2+AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES 493 ment of the air-oxidized sample at MH for 4 d resulted available per formula unit, nearly all were converted to in partial decomposition of the amphibole to liberate ap- o2-. proximately l0o/ohematite and quartz. In an 8-d run at the MH buffer, riebeckite decomposed Magnesio-hornblende.The magnesio-hornblendeis nearly completely to hematite, magnetite, quattz, and ac- somewhat similar in composition to the tschermakitic mite. Theseresults are in agteementwith those observed hornblende,containing 1.66 vs. 1.96total Fe per formula in the synthetic system(Ernst, 1962),inwhich riebeckite- unit, respectively. However, the magrresio-hornblende arfvedsonite solid solutions are unstable down to tem- contains 0.65 fewer Al per formula unit, and has a lower peraturesas low as 500'C at the oxygen fugacity defined initial ferric-ferrousratio of 0.07 vs. 0.21. As with the by the MH buffer. tschermakitic hornblende, a systematic variation in R was Runs carried out at the FMQ and CCH. buffers for 8 observed over the range of oxygen fugacities employed d resulted in slightly reduced ferric-ferrous ratios of0.28 (Table 3). and0.26, respectively,with no optical or X-ray evidence The most highly oxidized amphibole was obtained by for decomposition. Basedon the results of Ernst (1962), air-heating:R:0.31 after0.5 h, R:0.50 after 1.5h. the amphibole should lie within its stability field at these Very small amounts of opaques(< lolo),presumably iron conditions.In contrastto the resultsofErnst, Owen (1985) oxide, were observed optically in the air-heated runs. reported arfvedsonitic amphibole to be unstable at 750 Treatment at the MH bufer for 8 d resulted in the for- oC, I kbar, andfo, defined by the FMQ buffer, but the mation ofpyroxene and a spinel phase in the charge,as results cannot be directly compared to those obtained identified by X-ray difraction. Electron-microprobe here at 650 "C. The most reduced of the riebeckites re- analysis(AOE 19, Table 2) revealsthat the oxidized am- quires reduction of 0.27 Fe3*per formula unit relative to phibole is somewhatdepleted in Al and Fe, and enriched the starting material. in Si and Mg relative to the starting material. Whether this amphibole composition representsan equilibrium Unit-cell parameters value is unknown; treatment for longer times could result Unit-cell parametersof selectedamphibole run prod- in further decomposition and changesin composition. ucts determined from powder X-ray diffraction are given As with the tschermakitic hornblende, runs of 8 d at in Table 3. The numbers in parenthesesin the table rep- the NNO and FMQ buffers produced little changein R. resent one-standard-errorvalues calculated by the least- The resulting amphiboles are slightly more oxidized as squares computer program. The relatively larger stan- compared to the ferric-ferrousratio of 0.07 in the starting dard-error valuesfor the air-treated samplesprobably are material,i.e., R : 0. I I for NNO, and R : 0. l2 for FMQ. due to the factthal the crystallinity of those sampleshas Treatment of the original sample at the CCH. buffer been affectedby incipient stagesofdecomposition. for 8 d resulted in total reduction (R : 0.00) of the Fe in As a simple test for precision, unit-cell parametersfor the amphibole. Treatment of the air-heatedsample at the the grunerite, as determined by several different tech- same conditions resulted in run products with R : 0.09, niques and by severaldifferent individuals, are compared but the small amount of opaquephases present in the air- in Table 4. The grunerite usedfor this study was obtained heated sample was not resorbedduring the run at lower as a hand sample of schist from C. Klein, who has made oxygen fugacity. The presenceof l0lo magnetite in the the amphibole available to workers for a variety of dif- sample would increaseR of the bulk sample by 0.025. ferent kinds of analyses(e.g., Finger, 1969;Dyar, 1984). Thus, subtraction of the effect of the magnetite results in Shown in Table 4 are five different determinations, two an amphibolewith R in the range0.06 to 0.07, as com- obtained by different individuals using a powder diffrac- pared to the value of 0.00 obtained from treatment of the tometer as describedabove, two obtained from powder original starting material at the CCH. buffer. photographs utilizing an internal standard, and one ob- Riebeckite. The riebeckite-arfvedsonitesolid solution tained from an automated single-crystaldifractometer. had an initial R value of 0.33 as comparedto 0.20 for In general,the agreementamong the values in Table 4 is ideal arfvedsonite and 0.40 for ideal riebeckite. In addi- good, but the data suggestthat doubling ofthe standard- tion, it contained 0.98 F per formula unit, so that the error values, a practice advocated by some researchers, maximum possibleamount of dehydrogenationis limited gives better agreementfor comparing the results of the relative to the other three amphiboles studied here. different analysts. Treatment of the original sample in air for 0.5 h re- sulted in the liberation of a small amount of free quartz (<2o/o).The resulting amphibole may be slightly silica DrscussroN deficient (AOE 32, Table 2), but the difference is at the In the discussion that follows, it is assumedthat the level of detection. Becausethe anall'tical technique for readeris familiar with the basic featuresof the amphibole ferric-ferrous ratio is not affectedby the presenceof ad- crystal structure, the nomenclature for cation and anion ditional non-Fe-bearingphases (Fritz and Popp, 1985), a sites, and the general scheme of partitioning of cations value of R can be determined for the oxidized riebeckite. among the sites according to size and valence. Excellent The resultingvalue of R (0.56)requires that 0.97 atoms reviews ofthe subjectare given in Ernst (1968), Carneron of Fe2twere oxidized.That is, of the 1.02atoms of OH and Papike(1979), and Hawthorne(1981, 1983). CLOWE ET AL.: Fe3+/Fe2+AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES

Treue4. Comparisonof unit-cellparameters

Method of b R Analyst X-ray diffraction (A) (A) (A) f) (A") C. A. Clowe" diffractometer 9.556(2) 18.397(4) 5.347(7) 101.904(35) 919.855(961) M W. Phillips(pers. comm.) single crystal 9.570(2) 18.397(3) 5.342(2) 101.94(3) 920.2(8) Klein(1964) powder photo 9.s62(2) 18.380(7) 5.338(4) 101.86(3) 918.2(7',) Finger(1 969) powder photo 9.5642(7) 18.393(2) s.3388(3) 101.892(3) 919.0(2) M A. Cusimano(pers. comm.) diffractometer 9.568(5) 18.374(17) 5.33s(16) 101.828(48) 918.067(2.058)

Nofe.'Comparison of unit-cellparameters of grunerite (Klein,sample 1, 1964) determinedby severaldifferent methods and experimenters. - Valuesas givenin Table3.

shorter run times and the fluxing effect of water in the Ferric-ferrous ratios and phase relations hydrothermal runs. Becauseno singlevalue ofR has beenapproached from The NNO and FMQ buffers produce little, or only slight, opposite directions, the strict reversal ofequilibrium has variation in ferric-ferrousratio relative to all four starting not been documented in any of the runs in this study. materials. Most significant is the fact that grunerite, un- However, the relatively rapid changesin R, the time in- like the other three varieties, contains no Fe3*at the fo, dependenceoffinal R values in the hydrothermal runs, defined by NNO. and the fact that R values can be restored after having The different behavior of the four amphiboles may be been altered (Fig. l) lend support to the assumption that related partially to the two mechanismsby which Fe3*is an approach to equilibrium has been obtained. accommodatedwithin the amphibole crystal structure.In Clearly, there are differencesin the behavior ofthe four the first mechanism, formation of oxy-amphibole com- different amphiboles. Air oxidation produces the most ponent by oxidation-dehydrogenation (Eq. 1), the in- highly oxidized samplesin all four minerals. As hydrous creasedcharge that results from oxidation ofFe2* to Fe3* phases,amphiboles are unstable in air at 700 "C. The is compensatedby replacement of the same number of slight, but still significant broadening of peaksin the X-ray univalent hydroxyls by divalent oxygens.As discussedin patterns, particularly in runs longer than 0.5 h, may rep- detail elsewhere(Phillips et a1., 1988), dehydrogenation resentincipient stagesof decomposition.Longer-term air- producessignificant underbonding at the O(3) site, which heatingruns describedelsewhere (Hodgson et al., 1965; in turn, inducescompensational mechanisms to offsetthe Pattersonand O'Connor, 1966) have resulted in decom- underbonding. For example, changesin octahedral oc- position. It is, thus, concluded that equilibrium is not cupancies, shortening of M-O bond lengths, displace- approachedin any ofthe air-heating runs. The Fe3*con- ment of univalent cations from M(4) to the A site, and tent of the amphiboles should increasewith longer run increasedinteraction of A-site cations with O(3) accom- times until complete dehydrogenation occurs, complete pany oxidation-dehydrogenation. oxidation of Fe is achieved, or the amphibole decom- In the second mechanism, herein termed Al substitu- poses.Therefore, differencesin R values of the four am- tion, Fe3*behaves analogously to Al3* in octahedral cat- phiboles heated in air are most likely related to kinetic ion sites. Octahedral Al3* is accommodated in amphi- factors and to their different initial Fe2*contents. boles either (l) bv a coupled tschermakitic-type At the oxygen fugacity of the MH buffer, all the am- substitution in which replacementof divalent cations by phiboles excepttschermakitic hornblende decomposedto Al3* in octahedral sites is coupled with substitution of assemblagescontaining Fe3*-bearingoxides, or in the case Al3+for Si4+in the tetrahedralsites or (2) by a substitution of riebeckite, Fe3*-bearingpyroxene. The original tscher- in which replacementof Ca2*by a univalent cation in the makitic hornblende(R : 0.21)did not decomposein runs M(4) sites is coupled with substitution of Al for divalent of up to 8 d, but treatment of the more highly oxidized cations in the octahedral sites, as in the case of glauco- air-heated sample (R : 0.47) at MH did result in decom- phane (NarMg.AlrSirOrr(OH)r) or alumino-winchite position to an assemblagecontaining iron oxides. These (NaCaMg"AlSi8Orr(OH)r).The extent to which each of results suggestthat the amount of Fe3* required in am- the two mechanismsdetermines the Fe3*content of any phibole by this relatively high oxygen fugacity cannot be given natural amphibole is unknown, but published accommodatedwithin the crystal structure, and decomposi- chemical analysesof amphiboles suggestthat both pro- tion to other Fe3*-bearingphases results. The attainment cessesare involved in many natural samples(Popp and of an apparent metastable equilibrium in tschermakitic Phillips, 1987). hornblende indicates that the equilibrium ferric-ferrous Reduction of Fe3*present as oxy-componentin an am- ratio is achievedmore rapidly than the rate at which de- phibole can be accomplished by a hydrogenation reac- composition reactions occur. The fact that decomposi- tion; that is, the reverse of Equation l. The extent to tion is not observedin the more highly oxidized air-heat- which Fe3* that is present as a result of Al substitution ed samplesis consideredto be a result of the significantly can be reducedis less well known. Semet(1973) docu- CLOWE ET AL.: Fe3+/Fe2+AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES 495 mented the complete reduction of Fe in magnesio-has- 4, representthe activities ofthe appropriate components tingsite (NaCarMgFe3*Al,Si6Orr(OH)r)to l00o/oFe2* and in the amphibole solid solution (a."r*, the activity of Fe3* concluded that one excessH was presentin the structure component; 4oxy,oxy-amphibole component; 4r"z*,Fe2t in order to produce the required charge balance. The component; and 4hyd,o*y,hydroxy-amphibole component). presenceofexcess OH has also been reported in synthetic Becausethe values of/r, for the buffer assemblagescan amphiboles (Witte et al., 1969; Maresch and Langer, be calculated,Equation 2 canbe used to compare calcu- 1976).as well as in natural calcic and subcalcicvarieties lated and observedcompositions for a single amphibole (Leake,1968). treated over a range of buffers, or two different amphi- Some conclusionscan be made based on the two dif- boles treated at the same buffer, provided activity-com- ferent mechanismsfor incorporation of Fe3*into amphi- position relations for the amphibolesare known. The ma- boles. The experimental results indicate that grunerite can jor compositionaldiferences between the two hornblendes contain a considerableamount of Fe3*under short-term, are that the magnesio-hornblende(1) contains a larger nonequilibrium conditions (i.e., the air-heated samples), amount of F in O(3) (0.33vs. 0.06 per formula unit), and but can accommodateonly a very limited amount of Fe3" (2) contains a smaller amount of octahedral Al (0.64 vs. under equilibrium conditions. No Fe3*was observed to 0.83 pfu). Given these compositional differences,some be presentin any of the hydrothermal runs made in this semiquantitative conclusionscan be made, provided that study,although analysis of amphibole Fe3*content of runs the activity coefficientsfor the two minerals are unity or made at the MH buffer was not possible. Becausegru- are assumedto be similar. For unit activity coefficients, nerites,in general,contain only small amounts of Na and the activity of hydroxy-amphibole component can be ex- Al, the Al substitution mechanism cannot produce a sig- pressedas nificant Fe3*content. The extent to which oxy-amphibole an"o*": X'ot' (3) component can form in grunerite may also be limited. If the presenceofA-site cations is required to relieve a por- where X is the mole fraction of OH in the O(3) site. tion of the imbalance of bond strengthscaused by O'z-in Therefore, from Equations 2 and 3, for two amphiboles the O(3) site, the relative absenceof Na in this and most equilibrated at the same buffer, the lower activity of hy- naturally occurring grunerites may severely limit the droxy-component in the magnesio-hornblenderequires a amount of oxy-component that can form. The result of lower ferric-ferrousratio at constant activity of oxy-com- the local chargeimbalance may be that grunerites either ponent. That is, if the activity of oxy-component is the resist oxidation of Fe2*,as observedin the NNO run, or same in the two minerals, the lower activity of hydroxy- at higher oxygen fugacities, decompose to Fe3*-bearing component requires a lower ferric-ferrous ratio. The ac- oxides so that the Fe3t in the systemis accommodatedin tivities ofFe3* and Fe2*can be expressedas oxides rather than in the amphibole. Previous experi- Qp4* : X2prt*1yrrll4":*tr..rt:>Xo",*G.,rrrll (4) mental studies (e.g.,Popp et al., 1977) have documented : (5) that Fe-Mg amphiboles are restricted to very Mg-rich Qps2+ X2."r*1*1r;X2u.r*1u12;XEz*641t;, compositions at the oxygen fugacity defined by the MH where the X terms refer to the mole fraction of the ions buffer, which is likely to be related to the inability of this in the individual cation sites. Fe3* and Al are strongly group of amphiboles to accommodate Fe3*. Low Fe3t partitioned into the M(2) site in amphiboles (e.9., Haw- contents are also observed in naturally occurring Fe-Mg thorne, 1983), and thus effectively reduce the number of amphiboles (Fig. 2 in Robinson et al., 1982; Deer et al., sites on which Fe2* can mix. For example, if M(2) were l 963). totally occupied by Al and Fe3*,Fe2* could mix on only The magnesio-hornblendeis more reduced than the three rather than five sites, whereasFe3* could still mix tschermakitic hornblendefor all the buffer conditions and on all five. A consequenceof mixing on fewer sites is an is totally reduced (R : 0.00) at the CCH. buffer as com- increasein activity, i.e., the mole fraction is raised to a pared to the value ofR:0.15 obtained for the tscher- lower power. Therefore, if two amphiboles contained makitic hornblende. These results suggestthat the origi- identical amounts of Fe2*,that with the higher Al content nal Fe3* content of the magnesio-hornblendemay have would have a higher activity of Fe2*.From Equation 2, been virtually all oxy-component, whereas that of the a higher activity ofFe2* requires a higher activity ofFe3* tschermakitic hornblende may have been due to both oxy- at constant ahyd,o,y,a"*y, and fH2. Thus, increasedAl con- componentand Al substitution. tent favors a higher ferric-ferrousratio, given that all oth- A simple comparative thermodynamic analysis can er variables are constant. provide some information regardingthe comparative be- It can be concludedthat the higher OH and Al contents havior of the two hornblendes.The log form of the equi- of the tschermakitic hornblende are consistent,in a semi- librium constant for Equation 1 is quantitative sense,with its higher ferric-ferrousratios rel- ative to magnesio-hornblende.Without knowledgeof the log K: log 4.":* * log c.*, * r/zlogfr, ",gapg2+ actual site populations of the cations,more rigorous anal- - log anru.""r, (2) ysis is not warranted. where/r, is the fugacity of H, in the system,and the terms The fact that the air-oxidized magnesio-hornblende 496 CLOWE ET AL.: Fe3+/Fe2+AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES

cannot be totally reduced, whereasthe unoxidized mag- Mg in M(4) also exert control on b. The size of a sin B is nesio-hornblendecan, is consideredto be related to the related to the unit repeat across facing double chains. presenceof the iron oxides in the air-heated sample.The Given constant thickness of the tetrahedral chains (i.e., presenceof oxides suggeststhat mechanisms other than constant tetrahedral occupancy),a sin B is related to the solely oxidation-dehydrogenationmay have been opera- thicknessof the octahedral strip. For constant M(4) con- tive (e.g.,Addison eI al., 1962a,b; Hodgsonet al., 1965). tent, the occupancyof the M(l), M(2), and M(3) sites The possibility and identity of such mechanismsare the determines the size of a sin B. The c unit-cell edge is a subjectof continuedinvestigation. measure of the unit repeat parallel to the tetrahedral The results of the riebeckite experimentsare more dif- chains. Its size is apparently controlled by complex in- ficult to explain in some respects.The ferric-ferrous ratio teractions of the cations occupying all of the M sites, as of the air-oxidizedsample (R : 0.56)requires loss of 0.97 well as the Si:Al ratio. Finally, B is determined mainly by OH from the starting amphibole composition. Given the the occupancyof cations in M(a). Thesecontrols on unit- original F content of 0.98 F pfu (Table l), the maximum cell parametersdescribe the generaltrends observed,but original oxy-component can be no greaterthan 0.05 oxy- in some cases,the variations in unit-cell parameterscal- gen pfu. The problem arisesin that the riebeckite treated culated strictly on the size differencesbetween cations in at the FMQ and CCH. buffers is significantly reduced specific sites are significantly smaller than, or even op- relative to the starting material. For example, the most posite to, those observed in the actual amphiboles (e.g., reduced riebeckite (R : 0.26) requires 0.30 Or- to be Table 4 in Colville et aI., 1966). Such observations sug- converted to OH-. However, this number of oxygensis gestthat other crystal-chemicalfactors contribute to vari- significantly larger than the number presentin the chem- ations in amphibole unit-cell parameters. ical formula. Given the variations in unit-cell parameters(Table 3), Two possiblemechanisms could be responsiblefor the some interpretations can be made, but the interpretation apparent "over-reduction" of Fe3* in riebeckite. The is complicated somewhatby the relatively large standard presenceof excessH in the form of OH within the am- errors in the unit-cell refinement for the air-heated sam- phibole as proposedelsewhere (Leake, 1968; Witte et al., ples when the standard statistical test for significanceat 1969; Semet, 1973; Maresch and Langer, 1976) would the two-standard-error level is used. permit the existenceof excessFe2t. The likelihood of this a sin d. Figure 2 shows the variation in a sin B of the being the actual mechanism cannot be evaluatedwithout amphiboles normalized to the changein number of ferric quantitative analyses of water contents or without the irons per formula unit. Positive values represent in- application of techniques such as infrared spectroscopy creasedamounts ofFe3*, whereasnegative values repre- or single-crystal structure analyses.Alternatively, Reb- sent reduction of Fe3* relative to the unaltered starting bert and Hewitt (1986) reported results of a similar ex- material. The similar slopes for tschermakitic horn- perimental study of the variation of ferric-ferrous ratios blende,magnesio-hornblende, and riebeckite(0.084, in synthetic biotites in responseto changesin controlled 0.062,and 0.060A per atom ofFes*,respectively) suggest ,f"r. Basedon the direct measurementof H content of the that the expansion or contraction in the direction per- run products, they concluded that anion vacancieswere pendicular to the chains is approximately the same in all present in the more reduced samples. The presenceof three crystal structures as the amount of Fe3t changes. such vacanciesin amphiboles would allow for excessre- Assuming that the Fe content of the M(4) site is negligible duction ofFe3* over that presentas oxy-component, but for these three amphiboles, the changesin Fe3* content in the absenceofH analyses,such an explanation is also must be accommodatedin the M(l), M(2), and M(3) sites, totally speculative. which is then reflectedin the size of c sin B. The responseof a sin B to changing Fe3* content is Unit-cell parameters distinctly different for grunerite. The least-squarescurve Changesin unit-cell parameters with changing ferric- through the data points showsessentially no changeover ferrous ratio should be related to the size differencebe- the range of compositions obtained in the experiments. tween Fe2*and Fe3*,as well as to changesin cation site It is difrcult to envision a mechanism by which replace- occupancies.The observedvariations in unit-cell param- ment of a larger cation by a smaller one would result in eters can be interpreted to some extent based on the re- no changeor an increase,but given the very large relative sults of Colville et al. (1966),who investigatedthe rela- error for the most oxidized air-treated run, a decreasein tion between unit-cell parameters, bulk chemical a sin S with increasingFe3* content is certainly possible. composition, and site occupanciesin synthetic and nat- Part of the difference between grunerite and the other ural clinoamphiboles.They concludedthat the size of the three minerals may be due to the fact that in grunerite, D unit-cell edge is related to the lateral linking of the there is occupancy of Fe in the M(4) site, and thus the tetrahedralchains by the cations occupying the M(4) and occupancyof M(4) is involved in controlling a sin B. Klein M(2) sites.In the caseof the calcic and sodic amphiboles, and Waldbaum (1967) calibrated the variation in unit- in which M(a) is occupied mainly by Ca and Na, the cell parametersas a function of Fe-Mg ratio for the cum- occupancyof M(2) exerts the major control on b. In the mingtonite-grunerite series.The dashed line in Figure 2 Fe-Mg amphiboles, however, the proportions of Fe and shows the variation in a sin B calculated from the equa- CLOWE ET AL.: Fe3+/Fe2+AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES 497 tion of Klein and Waldbaum for replacementof Fe'z*by 9.600 Mg. The variation observed for grunerite is in better agreementwith the calculatedcurve than with the curves for the other three amphiboles,but the analogousbehav- 9 550 ior of Fe3*and Mg2+is questionable.Colville et al. (1966) were able to explain observed variations in unit-cell pa- Riebeckite 9.500 rameters with the assumption that the two cations are m=-0060 similar in radius. However, in more recent compilations ofionic radii (Shannonand Prewitt,1969), the radius of Mg-hblnde e 4s0 m = -0.062 Fe3t is approximately 100/osmaller than that of Mg. Ideal "? Tsch-hblnde \ (M-O) bond lengths in amphiboles (Table 15, Haw- m = -0.084 thorne, l98l) also reflectthe smallerradius of Fe3*rela- f nooo tive to Mg. Increasing Mg content in the natural cum- mingtonites-grunerites is strongly partitioned into the M(l), M(2), and M(3) sites,whereas changes in site oc- cupanciesin the oxidized grunerites are unknown. The significantly smaller responseof a sin B relative to Fe3* as compared to Mg may indicate that oxidation occurs 9.300 -0.5 0.0 0.5 1.0 1.5 2.0 with little repartitioning of Fe within the cation sites. In Change in Atoms of Ferric lron Pfu summary, the observed behavior of a sin B in grunerite is considerably different from that of the three amphi- Fig.2. Variationin a sin B of the amphibolesas a function boles with negligible Fe content in M(4), but an expla- of thechange in thenumber of ferricirons per formula unit. The material. nation for the differencecannot be verified without data value0.0 on theabscissa represents the original starting Positivevalues indicate increase in Fe3*content, whereas nega- on site populations. tive valuesrepresent reduction of Fe3*,relative to the starting The for grunerite (Table Dunit-cell edge. data 3) suggest material.Slopes (z) arein A/atom of Fe3*.The dashedline for a decreasein b as R increases.Such a decreaseis to be gruneriteshows the changein a sin A that would resultfrom expected because,in grunerite, there is significant Fe2* substitutionof an equivalentamount of Mg for Fe'?*,rather than content in both M(2) and M(4), which, presumably, is Fe3*for Fe2'.See text for details. oxidized to the smaller Fe3*.As was the casefor a sin 0, the observed change in b is considerably less than that observedfor an analogousamount of replacementof Fe2* in c are understood more poorly on a crystal-chemical by magnesium. The observed decreaseof 0.020 A for basis than for the other two unit-cell edges(Colville et replacementof the 1.62 atoms per formula unit compares al.,1966), so that interpretation ofthe results may be less to the value of 0. I 20 A for replacementof I .62 atoms in meaningful than for the other parameters. In addition, the Fe-Mg system (Klein and Waldbaum, 1967). In the the data for the c unit-cell edge are consideredto be of natural cummingtonites-grunerites, the increased Mg lesserquality becausethe least-squaresrefinement ofc is partitions very stronglyinto M(2); thus, the smallerchange basedon a relatively small number of peaksas compared in Dobserved here could be explainedif Fe3*is not strongly to a and b. A decreasein c with increasingFe3* content partitioned into the M(2) site. should be expectedin grunerite, based on the behavior In the two hornblendes,there is no evidencefor a change of Fe-Mg substitutions (Klein and Waldbaum, 1967),but in D. For both amphiboles, all values of D except the two the observed difference is several times larger than that extremesare equal within one standard error, and those in the Fe-Mg system. The relatively imprecise value for of the extremes are the same well-within two standard the most oxidized sample may contribute to the differ- errors. In general, it has been observed that Fe3* parti- ence. tions strongly into the M(2) site in hornblendes(Cameron There is no evidencefor variation ofthe c unit-cell edge and Papike, 1979; Hawthorne, 1981, 1983). The con- of the tschermakitic hornblende, but that of the magne- stancy of b observed here is interpreted to indicate that sio-hornblendeapparently decreaseswith increasingFe3* Fe3*is not strongly partitioned into that site in the oxi- content. Given the similarities in compositions ofthe two dized hornblendes.This conclusion is confirmed by Phil- amphiboles, significant differenceswould not be expect- lips et al. (1988), who have shown that both total Fe ed. Explanation ofthe behavior, ifnot related to the pre- content and Fe3*content of the M(2) site decreasein the cision of measurement,awaits further crystallographicin- oxidized tschermakitic hornblendesof this study. vestigation. Given the large magnitude of the errors, there is no Within the standard errors of the unit-cell refinement, significant changein b for the riebeckite, but Ernst and c of riebeckite does not vary with R. Wai (1970)have documenteda generaldecrease in b of p angle. With the exception of grunerite, the value of sodic amphiboles with an increasein oxidation-dehydro- the interaxial angle B does not vary significantly for the genation. amphiboles (Table 3). This observation is consistentwith c unit-cell edge.The factors that control the variation the resultsof Colville et al. (1966),who concludedthat B 498 CLOWE ET AL.: Fe3+/Fe2+AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES is determined mainly by the identity of the cation in the - (l 962b) Amphiboles. Part II. The kinetics ofoxidation ofcrocido- M(4) site, Ca vs. Na vs. Fe-Mg. lite. Joumal ofthe ChemicalSociety, 1962, 1472-1475. Bames, V.E (1930) Changesin hornblende at about 800 {. American Unit-cell volume. The unit-cell volume is the product Mineralogist, 15, 393417 ofthe other parameters,and thus reflects the variations Borg, I.Y., and Smith, D.K. (1969) CalculatedX-ray powder patternsfor of the individual unit-cell parameters discussedabove. silicate minerals. GeologicalSociety of America Memoir 122' Except for riebeckite, for which the standard errors are Cameron, M., and Papike, J.J (1979) Amphibole crystal-chemistry:A Fortschritte der Mineralogle, 57, 28-67 very large, unit-cell volumes of all the amphiboles de- review. Clowe, C A. (1987) The relation of/o,, ferric-ferrousratio (R), and phys- creasewith increasingcontent of the smaller Fe3*. ical propertiesoffour natural clinoamphiboles.M S. thesis,Texas A&M University, CollegeStation, Texas. CoNcr,usrous Clowe, C.A., and Popp, R.K. (1987) Relation of alpha refractive index Basedon the results obtained in this study, the follow- and optic 2V angleto ferric-ferrousratio in a hornblende.Geological Societyof America Abstracts with Programs, 19, 148 ing conclusionscan be made. Colville, P.A, Ernst,W.G., and Gilbert, M.C. (1966)Relationships be- l. Significant variation in Fe3*content can be achieved tween cell parametersand chemical compositions of monoclinc am- in amphiboles by variation in oxygen fugacity. Oxida- phiboles.American Mineralogst, 5 l,l7 27-17 54. tion-dehydrogenationequilibrium as describedin Equa- Deer, W.A., Howie, R.A., and Zussman,J (1963)Rock-forming min- vol. 2: silicates, cummingtonite-grunerite, p 234-248. I for most, not all of the observed erals, Chain tion can account but Longmans,Green and Co. Ltd., London. behavior. Dyar, M.D. (1984) Precision and interlaboratory reproducibility of mea- 2. Steady-statevalues offerric-ferrous ratio, which are surementsof the Miissbauereffect in minerals. American Mineralogist, presumedto approach equilibrium, are attained relative- 69,112',7-t144. rie- ly rapidly as compared to rates of decomposition reac- Emst, W.G. (1962) Synthesis,stability relations, and occurrenceof beckite and riebeckite-arfvedsonitesolid solutions.Journal of Geology, tions. In some cases,metastable Fe3r-Fe2+ equilibrium is 70,689-736. achievedprior to decomposition. -(1968) Amphiboles: Crystal chemistry, phase relations, and oc- 3. The Fe3+content of the magnesio-hornblendeis currence.Springer-Veilag, New York. present mainly as oxy-amphibole component, whereas Ernst, W.G , and Wai, C M. (1970) Mdssbauer,infrared, X-ray, and op- ofcation ordering and dehydrogenationin natural and heat- that of the tschermakitic hornblende is present as both tical study treated sodic amphiboles American Mineralogist, 55, 1226-1258' oxy-component and as the result of Al substitution. The Evans,H.T, Appleman,D.E., and Handwerker,D.S (1963)The least- possiblereduction ofFe3* either by the presenceofexcess squaresrefinement ofcrystal unit cells with powder diffraction data by H over two atoms per formula unit or by the formation an automated computer indexing method (abs.).American Crystallo- ofanionic vacanciesis possible,but cannot be evaluated graphic Association Meeting, P. 42 Finger, L.W (1969) The crystal structure and cation distribution of a from the results obtained here. grunerite.Mineralogical SocietyofAmerica SpecialPaper 2, 95-100' 4. Under equilibrium conditions, grunerite is inherent- Forbes, WC. (1977) Stability relations of grunerite, Fe'SLOz(OH)r. ly lessable to accommodateFe3* than are the other three American Journal Science,27 7, 735-7 49. amphiboles. Grunerite respondsto increasedoxygen fu- Fritz, SJ., and Popp,R.K. (1985)A single-dissolutiontechnique for de- FeO and FerO, in rock and mineral samples.American Min- gacity resisting or by expelling the termining by either oxidation, eralogist,70, 961-968. oxidized Fe from its crystal structure. Gilbert, M.C., Helz, RT., Popp, R.K., and Spear,F.S. (1982) Experi- 5. Unit-cell parameters respond to changesin ferric- mental studiesof amphibole stability. Mineralogical Society of Amer- ferrous ratio, but the large errors of refinement of some ica Reviews in Mineralogy, 98,229-353. (1981) amphiboles. Mineral- sampleslimit crystal-chemical interpretation. The most Hawthorne, F.C. Crystal chemistry of the ogical Societyof America Reviews in Mineralogy, 9.A, l-95' significant changeobserved is a decreasein a sin B ofthe -(1983) The crystal chemistry of amphiboles.Canadian Mineralo- two hornblendesand riebeckite with increasingR, which gist,2l, l?3-480. representsincreasing overall content of Fe3*in M(l),M(2), Hodgson,A.A., Freeman,A.G., and Taylor,H.F.W (1965)The thermal Koegas,South Africa. Mineralogical and M(3). The variation of a sin B with R in grunerite is decompositionof crocidolite from Magazine,35, 5-30 different from that of the other three am- significantly Huebner, J.S. (1971) Buffering techniquesfor hydrostatic systemsat ele- phiboles. There is no strong evidence for partitioning of vated pressures.In G.C. Ulmer, Ed., Researchtechniques for high pres- Fe3*into the M(2) site of the oxidized amphiboles,based sure and high temperature,p. 123-177. Springer-Verlag,New York. on the variation in b. Klein, C. (1964)Cummingtonite-grunerite series: A chemical,optical, and X-ray study American Mineralogist, 49, 9 63-9 82. Waldbaum, D.R. (1967) X-ray crystallo$aphic properties AcxNowr,nocMENTS Klein, C., and of the cummingtonite-gruneriteseries. Joumal of Geology, 75, 379- We are indebted to P. Dunn, M. C. Gilbert, C. Klein, and F S. Spear 392. for providing the amphibole samples,and in some casesanalytical data, Leake, B E. (1968) A catalog of analyzedcalciferous and subcalciferous usedin this study. The manuscript was improved considerablyby reviews amphibolestogether with their nomenclatureand associatedminerals. of M. D. Dyar, M. C. Gilb€rt, M. W. Phillips, M R. Scott, F. S. Spear, GeologicalSociety ofAmerica SpecialPaper 98. and panicularly M. Cho. This researchwas supportedby National Science Maresch, W.V., and Langer, K. (1976) Synthesis,lattice constants,and Foundation grant EAR-83 12878. OH-valence vibrations of an orthorhombic amphibole with excessOH in the systemLirO-MgO-SiOr-HtO. Contributions to Mineralogy and RnrnnnNcns crrED Petrology,56,27-34. Oba, T. (1978) Phaserelationship of Ca:MgrSLAlrOrr(OH),join at high Addrson,C.C., Addison, W.E., Neal, G.H., and Sharp,J.H. (1962a)Am- temperaturesand high pressure-The stability of tschermakite.Journal phiboles. Part I. The oxidation ofcrocidolite. Journal ofthe Chemical ofFaculty Science,Hokkaido University, Series4, 18, 339-350. Society, 1962, 1468-14'7l. Owen, C. ( I 985) Constraintson the stability of riebeckite.EOS, 66, I I 3 l. CLOWE ET AL.: Fe3+/Fe2+AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES 499

Patterson,J. H., and O'Connor, D.J. (1966) Chemical studiesof amphi- Semet,M P. (1973) A crystal-chemistrystudy ofsynthetic magnesio-has- bole asbestos-Structural changesof heat-treatedcrocidolite, amosite iingsite. American Mineralogist, 58, 48G494. and tremolite from infrared absorption studies.Australian Journal of Shannon, R.D., and Prewitt, C.T. (1969) Effective ionic radii in oxides Chemistry,19, 1155-l164. and fluorides.Acta Crystallographica,B25, 925-946. Phillips,M.W., Popp,R.K, and Pinkenon,A.A. (1986)Structural inves- Solberg,TA, and Speer,J.A. (1982) QALL, A l6-elementanalytical tigation of oxidation-dehydroxylation in hornblende (abs.). EOS, 67, scbemefor efficient petrologic work on an automated lnr-ser.rq: Ap- 1270. plcation to mica referencesamples. In K.F.J. Heinrich, Ed., Micro- Phillips,M W, Popp, R.K., and Clowe,C.A. (1988) Structuraladjust- beam analysis,p 422426. San FranciscoPress, San Francisco. ments accompanyingoxidation-dehydrogenation in amphiboles Spear,F.S. (1982) Phaseequilibria ofamphibolites from the Post Pond American Mineralogist, 73, 500-506. volcanics, Mt. Cube quadrangle,Vermont Journal of Petrology, 23, Popp, R.K., and Phillips, M.W. (1987) Accommodation of ferric iron in 383426. calciferousand subacalciferousclinoamphiboles. Geological Society of Vedder, W., and Wilkins, R.W.T (1969) Dehydroxylation and rehydrox- America Abstracts with Programs, 19, 808. ylation, oxidation and reduction in micas. American Mineralogist, 54, Popp, RK., Gilbert, M.C., and Craig, JR (1977) Stability of Fe-Mg 482-509. amphiboles with respect to oxygen fugacity. Amencan Mineralogist, Witte, P, Langer, K., Seifert, F., and Schreyer,W. (1969) Synthetische 62, r-12. Amphibole mit OH-uberschussim SystemNa'O-MgO-SiO'-H'O. Na- Rebbert,C R., and Hewitt, D.A. (1986)Biotite oxidation in hydrothermal turuissenschaft. 56. 4144 | 5. systems:An experimental study Intemational Mineralogical Associa- Wones,D.R. ( I 963) Physicalproperties ofbiotites on the join phlogopite- tion Abstracts with Progam, 207. annite.American Mineralogist, 48, I 300-l 321. Robinson, Peter, Spear,F.S., Schumacher,J.C., Laird, J., Klein, C., Ev- ans, B.W., and Doolan, B L. (1982) Phaserelations of metamorphic amphiboles:Natural occurrenceand theory. Mineralogical Society of Amenca Reviews in Mineralogy, 98, l-211. Scofield,N, and Gilbert M.C. (1982)Alkali amphibolesof the Wichita MeNuscnrgr REcETvEDMev 28, 1987 Mountains. Oklahoma GeologicalSurvey Guidebook, 21, 60-64. M.r.r-uscnrpr AccEprED Jer.rulrv 14, 1988