American Mineralogist, Volume 58,pages 480-494, 1973
A Grystal-GhemicalStudy of SyntheticMagnesiohastingsite
Mrcnrr P. SeIvrBrl Departmentol Geology, Uniuersityol California, Los Angeles,Cal"ifornia 90024 Abstract
Mcissbauerand infrared spectra of synthetic magnesiohastingsite(Mh), NaCa2Mg4Fea+SisAlzOzz (OH)2, indicate that, except at high values oflo,, Fe occurs both as Fe2+and Fe3+contrary to the ideal formula. The amount of Fe2+produced is a function of the oxygen fugacity of synthesis.Oxida- tion and reduction of Fe may be carried out reversibly under hydrothermal conditions at relatively fast rates. Apparently this does not involve gain or loss of O(3)-H hydroxyls contrary to the case ofFe2+ bearing sodic amphiboles. Octahedrally coordinated cations are significantly ordered among the M(1), M(2), and M(3) sites in agreement with recent crystal-structure determinations. The distribution schemeis dependenton the amount of Fer+ and Fea+ present. Refractive indices (c = 1.642-1.657t :1.653-1.672),color, and unit-cellparameters (a:9.928-9.933 A; b: 18.015- 18.029A; c : 5.282-5.257A;9 : 105.43-105.46o)also reflectthe lo, of synthesis.Physical proper- ties agree well with a sample of natural Mh similar to U-1236 of Larsen (1942). The experimental data suggesta mechanismto produce Fez+in synthetic Mh which involves an excessof two protons (and/or anionic defectsat low lo,) per unit formula.
Introduction or other phenomenaat the nuclear or atomic level (Colville et aI, 1966). As a preliminary step in the study of the phase Mcissbauer and infra-red spectra of synthetic relations of the calcic amphibole end-membermag- magnesiohastingsitesreveal that Fe occurs as Fe3* nesiohastingsite,'z(NaCa2MgaFes*Si6Al2O22 ( OH ), ), and Fe2*in variable proportions accordingto experi- amphibolesof this bulk composition were synthe- mental conditions.The mere existenceof Fe2*ions sized hydrothermallyat a variety of temperatures, in the structure suggestedthat a mechanism,as yet pressures,and oxygen fugacities. Significant and poorly in the production systematic variation in physical properties of the known, might be operative magnesiohastingsite." amphibolesproduced led to a detailed investigation of "reduced This study presents an attempt at correlating of the influence of the intensive variables (mostly physico-chemicalchanges in synthetic amphiboles oxygen fugacity) on the chemistry of the synthetic environment,and an minerals. with the variable crystallization resolving the of the During the last decadethe applicationsof infrared attempt at nature changes. absorptionand Mcissbauerspectral techniques (e.9., Experimental and Analytical Techniques3 Burns and Strens, 1966; Whitfield and Freeman, Hydrothermal runs. Synthesis.,ofmagnesiohastingsite was 1967;Bancroft et al, 1.967a, b; Bancroft and Burns, carried out using conventional hydrothermal apparatus, 1969; Ernst and Wai, 1970; Burns and Greaves, and the oxygen buffer techniqte (e.g., Eugster and Wones, l97I), coupledwith precisethree-dimensional crys- 1962). Abbreviations for the buffers are listed in Table I, tal structure determinationsof the clino-amphiboles and the necessary thermodynamic data in Table 2. Pt sample were NNo Ag-Pd (e.g., capsules used with the bufier and Papike et aI, 1969, to whom the reader is capsules with the other bufters. Pressures were measured referredfor site nomenclature),have addedgreatly with calibrated Bourdon tube gauges, temperatures with to our understandingof one of the most complex chromel-alumel thermocouples. Presumed accuracy is -+ 10 mineral series.Before the adventof suchtechniques bars, and -16"C, respectively. Starting material consisted the intracrystalline chemical behavior of synthetic of mixtures of reagent grade.oxides, hydroxides, and car- bonates in the stoichiometric proportions for Mh. amphibolescould be delineatedonly by physical Optical and X-ray measurements. Optical properties were measurementsrelativelv insensitiveto order-disorder obtained on a flat stagepetrographic microscope using refrac- tive-index oils of .0O2 interval. Averaqe values of several 'Present address: c/o H. Raquillet, Bd Graville, 84200, Carpentras,France. " Detailed description of the experimental apparatus and ' End-membersnomenclature follows Ernst (196E). the analytical techniques are presented in Semet (1972).
480 CRYSTAL.CHEMISTRY OF SYNTHETIC MAGNESIOHASTINGSITE 481
TesI-B l. Abbreviations and Symbols" Trsru 2, Thermodynamic Data on the Oxygen Fugacity Buffers oxygen fugacity buffers other synbois
= - IQF 2Fe+Si0Z+02 Fe2Si04 n Refractive index A, a , C(P l) rog Io, T Fl,lQ 3Fe2Si04+02= 2Fe304+3Si02 Mh Magnesi ohasti ngsi te Reference NNO 2Ni+02= 21i9 IO.- oxygen fugacity IQF 29382. 7.51 .050 Eugsterdnd l,lones,1962 MMo 6Mn0+02= 2t4n304 Fr'lQ 25660. 8.92 .092 lionesand Gilbert, 1969 Cco 4Cu+02= 2CuZ0 NNO 24709. 8.94 .046 Eugsterand Wones, 1962 HM 4Fe304+02= 6Fe203 MMO 25680. I 3.38 .0807 Huebnerand Sdto, 1970 CT 2Cu20+02= 4Cu0 cc0 I 7050. 6.85 .096 Semet,nanuscript 'Numbers Hr,l 24912, 14.41 in parenthesesin tables and figures represent one standard ,0i9 Eugsterand Wones, 1962 deviation unless othemise stated and apply to the last significant 'l.966 CT I 3928. t0.34 .0105 Senet,manuscript djgit. Example:2.35(9) = 2.3s f 0.09, or'1.996(12)= r O.Ot2.
* T is the temperaturein oK; P is the total pressurein bars determinationsare presentedin Table 3. The resultsare in accordwith an earlierdetermination by Colvilleat ul (1966). Refinedcell parameterswere computed from powderdiffrac- of startingmaterials (Semet, 1972). Yields of 90- tion data (Cu-Ka radiation)by a least-squareprogram (Burn- 100 percentare common on initial synthesis.At ham, 1962)using Si (u : 5.43054A) as an internal standard. Indexingwas done by comparisonwith singlecrysral (U-1236)1 low oxygenfugacities, however, significant amounts intensitydata and by usingthe Fortran lV programof Evans (up to 10 percentvol.) of the high temperature et al (1963). Typical powder data are presenredin Table 4, assemblageof equivalentbulk composition(clino- Average unit-cell dimensionsobtained for the syntheticMh at pyroxene* olivine * nepheline* spinel ! meli- various/,,, and sampleU-1236 are listed in Table5. Miissbauerspectra. Data collectionwas done on a con- lite) alwaysappear on first synthesis;repeated grind- ventional loud-speakerdrive spectrometer( Kiindig cr a/, ing and recrystallizationyield a product which con- 1966).Sample preparation and analysisof the spectrafollow tains virtually only amphibole.The amphibolesyn- procednresoutlined by Ernst and Wai (1970). More re- thesizedfrom an oxide mix is always very fine cently, computer processing of the data was accomplished grained(10 x 5 x 5 pm) and prolongedrun time utilizing a Fortran IV least-squarecurve fitting program (Dollase, 1972). All isomer-shiftsare quoted relativeto 0 doesnot bring about significantgrowth. Crystalsof mm/sec for Fe;i in metallic iron. a largersize (100 x 20 x 20 y"m)can be obtained Irtlrared spectro. A Perkin-Elmermodel 421 was used when amphiboleis synthesizedfrom the high tem- in obtainingthe infrared spectraof the amphibolesby the peratureassemblage of equivalentbulk composition. KBr pellet method. The fundamental O-H stretchingregion The amphibole presumablynucleates very easily (3800-3600 cm-') was covered in more detail with the abscissaexpanded 2X, 5x, and l0X. High resolutionspec- from the highlyunstable oxide mix, whereasit does tra were obtained on carefully dried specimensand show not from the metastablehigh temperatureassem- very little absorptiondue to absorbedH,O. blage. In the latter casethe clinopyroxenecrystal- Electrott nticroprobe analysis. Chemical compositionsof lites, becauseof their similar atomic configuration, selectedsamples of syntheticMh and sampleU-l236 were may serve as nucleationcenters for the growth of obtained on an ARL-EMxelectron microprobe. Analyzed hornblendeswere used as standards.Empirical correction the amphibole(e.9., see Greenwood, 1963). factors(Bence and Albee, 1968) and a Fortran IV program A temperatureof approximately850"C, and a providedthe refinedanalyses presented in Table 6. Accuracy pressureof 2000 bars,were found to be the experi- is consideredto be better than 3 percent of the amount present for sample U-1236, and approximately 7 percent (due to relativelylarge count rate variation on the n)inute Tesre 3. Optical Properties of Synthetic Magnesiohasting- syntheticcrystals) for the other samples. site*
_1og Synthesisof Magnesiohastingsite Buffer fg, zAc Magnesiohastingsite can be readilysynthesized at IQF '13.76I 8.56 1.642(4) r.653(4) 24" FMQ l.6so(5) 1.662(4) 26" a variety of experimentalconditions from a number NNO I 2.98 1.637(5)** l.6ae(6)** 23" -10.50 'The r.653(8) ',r.669(4)r.668(8) 25' samplereferred to as U-1236 was iu 7.74 'l.6s7(3)r.6s4(4) 25" collectedby the CT 2.03 1 672(4) author from the samesite (Iron Hill, GunnisonCo., Colc.) as U-1236 from Larsen (1942). A new microprobeanalysis * Samplessynthesized at 850'C,2000 bars. is presentedin Table 6. ** Theanonalously low indicesare dueto Fe loss to the Pt samplecapsule, 482 MICHEL P. SEMET
Tenr-e 4. X-ray Powder Data for Synthetic and Natural Tesre 5, Cell Parameters of Synthetic* and Natural** Magnesiohastingsite Magnesiohastingsite
ab p ;3 Buffer AA (degrees )
dcalc dobs t/lo s.s21(2) t8.ol5(e) 5.2s2(3) ros43(4) 9r0.7(8) avq]al or" u" 0 9.01 9.00 2 9.03 9.05 4 I 8.45 8.40 20 8.47 8.46 33 s.e3o(s) 18.025(8) 5.zeo(4) r0s.43(4) er20(r0) I 5.09 5.06 I !t4avg or 5 -l 4.91 4.87 ? 0 4.51 4.49 5 4.51 4.51 9 s.e33(2) 18.02e(4) s.2e3(r) r05.43(r 9r3.0(s) avgfl9 oT.," ru ) 2 4.22 4.22 3 4.23 4.24 2 I 3.975 3.978 2 -l ll9 . s.s3z(2\ r8.or5(4)s.z8e(l) r05.43(2) 912.2(3\ 3.891 3.883 3 avg or J" 1 3.372 3.369 31 3.379 3.378 21 2 3 279 3 273 3.286 3.283 3? s.e33(r) rs.ozs(3)5.2s7(r') r05.44(2) 9r4.3(3) avgll or-. o 3 3.138 3.133 r00 3.147 3.144 > 100 -3 3.056 3.050 2 s.s26(5) r8.02e(e) 5.zsi(4) r05.46(5) 9r3.7(8) 2 2.933 2.932 37 2.939 2.940 l8 avg!T ol -," lz 3 2.815 2.8il t6 2.822 2.820 26 2.755 2.752 2.761 2.759 l5 Nat. r4h.** 9.940(3) 18.050(4) 5'310(4) r0s.45(4) 9r8.4(4) 2A ' l 2.700 2.696 48 2.705 2.704 samplessynthesized at 850'c, 2000bars. -1 2 2.621 2.626 ** Iron Hil1 colorado. 0 2.589 2.586 3l 2.593 2.592 , 2.553\ 2.548 35 2.558I 2 qqo 1l 0 2.5581 -2 2.456 2.450 3 -2 2.418 2.410 4 mental conditions combining a fast recrystallization 3 2.387 2.387 7 2,393 2.393 ?zqll 2.353 of the starting material to an amphibole, a high 2.344 39 2.3501 ,Z 2.344t yield (close to 100 percent), and a good crystal- -l 2.299 2.295 14 2.303 2.302 ,' 3 2.251 2.250 3 linity. Run conditionsfor synthesisof the samples -2 2.221 2.219 6 2 2.158 2.156 30 2.162 2.162 zo cited in the text are presentedin Table 7. 2 2.037| 2.042| ) 2.035 24 ) 2.042 e Oxygen fugacity has a pronouncedeffect on the 2 2 z.oztI z.oqzl 3 I 2.014 2,012 2.018 2 017 q color of synthetic magnesiohastingsite.At high 'l3 0 2.003 2.002 2 2.007 2.009 0 I .9599 I .9564 '102 I .9632 I .961l oxygenfugacity (cr bufter) the color of the amphi- 5 0 1.901 5 I .9012 l 9068 I .9062 ,i
'II .8882 1.8876 I 8923 | .8947 2 -l .8643| r .8581 T.lsLp 6. Chemical Composition of Synthetic and Natural 2 l.d5of, I -1 I .8465 ' Magnesiohastingsite
3 1 .8221 'lI .821 0 |.8270 1.8264 4 'I 0 l0'l 1 .8022 .7998 | .805] I .8039 3 2 3 4 -5 I .7651 1.7648 -l '1 'I .7s05 1 7499 si02 42.OA 40.94 41.8 41.2 0 l0 r .6995 | '12.85 I 1,7017 Al 11.90 12.1 lt.6 0 0 r .7023I 203 ' 3 9 1.6956 Ti02 0.58 I 1.6947 -2 8 'l1.6893 \ I .6874 Fe0 8.39 r0.57 8.4 8.5 -l 3 .6880t '19.1 lt1g0 18.82 15.01 18.5 0 2 | .6727 I.6741 '19 'I .6524 .6529 't3,2 4 6 'lI .6486 1.5476 1 Ca0 I 3.09 12.93 12.9 4 I .6394 1.6401 1 I .6429 1.6428 2 I .617l 5 1 il 0 1.6i49 I .6140 7 1,6t75 ||ln0 0.26 6 0 0 I .5035 1.5926 5 I .5979 I s978 I -l Na20 3.62 2.59 5 r . s808 I .5780 7 '].5543\ 4 0 2 I .5507\ r2 '| -6 1.5490 '' .5538 7 Kzo 1.24 0 2 I .5505t '1 ']3 1.5542 | 'I .5197 4 I 9 2 I .5152 .5153 'I 1.5202 2,10 2.03' 2,1*r 2,1** 0 12 0 1.5019 1.5022 I I .5043 I .5043 9 't00.00 '100.2 't2 Total I 00.00 98.? 2 1.4724 1.4762 2 -6 4 2 1.4561 I r .4683 4 4 2 1.4663t I l Theoreticalnagnesiohastingsite, Fe recalculatedas Feo. I 5 3 I .4652\ r .4641 I 2 2 3 1.4637 | 2. U-1236,see Larsen(1942); the structural fornula pcalgulated on the basis of l5 si, Al, Ti, Fe, i4g,Ca, andMn, the Fe2+/Fe3+ratio given ',t 3 ll 0 1.4571 1.4565 I .4598 I.4615 Z by Miissbauerspectra, and (0H)- as required to compensatefor total 6 I 1.4473 1.4467 26 I .4508 I .4509 28 cationic chargegives: 4 l0 0 r .4390I -5 'I I .4391 '|.4400 f K.23Na.73ca2.61r,rn.o3Ms3.47Fe2t7Fe3t2ntv{5ri.s6si5.95ntlvs5o22{oft)1.se -3 .4199 1.4213 'l 3. A3-llT, syntheticmagnesiohastingsite, 850'C, 2000 bars, cT buffer. r .4146 .4t28 6 1.4077 1.4072 4. A4-11C,synthetic mgnesiohastingsite, 850'C, 2000 bars, IQFbuffer. 6 'L1.4002 I .4030 3716 1.3717 * H20by difference. 7 r.3619\ I . 3521 l0 I . 3651 1.3517' I .3657 ** Theoretical Hro fron colunn I CRYSTAL-CHEMISTRY OF SYNTHETIC MAGNESIOHASTINGSITE 483 bole is pale golden yellow. The colors referred to TesI-e 7. Experimental Run Conditions for SelectedSamples here were observedwith a hand-lens on a sample of Synthetic Magnesiohastingsite* of the charge;under the microscopethe small crys- -lnn'-' 'v2fa T Tine tals are only very faintly colored or colorless.At sanple starting material Buffer ("c) (ht IQF-A3 oxide mix IQF 18.56 850 50 intermediateoxygen fugacities (ult, NNo, rtrQ buf- 0x to IQF oxlde mix IQF 18.56 850 2a fers), the color rangesfrom pale yellowish green FI4Q-A2 oxide mix Fr4Q I 3.76 850 46 CI-42 oxide nix CT '18.2,03 850 26 to pale blue-greenfrom higher to lower fugacities; CT to IQF l'lh (CT buffer) IQF 56 850 l0 at low oxygenfugacity (lqn buffer), it is very pale IQF to CT l,lh (IQF buffer) .T 2.03 850 l0 NNo-AI oxide mix NNO 12,98 850 50 grayish green to white. Hl'l-42 oxide mix Hltl 7,74 850 50 Studies of a number of synthetic iron-bearing 0A5-19-7lU ilh (unbuffered) cc0 I 0.36 707 I 50 AA9-llT I'lh (IQF buffer) CT 2.03 850 19 hydroxylatedsilicates have consistentlyshown a reg- M9-l2T r'irh(IQF buffer) CT 2.03 850 42 ular variation of refractive indices with oxygen * fugacity (e.9., Ernst, 1960, 1962; Eugster and Total Dressurewas maintained at 2000bars + l0 bars for all runs. Wones,1962; Wones,1963; Forbes,1969). This variation, also observedin this study (Table 3), ratiosof the peak areasin a multisitecompound are, has been variouslyinterpreted as representingsolid in general,directly proportional to the absolutenum- solutionwith other end-membersof the seriesor the bers of contributingFe57 nuclei. To be rigorous,the production of oxyphases.However, the specific re- ratio of the recoil-freefractions f/5 of Fe57at the fractive energyfor FezOgbeing larger than for FeO different sites should also be taken into account; (Larsenand Berman,1934, pp. 30-31), oxidation however,this ratio doesnot, in general,depart sig- of iron in a crystal concomitantwith minor changes nificantlyfrom unity. Detaileddiscussion of the prin- in the structuredue to the redistribution of electrical ciples involved is readily availablein the literature chargescould accountfor the whole eftect (or part (e.s., Wertheim, 1964; Goldanskii and Herber, of it). 1968). The variability of both color (e.9., Burns, 1970, The amount of Fe enteringeither the eightfold- pp.63-75) and refractiveindices with oxygen coordinatedM(4) site and the Si I and Si II tetra- fugacity, which presumablyindicates a changein the hedral sites in magnesiohastingsiteappears to be valence state of iron in magnesiohastingsite,negligible.If enoughCa is presentto fill the M(4) prompted a detailed investigationof the synthetic site, solid solution toward cummingtonite-grunerite productsto determinequantitatively the nature of should be non-existent,as disorderingof Ca and the variation. Fe2*between M(4) and,M(l) + M(2) + M(3) hasnot provensignificant in the recentcrystal struc- Interpretation of the Spectra ture determinationof C2/m amphiboles (Papike et aI., 1969). Similarly,when enoughAl is present Miissbauer spectra of magnesiohastingsile to preservethe electricalneutrality of the structure The parameterswhich can be obtained from a by substitutingfor Si in the tetrahedral sites, it Mcissbauerspectrum of an iron-bearingparamag- doesso preferentiallyto Fe3*.Ernst andWai (1970) netic compound are: the isomer shift (r.s.), the have reporteda case (sampleC-4980) where Fes* quadrupole splitting (q.s.), the width (at half proxying for Si seemedto be requiredon the basis height) or the half width at half height (uwnH), and of the chemicalanalysis. Mijssbauer spectra of this the ratios of the areasof the contributingpeaks (or sample of magnesioriebeckiteand its heat-treated dips). Both r.s. and e.s. are sensitiveto the valence equivalentsshow absorptionattributable rto FeS*in state of the Fe ion and, to a lesserdegree, to the tetrahedralsites. Furthermore, synthetic stlrdies have crystal-chemicalcharacter of the site. The observed shownthat significantFe3* can enter the tetrahedral half-width of the spectrum lines always departs site in the related diopsidestructure (Huckenholz significantlyfrom the theoretical0.105 mm/sec be- et al, 1969; Hafner and Huckenholz,1971). It is causeof thicknessbroadening, and/or scatterof the thus possiblethat, evenwhen Al is presentin excess electricfield gradientsat the differentnuclear sites, u l' is the fraction of Fe"' nuclei which absorb the 7 ray and/or diftusionaleftects. The values of r.s., q.s., resonantly without recoil. It is dependent on temperature and nwnn are generally given in units of mm/sec and on the vibrational spectrum at each iron site in the (1 mm/sec = 7.694 x 10-,0ergs). Finally, the structure. MICHEL P, SEMET of what is required to fill the Si I and Si II sites, lessintense dip (D'). It is not obvious,however, that randomizationof Al and Fe'- amongthe M(2) and the Fet* absorption areas represent overlapping tetrahedralsites could occur at high temperatures, of at leasttwo dips (AA' and BB'). This conclusion but to the author'sknowledge such an occurrence was arrived at by comparingthe spectraof magnesio- has not yet been documented.In the generalcase, hastingsite synethsizedat higher oxygen fugacity then, the Mcissbauerspectrum of calcic amphiboles (samplect-t2, cr buffer) wherethe overlapis more containing both Fe'9* and Fes* would consist of easilyperceived (Fig. 2). Furthermore,the availability absorption peaks contributed by six different octa- of spectraof amphibolessynthesized at a variety hedrally coordinatedFe "species,"namely Fe2* and of f o, valueshas led to an unambiguousidentification Fe'* in M(l), M(2), and M(3). Greaveset aI and assignmentof the peaks.The peak locationsdo (197I) and Burns and Greaves(197I) have,for not vary greatly in the series,so that the complex instance, resolved the Fe2* spectrum of actinolites spectrumpresented in Figure I can be fitted to over- into three quadrupole split doublets, the contribu- lapping doubletswith reasonableconfidence. Even tions arisingfrom Fe2*in M(1'), M(2), andM(3). then convergenceand a reasonablex resultedonly It is, however, very difficult if not impossibleto after severalcomputer runs that releasedone by one resolvethe Fe3*spectra into three different doublets the positions,approximate area ratios, and half-width becauseof the smallersplittings and the similarityof constraints.The valuesof the Mcissbauerparameters the M sites.Greaves et al (1971) and Burns and of sampleoAb-r9-71u are comparedin Table 8 with Greaves (1971) observedthe Fe3* spectrum of those obtained from selected spectra of natural actinolitesto consistof a singlebroad doublet. magnesiohastingsiteand of amphiboles synthesized The M FeZ*tr pg5+n----_----_------Fe3+I c) o oA5-19-7tU olt AA=54% c BU=22% c :t CC'=197" o o DD=5% c) o o) E. Velocifvin mm/sec relofivti lo met Fe57 FIc. 1. Mcissbauerspectrum of sampleoa5-19-71u ("annealed Mh,707oC, 2000 bars,cco buffer). 0 mm/seccorrespondstothe rsomer shift of metallic Fe57. CRYSTAL-CHEMISTRY OF SYNTHETIC MAGNESIOHASTINGSITE 485 366S CHRNNELNUMEEF 75 100 S LLI f{ J G= ctr E Zn qq LLI F CE t (-] = - z 0.98 f E (-J Cf-42 2 kb PHro,85O' C VELOCITT]N IlM/SEC Frc. 2. Computerplotted M The spectraof the Iqr-bufferedMh (labeledIqr spectrum (rqn to cr) was also fitted with two doub- in Table 8) were fitted to three quadrupolesplit lets of equal half-widths (Kiindig's program) but doublets,with widths constrainedto be equal. The gave a much less satisfactoryx (1.49), although the parametersdeduced from the spectradid not dif- other parameters were within one standard deviation fer from the averagemore than two standarddevia- of the values reported in Table 8. Attempts to fit the tions. The same procedurewas followed for the natural magnesiohastingsitespectrum to two FeS* ruq-12 spectrum.Two resolvedFeS* doublets with doublets did not result in convergence, indicating unequalwidths gave the best 1 for samplecr-A2.6 that if more than one Fe3* "species" existed, it must When the samespectrum was fitted with two doub- be only minor. The three quadrupole doublets fitted lets of equal widths (Kiindig's 1969 program), the with equal Fe'- half-widths and an independent Fe3* samepeak positionsresulted (x = 1.15), but the half-width gave a good resulting 1. It is apparent area ratio differed signifi.cantly,being 1.78 instead from the table that, in general, the peak positions of 1.17;however, 1.17 is consideredmore reliable. agree well. The Fe3* quadrupole splitting in samples The spectrumrefinement for samplecr to reF con- nrraq-a2and cr to IeF are, however, slightly larger verged only when the three doublets were con- than the average. This may be due to an unresolved strainedto be of equalwidth. The rer to cr spectrum contributionof a secondFe3* "species." was also resolvedinto two Fe3.doublets of unequal On the basis of the foregoing discussion of in- half-widths.Except for the arearatios, which reflect dependent crystal-chemicalevidence (e.g', Ghose, different experimentalconditions, the parameters 1965; Ernst, 1968; Papikeet a\,1969) and of pub- agreewell with thoseobtained for cr-e,2.The same lished spectra of calcic amphiboles (Bancroft et al, 1967a; Greaves et al, l97l; Burns and Greaves, uA 797l), the following peak assignmenthas been made. value of 1 less than 1.00 means that the rnrs de- to Fes*in viation per channel is less than the variance at a single Doublet AA' (Figs. 1 and 2) is attributd channel but is a statistically acceptablevalue if the obser- the M(2) site; it represents the major contribution vations consistof a limited number of channels. of Fe3- to the total absorption and confirms the gen- 486 MICHEL P. SEMET Tasr-r 8. McissbauerParameters of Synthetic and Natural Magnesiohastingsite* Outer Fe2+D,oublet Inner Fez+ Doublet outer Fe3+Doublet Inner Fe3+ Doublet Smple ** x t. s. Q.s. Hl.lHH I I.S Q.S. H}IHH I LS. Q.S. HUHH % Q.s I+IHH IQF avg ofz r r3(r)2.73(2) .le(t) 53(3) r.r4(2) 2.il(3) re(r) 24(3) 26(el .7s(6) .re(r) r3(r) I .55 Filq-A2 r.r0(r ) 2.74(2) .re(r) 37(2) r.rs(2) 1.92(4).re(r)17(21 36(r) .84(r) .re(r) 45(r) 35(3) r.28(7) .3r(2) 46(2, 35(r) .73(3) .25(r) s4(2) .90 0A5-t9-7lU r.12(r 2.67(2) .23(t le(l 17lr\ r ?o/11 ,al1\ ,rlt\ ) ) ) l.r3(3) 1.87(7) 23(r) 5(t) 3e(1) .7e(r) .26(1)54(21 'I.94 CTto IQF r.l2(r)2.7'tl2) .2r(r ) 47(21 r.r7(2) 2.01(4) .2r(l)r8(3) 37(2) .8s(2) .2r(r ) 3s(r) .08 IQFto CT 37(2) r.23(4) .34(l) 66(3) 37(r) .7e(2) .24(1\ 34(2\ I .13 Nat. ltlh. Colorado r.r2(r) 2.64(l) .21(l) 37(l) r.r0(3) 2.0r(5) .21(t ) 7(l) 3e(r) .80(r) .2s(r) s6(l) 105 * A'll Isomer Shifts are quoted relative to Fe57 in netallic jron. *All Isorer Shifts, Quadropole Splittings, and Half lidths at Ha'lf Heiqht are quoted ln m/sec. Numbersin parentheses indicate precision on the last digit; accuracy-nay be iignificantly louer for strongly overlapping peaks. erally acceptedview that trivalentM cationspresent Ito, 1967; Vedder and Wilkins, 1.969) are directly in clinoamphibolesare concentratedin this site. The applicableto amphibolesin view of the close simi- remainingFe3- doublet BB' is attributed to unre- larity of their proton environment(e.g., Burns and solvedabsorption from Fe3*inthe M(l) andM(3) Strens,1966; Burns and Prentice,1968; Ernst and sites. The fact that the width of the latter doublet is Wai, 1970). In amphibolesthe hydroxyl is directly larger in those spectrawhere the equal width con- coordinatedto two M(l) and one M(3) cations, traint was not applied (cr-t2 and rer to cr) also the proton extending toward the tetrahedral chain seemsto favor such an assignment.The two Fe2* cavity (Papike et al., 1969). Combinationsof ca- doubletsCC'and DD' arevery similarto thosefound tionswith differentelectronegativities and/or polariz- in actinolite (Bancroft et al, 1967a; Greaveset al, abilities(mostly Mg, Fe2*,FeS., Al) in M(1) and 1971; Burns and Greaves,l97l). The outer doub- M(3) influence the bond-strengthof the hydroxyl let CC', which is alsothe mostintense, is due to Fe2* ion. This, in turn, is reflectedby slightly different i.r. in M(l) and,M(3) (unresolved);the innerdoublet absorptionfrequencies. In the talc-minnesotaiteseries DD' representsFe2* in M(2). Assumingthat Fe2* (Wilkins and Ito, 1967) and,in the tremolite-actino- was randomly distributedamong the M sites, and lite series(Burns and Strens,1966; Wilkins, 1970; that the recoil-freefraction / is the samefor all sites, Burns and Greaves,l97I), the OH stretchingfre- the arearatio shouldbe quencies corresponding to MgMgMg, MgMgFe2., MgFe2*Fe2*,Fe2+Fe2+Fe2+ groups coordinatedto the zM(r)+ M(3) _3. hydroxyl are equal within a few cm-1. Here, the 2M(2) 2', proton is vibrating in a cavity closedby Si6Oparings. this clearlyis not the casefor most of the specimens In trioctahedralmicas of the phlogopite-biotiteseries, studied.For Fe3*,this ratio differs even more from however, the O-H bonding is perturbed by the Al- that calculatedfor randomdistribution. for-Si substitutionin the tetrahedrallaver. Additional spectranot reportedin Table 8 were taken on samples synthesizedemploying the NNo and Hvt buffers to determinethe Fe3-/Fe2*ratios. Tenre 9. Oxidation Ratios in Synthetic Magnesiohastingsite Table9 presentsthe pertinent datafor thosesamples * ** -1og f^ Fe3+ FeZ* and a summaryof the dataof Table 8. Sample Percent Percenr I nlrared spectra ol magnesiohastingsite IQF*$ 18.56 13 87 FMQ-Az 13.76 46 54 The investigationof infraredabsorption of mineral- NNo-AI 1?.98 57 43 ogicallyimportant compoundshas proven 045-19-7lU 10.36 76 ?4 usefulin Hr,t-42 7.74 89 11 characterizingstructural units and in quantitative cT-42 2.03 100 studiesof solid-solutionseries or mineral mixtures CTto IQF 18.56 35 65 IQFto CT 2.03 100 (e.9., Lyon, 1967). In particular, studies of hy- droxyl-stretchingabsorption in mineralsof the talc * Ratio of the Miissbauerabsorption area due to Fe3+nuclei to the tgtal absorptionexpressed in percent. and mica groups (e.g., Farmer and Russel, 1964, ** l0O - Fe3+- *** 1967;Vedder, 1964; J/rgensen, 1966; Wilkins and Averageof two. CRY STA L-CH EM I STRY OF SYN THET I C M AG N ES IO H A STI N GS IT E 48'7 A similar situationoccurs in syntheticmagnesio- ratio hastingsitewhere the Al for Si substitution in the lx * ( * tetrahedralchains parallelsthat of the phlogopite- -'t-"'r[-*/:\r-"/,. ."( \' \' biotite seriesand where the I site. sandwichedbe- tweenopposite tetrahedral chains, is occupiedby Na. (Vedder,1964), where x is the concentrationof the Spectra of the synthetic amphibolesare compared substituting ion. Spectra of synthetic magnesio- with a spectrumof natural magnesiohastingsite(U- hastingsitetaken at high resolution have been fitted 1236) in Figure 3a and b (abscissaexpanded 2x, by hand to Gaussianlike curves after normalization no special precautionsregarding HrO). Selected spectrataken at higherresolution (abscissa expanded 3800 3700 3600 10x, dried specimens)are comparedin Figure 4 to a spectrumof syntheticpargasite (NaCazMgrAlSia- A12O22(OH)2, synthesizedhydrothermally at 800oC, 2000 bar), The spectraare seento consistof two major bandsand severalminor peaksand shoulders. The major bandsat 3705,3675, and/or 3660 cm-l can be assignedwith reasonableconfidence to OH ions bonded to MgMgMg, MgMgFe2*, and/or MgMgFes.(MgMgAl for syntheticpargasite) groups in the zM(l) + M(3) sites.The minor peaks around 3645-3620 cm-1 most probably represent MgFe2*pez*-OH,MgFe'?*Fe3*-OH, and MgFes--OH (MgAlAl-OH) groups.The observedfrequencies for those groupingsare similar to what is observedin phlogopite-biotitemicas, although the assignmentdif- fers somewhatfrom thoseof J@rgensen(1966) and Wilkins (1967). The limited substitutionof Fe (or o A1) for Mg in magnesiohastingsite(or pargasite) H precluded observation of bands corresponding to o|l o FeFeFe-OH (AlAlAl-OH). The significanceof the J shouldersin the spectraof the oxidizedand reduced samples(cr to reF, reF to cr) as well as the broad, almostfeatureless spectrum of the natural sampleis lessclear. In samplecr to reF,where significant Fe2* and Fe3*occur, the broadeningof the band at 3670- 3660 cm-' may arise from MgFes-Fe3--OHand/or MgFe2*Fer*-OHgroups; in samplereF to cr, where only Fe3*was detected in the Mcjssbauerspectra, the 3660 cm-' shoulder is of unknown origin (Si-Al orderingin the tetrahedralchains may causesuch a splitting). The spectrum of the natural specimen (U-1236) most probably representsa combination of severalbands arising from the complexsubstitu- 3800 3700 3600 tion of differentcation species(Mg, Fe2*,Fe3*, Ti4*, Cm-l Al) among the M sites as well as the influenceof natural magnesio- Al-substitutionin the tetrahedralchains and of in- Frc. 3. Infrared spectra of synthetic and hastingsite completeoccupancy of the .,4site. Where ordering among the three cationscoordi- (a) 1. ox to rer (rer buffer) rer (reduced nated to the hydroxyl in binary solid-solutions 2. cr to Mh) does L U-1236(natural Mh) not occur, the intensitiesof the absorptionbands (b) l. ox to cr (cr buffer) correspondingto the possiblegroupings are in the 2. tqr to cr (oxidized Mh) 488 MICHEL P. SEMET of the non-uniform background; then the ratios of 38oo 37oo Cm-l 3600 width-at-half-height X peak-height products have been calculated.The results obtained are presented in Table 10 and compared with the same ratios Cuprile-TenorileBuf fer 850'c. 2 Kb obtained from Mcissbauerspectra where available. When allowance is made for the uncertainty in background position and the strong peak overlap, the agreementis rather good. The observedratios for samplesee9-11r and aa9-12r,synthesized under reducing conditions (tOr buffer) and rerun under o H oxidizing conditions(cr buffer, 850oC,2000 bars) o o for 19 and 42 hours respectively,do not differ signifi- J cantly from each other; neither do they differ from the ratios obtained from the Mcissbauerspectrum of samplereF to cr (10 hours), indicatingthat no substantialcation migration has taken place in the longer runs. Using samplesprepared as for the high resolution spectra,repeated spectra of magnesio- hastingsitesynthesized at low oxygen fugacity (rqr buffer) or synthesizedat high lo, but reducedat low 38 00 f6, consistentlyshowed a broad, flat, and featureless absorption band developedbetween 3640 and 3540 Frc. 4. Infrared spectra of synthetic magnesiohastingsite cm-'. The possiblesignificance of this feature will and pargasite(high resolution). be discussedbelow. Comparisonof band intensitiesin the spectrum sampleoA5-19-71u, which was synthesizedat lower of syntheticpargasite (Fig. a) indicatesthat Mg and temperatureand for which a = 1.653, y = 1.668, Al apparently are randomly distributed among the Fe3*/(Fe3.* Fe'z*)= 76 percent.Except for sam- M(l), M(2) and M(3) sites. ples synthesizedunder NNo bufter conditions,which Discussion show anomalouslylow indices (probably becauseof Optical properties. Variations in refractive in- significantloss of Fe to the Pt samplecapsule used dicesof magnesiohastingsitesynthesized directly from an oxide mix (Table 3) havebeen plotted (Fig. 5) againstthe Fe3-/(Fe3*+ Fe2*)ratios calculated from Mcissbauerspectra (Table 9) of the sameor similar material.Also includedin Figure 5 are the data for Tesre 10. Ratios of Band Areas in the Infrared Spectra of Synthetic Magnesiohastingsite Calcul ated** IQF-A3 I : .83 : .20 1 : .80 : .21 CT-A2 I : .46 : ..|0 I : .54 : ,10 IQF FMO ONNO OA5.I9-7IUHM AAg-liT I : .80 : .19 I : .85 : .24*** M9-l2T I : .87 : ,22 l:.85:.24*** Fro. 5. Variation of refractive indices of synthetic mag- * = x mole fraction of Fe in the M(l), M(3) sites. nesiohastingsite with oxidation ratio. The size of the sym- ** Calculated from data of HiissbauersDectra. probable (a *** Calculated for SampleIQF to CT. bols indicates the error different symbol has beenused for rqNosamples because they are depletedin Fe). CRY STA L-C H EM I STRY OF SYN TH ET I C M AGN ESIO H A STI N GS IT E 489 in conjunctionwith NNo buffer), a regularvariation Tesre 11. Octahedral Site Occupancy in Synthetic and of refractive indices is apparent.A linear fit to the Natural Magnesiohastingsite data indicatesan increaseof both c and y with the r"r(r), 14(3) 14(2) oxidationratio. As pointedout earlier,such a rela- Sample -2+ fe_' tion would obtain where the rule of Gladstoneand Dale is followed and the specific refractive energy IQF (avs) .79 .21 .81 .07 0A5-l9-71 U .87 .06 .07 .70 .27 for Fe2O3is larger than for FeO (Larsen and Ber- CT-A2 .85 - .15 .73 ,27 man, 1934,pp. 30-31). The systematicchange in IQF to CT .78 - .22 .83 CT to IQF .84 .16 .73 color also follows directly the variation in Fe3*pro- u-l 236 .86 .14 .46 portion. The refractiveindices for samplereF to cr ' .08 Al + .04 Ti not included. and cr to reF-d - 1.651(4), -y = 1.669(5), Fe3-/(Fe3-+ Fe'-) = 100 percent,and 4 = 1.640(5),y = 1.660(6),Fe3*/(Fe3* p"z*) - * 35 In magnesiohastingsitedirectly synthesizedfrom percent respectively-also are in good agreement an oxide mix, the number of Fe2*,Fe3*, and Mg with the valuesobtained for direct synthesis.This ions per site clearly does not reflect random dis- indicatesthat the observedvariation can be attrib- tribution of theseions (0.20Fe * 0.80Mg per site uted primarily to changesin Fe3*/(Fe3* Fe2*). * if random). Evidently Fe3*has a strongpreference Distribution of octahedral cqtions in synthetic for the M(2) site overM(l) and M(3), whereas Mh. Recent crystal-structure determinations (for Fe2*is lessstrongly concentrated in M(1' ) andM(3 ) ' a reviewsee Papike et al., infrared 1969), and Mtjss- Although the observedcation distributionmay be bauer studies (Bancroft et al., 1967a; Burns and different from the equilibrium distribution at the Prentice, 1968; Bancroft and Burns, 1969; Ernst conditions of synthesis,the ordering schemeob- and Wai, 1970;Wilkins, 1970;Buckley andWilkins, servedcompares favorably with the resultsobtained 1971 Burns and Greaves 197 ; , | ) havedemonstrated for sampleU-1236. For this sampleFe3* seems to that extensivecation ordering occursin naturalclino- be restrictedto M(2), whereasFe'?- occupies all amphiboles.Ordering in syntheticclino-amphiboles, octahedralsites but is relatively concentratedin however,has not beenconclusively pre- documented M(1) and M(3). The cation distributionscheme viously. Ernst (1963) has suggestedthat a second observed is also in agreementwith the crystal- order polymorphic transition involving Al ordering chemicalbehavior of other naturalcalcic amphiboles in the M(2) site occursin syntheticmembers of the (Zussman,1955; Ghose, 1965 Ernst, 1968;Papike glaucophane Attempts series. at disorderingnatural et al,1969). sodic amphiboles(Ernst and Wai, 1970) by one In pargasitesynthesized at similar pressureand atmosphere and hydrothermal heat-treatment re- temperatureconditions, Al occupancyof M(1 ) and sulted in oxidation Fe2* of ions and concomitant M(3) deducedfrom the i.r. spectrumis 0.23 + loss H2 of andfor H2O. Significantoctahedral cation 0.05 ions per site and thus undistinguishablefrom migration occurredin runs of 94 hours at 700'C random distribution. Apparently ionic size differ- and atmosphericpressure. A year two long hydro- encesbetween Fe3* and Al play a lesserrole in the thermal heat-treatment of a natural iron-bearing octahedralordering processthan the higher electro- glaucophaneproduced partly a oxidized amphibole negativity of the former ion over the latter. The "anhydride" which was interpretedas being the dis- process presumably involves the rectification of orderedequivalent of the startingmaterial. The cationoccupancy of the M(I), M(2), and andReduced Mag- M(3) sites in syntheticmagnesiohastingsites pro- Trst-z 12.Cell Parameteff.1,:#1f* duced in this study has been deduced from the Mcissbauerdata of Table 8 (assuming4Mg * Fe per 2M(l) a 2M(2) + M(3) sites) and is s"rot"* i i i (a"o"""s) i3 presented in Table 11,. The results for natural r 9.e80(4 ) l 8.Ol l (9) s.287(21 105.40(4) 908.0(7) magnesiohastingsitebased on the new microprobe 2 9.955(2) 18.053(5) s.293(l) 10s.46(3) e16.8(4) analysis (Table 12) and the Mdssbauer -Identities spectra, of sanp'les: (l) mgnesiohastingsitesynthesized at low oxygen-. assumingTia- and Al3*to be concentratedinM(2), fuoacitv (IOFbuffer) andrerun at high oxygenfugacity (average0f 4); (z) minesi6hisi'inqsite synthesizedat high oxygenfugacity (cT buffer) and re- are includedfor comparison. run at low oxygenfugacity (averageof 2). MICHEL P. SEMET chargeimbalance caused by Al for Si substitution in Fe3-/(Fe3** Fe'*) ratio with run conditions. in the tetrahedralchains. If similar considerations To test this hypothesistwo amphibolessynthesized (here the local chargeimbalance may be causedby at high (cr buffer) and low (Iqr buffer) oxygen the more or less complete occupancy of the "4" fugacity have been analyzedwith an electronmicro- site by monovalentions and/or the occupancyof probe.The data presentedin Table 6 showthat the Ihe M(4) site by monovalentions) can be extended oxide percentagesare equal, within experimental to the glaucophane-magnesioriebeckiteseries, then errors.to thoseof the ideal end-member.This seems the lack of polymorphism in the iron-rich portion, to rule out the possibilityof significantcompositional noted by Ernst ( 1,963),could be explainedby the variation. Furthermore,refractive indices and cell fact that Fe3*is alreadyconcentrated in M(2) for parameterspresented in this work were obtained all pressure,temperature conditions investigated. from specimenssynthesized from three batchesof When comparedto their initial values,the site oxide mixes prepared at severalmonths' interval. occupanciesof the oxidized and reduced samples No systematicdifterences in those propertieswere (tqn' to cr and cr to IeF, respectively,Table 11) observed,indicating that errors in startingmaterial are seento remain essentiallyunchanged except for preparationwere probably kept to a minimum.Solid- in situ oxidation or reduction of Fe ions. Further- solutionwith other amphiboleend-members (mostly more, it appearsthat Fe ions in M(l) and M(3) ferropargasiteandfor ferroedenite)to produce the are preferentiallyreduced (in the reducedsample) observedhigh Fe'. proportionat low oxygenfugacity but that Fe ions in M(2) are also affected.Similar (lqr buffer) would require that at least 10 percent relationsare also suggestedby the infrared spectra by volumeof otherphases be presentin the synthetic of sampleshydrothermally oxidized for longertimes products.Such phases(clinopyroxene, olivine, pla- (eee-ttr and ee9-12r,Tables 7 and 10). Redox gioclase,elc. ) would most certainlyhave been de- reactionsapparently occur at leastan order of mag- tectedwith the petrographicmicroscope or by X-ray nitude faster than octahedralcation rearransements powder diffraction.Variations or systematicdevia- at thesetemperatures. tions from the ideal bulk compositionmay still ac- Mechanism of Fss* reduction in synthetic Mh. countfor a minor part of the redoxphenomenon but The structural formula of the end-membermag- are certainlynot the major factor in this effect. nesiohastingsite,NaCa2MgaFe3-Si6AI2O22 (OH)2, in The feasibilityof the secondmechanism proposed which all sitesare filled, doesnot theoreticallyallow is very difficult to assessby direct analyticalmeans for the occurrenceof Fe2*ions without the addition becauseof the lack of reliableoxygen determination of postiveions or the subtractionof anions.Three methods.Anion vacancies,especially at low oxygen mechanismsthat would producethe observedeffects fugacities(Iqr buffer) could conceivablyestablish havebeen considered: electricalneutrality in reducedmagnesiohastingsite. If, however,this mechanismwere solelyresponsible (l) The amphiboles synthesized are not "on it would require that close to one oxygenposition composition,"or the composition variesas a per unit cell (on the average)be vacant (Na2Caa functionof run conditions. MgsFe22*Sir2AllO4Bn(OH)r) ; such a structure (2) The structure allows anion vacancieswhen would most probably be quite unstable.Moreover, exposedto low oxygenfugacity. the oxidation of Fe2* (sampleIQF to cr) without (3) Protonsin excessof two per unit formula can concomitant diftusion of oxygen into the vacant be accommodatedin the structureat low values anion position should lead to a loss of positive of high lo, 1: l',). charge(presumably mostly H* or Na-); no evidence Each involves the diffusion of some speciesin or for such a loss has been obtained.On the other out of the amphibolestructure. A model whereno hand, if rapid (lessthan a few hours) O'- diffusion diffusion occurs but chargebalance is attained by on oxidation or reductionwere possible,it would the change in valenceof ions other than Fe seems likely involve cation rearrangementtoward the more to be out of the question. stabledistribution (Fe" in M(l) and M(3), Feg' If the proportions of oxides in the starting in M(2)). Suchcation redistributionswere not ob- material were not equal to what is required for servedin samplesIeF to cr andcr to IQF. magnesiohastingsitesynthesis, solid solution with A model involving the reduction of "theoretical other end-memberscould have producedvariations magnesiohastingsite"by the formation of hydroxyl CRYSTAL-CHEMISTRY OF SYNTHETIC MAGNESIOHASTINGSITE 491 groupsin excessof two per unit formula was tested attractive. The fast diffusion rates would be in ac- with the availabledata. From the equilibriumcon- cordancewith a mechanismfirst proposedby Brindley stantfor the reaction: and Youell (1953)for chamosite.They showedthat 2 NaCazMg,rFes.SioAlzOtz(OH),+ Hs = 2 NaCaz for this structurethe reaction: Mg1Fe2-Si6Al2O22H(OH)2we obtain: Fe'*+ oH-=Fe'* + o'- + H* * e- RZln : 2(pr.,+o- ls" ;,rp""*o) That is, oxidationor reductionof Fe occursby the + 2RTln 4p""*- zRTln 4p""* break or formation of OH bonds from or to Fe ions (by diffusionof H*, e- pairs) and providesan easily where is the hydrogenfugacity in the gas phase, fs" reversibleprocess even at rather low temperatures. are the standard chemical potential /.rF",+oe 1lFu"*o An identicalmechanism has sincebeen invoked to of the reduced and "ideal" magnesiohastingsite accountfor similar phenomenain micasand amphi- respectively,ard ap.,*, as..+ ara the activities of boles (Addison e/ al, 1962a;Addison et al, 1.962b; the "end members" in the amphibole solid solution. Eugsterand Wones,1962; Wones, 1963; Newman For an ideal solid-solution(the data do not warrant and Brown, 1966; Pattetsonand O'Connor, 1966; a more rigorous treatment),the activity 4p""+ of Vedderand Wilkins, 7969;Ernst and Wai, 1970). ideal magnesiohastingsitein the solid-solution is Wilkins and Vedder (1,969) presenta reviewof the equal to xp. (hereafterdenoted x), the molar "* phenomenon for micas and amphiboles. Further- concentration.4p",+ is also equal to the molar more, Newman and Brown (1966) and Newman concentrationof the "reduced end-membero'and (1967 970 havepresented evidence which strongly to I - x. Then for the equation above ,1 ) log f,',: 2loe- * constant. Hydrogen fugacity in the gas phase may be cal- culated as a function of total pressure,temperature, and oxygen fugacity, as defined by the different buffers(Eugster and Wones, 1962): ^ : P fht"'Yrt"o tE' KwfoYT'.'4 ^t*"o whereP, is the total pressure,Kw is the equilibrium constantfor HrO dissociation,It,, f o" are the H, N - and O, fugacities, and 7s,, 711,s&te the fugacity coefficientsfor H, and HrO. At 850oC, 2000 bars orl total pressure,the following values obtain: Kw : 4.387 X 10' (rlNm Tables, 1965); :'lt. 1.403 (Shaw and Wones, 1964);tr,,o : 0.8760(Burnham et al, 1969); and 2806 ,t,, -_ 7.026x lo8 x fo"rr"* l The valuesobtained for log fn" and loeQ - x)/x for the amphiboles directly synthesizedfrom an oxide mix are presentedin Figure 6; the size of the symbols representsthe estimatedaccuracy based on . l-x 5 + 0.05for x, and + 6'C from the nominal850'C. tog -l- A straight line has been fitted visually to the data Frc. 6. Hydrogen fugacity of the gas phase coexisting with points; the slope obtained(2.5 + .4) is consistent "magnesiohastingsitesolid-solution" containing .r (mole frac- with (but far from proving) the model proposed tion) ideal Mh and 1 - x reduced Mh' The size of the which requires a slope of 2. Other aspects of the symbols indicates the estimated accuracy. Value for cr redox process make the latter model particularly extends to -oo. 492 MICHEL P. SEMET suggestsprotonation of oxygens in the tetrahedral 5). If such vacancieswere retained on oxidation sheetsof artificially altered micas. During heat-treat- (they are, then, no longer compensatedby a neigh- ment in H2 of experimentally oxidized or outgassed boring Fe2* ion but by some other process), the crocidolites, Addison and Sharp (1962) produced slightly enlarged a repeat in oxidized samples (Table strongly reduced amphiboles. At a relatively low 13) could be attributed to a small dilation of the temperature(415"C), the structureis not destroyed structure acrossthe tetrahedral chains and octahedral and the measured intake of H2 led the authors to strips by cation-cation repulsion. In contrast to this, postulate proton fixation by oxygens coordinated to comparisons of the b parameter in the Fe'r- samples the tetrahedralchains. The oxygen O(4) is a likely (cr buffer, Table 5) and in the reduced samples candidate for protonation due to its strong under- (Table 13) show a normal trend, b being enlarged bonding (loc.cit.; Brown and Gibbs, 1969). The when some Fe'r. in M (2) is reduced to the larger presence of hydroxyls in excessof two per formula Fe2. ion, while the other dimensions remain con- unit is also suggested by a number of amphibole stant. analyses (Zussman, 1955; Gillberg, 1964; Leake, 1968; Witte et al, 1969) and may not all be due to Conclusions inaccurate H2O determination. The broad absorp- Saxenaand Ekstrijm (1970) have shown that the tion band in the i.r. spectra of the Fe2*-rich syn- variability of (OH) content in amphiboles is a sig- thetic magnesiohastingsitesin the OH-stretching re- nificant parameter of this mineral group's crystal- gion may be attributed to extra hydroxyl ions chemistry. Inaccurate H2O analysis, however, has a (protons coordinated to oxygens of the tetrahedral notable influence on the computation of structural chains). In this case oxidation would only involve formulae (Leake, 1968), particularly the distribu- the diffusion of the extra protons and would not tion of tetrahedral and octahedral cations. As this require the lossof normal O(3)-H hydroxyls coordi- study suggests,computation of structural formulae nated to the octahedral cations, in agreement with on the basis of 23 anions or 24 (O, OH, F, Cl) the major featuresof the i.r. spectra. may not always be appropriate. However, this may Most of the experimentaldata suggestthat redox be the only alternative in the absence of accurate in magnesiohastingsiteinvolves the protonation of density and cell parameter measurements, and of oxygens belonging to the tetrahedral chains,? but accurate chemical analyses that include oxidation this may not be the only mechanism responsible. ratios for elementswith variable valence. For instance, it is not clear why magnesiohastingsite Although it is not known whether "reduced with = Fe'2./(Fe'. * Fe3.) 87 percent can be syn- magnesiohastingsite" would be stable in systems thesized directly from an oxide mix at low oxygen differing chemically from its own bulk composition, fugacity, whereas the reduction of a previously syn- this amphibole could indicate the existence of a thesized Fe3. amphibole cannot be carried further stability field for similar hornblendes occurring in = than Fe'!-/(Fe'. f FeS-) 65 percent under the igneous or metamorphic rocks. Natural and synthetic same conditions. Furthermore. the variation of cell amphiboles experimentally studied so far have in- parameters with the proportions of Fer- and Fe3- dicated that redox reactions occur in them at a very are not always in the expected direction. For ex- fast rate due to the high mobility of protons and ample, the relationship of the b cell dimension with electrons. It is, therefore, unlikely that the measured the mean radius of cations occupying M(2), as ob- oxidation ratios in metamorphic and igneous horn- served by Colville et al (1966), does not seem to blendes will reflect the redox potential at the time of hold in detail for the amphiboles produced in this crystallization. They will instead carry the imprint study. A small proportion of anion vacancies com- of conditions existing at some late cooling stage. pensatedby Fe reduction (in addition to OH) may Within certain limits, compositional variations in explain the relatively shorter a and b parameters in natural amphiboles may not significantly affect the samples synthesized at low oxygen fugacity (Table oxidation ratios (the other variables being held constant). This is suggestedby the behavior of the 7 Volumetric H, analyseskindly performed by H. Taylor iron-depleted samples (NNo buffer). The distribution (California Institute of Technology) on synthetic samples of the absolute amounts of Fe ions per site is a definitely prove that Mh annealedat low le" contains H in amounts greater than 2 per unit formula and sufficient to function of the intensive parameter controlling account{or Fez+charge imbalance. crystallization, especiallyoxygen fugacity. Therefore, CRYSTAL.CHEMISTRY OF SYNTHETIC MAGNESIOHASTINGSITE 493 (1970) applications ol crystal the accuratedetermination of site occupancyfor Fe BunNs, R. G. Mineralogical theory. Cambridge University Press' 224 pp. intensivevariable field ions could, in principle, serveas an eNo R. G. J. SrnrNs (1965) Infrared study of the indicator (mostly f6" sensitive),providing the Fe hydroxyl bands in clinoamphiboles. Science, 153' 890- distributionhas not changedon cooling. 892. eNo F. J. Pnr,Nrrce (1968) Distribution of iron Acknowledgments cations in the crocidolite structure. Amer. Mineral. 53, Miissbauer and infrared spectra were obtained through 770-776. the courtesy of the Department of Physics and Chemistry, eNo C. Gnr,,lvrs (1971) Correlations of infrared respectively, University of California, Los Angeles. W. and Mcissbauersite population measurementsof actino- Ki.indig provided liberally of his time in instructing the lites. Amer. Mineral. 56, 201U2033. author in the principles and applicationsof M HeENEn,S. S., rNn H. G. HucxEvuotz (l9il) Mijssbauer heat-treated crocidolite, amosite, and tremolite from spectrum of synthetic ferri-diopside.Nature (Phys. Sci.), infrared absorptionstudies. Aust. L Chem. 19, llSS-1164. 233,9_lO. SexrNl, S. K., eNo T. K. Exsrncirvr(1970) Statisticalchem- HucKENHoLz,H. G., J. F. SnernEn,eNo H. S. Yonrn ( 1969) istry of calcic amphiboles. Contrib. Mineral. Petrology, Synthesis and stability of ferri-diopside. Mineral. Soc. 26,276-284. Amer. Spec.Pap. 2, 163-177. SeuEr, M, P. (1972) Stability relations and crystal chem- HuE.nNrn, J. S., eNn Moroexr Sero (1970) The oxygen istry of the amphibole magnesiohastingsite,NaCa"Mg, fugacity-temperature relationships of manganese oxide F d. s i"AI.p -( o H h. ph.D. Thesis,University of california, and nickel oxide buffers.Amer. Mineral.55r934-952. Los Angeles,California. JrNrn Tables (1965) loint Army, Nauy, Air Force Tables Surw, H. R., rlto D. 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