American Mineralogist, Volume 73, pages 1335-1345, 1988 Ostwald ripening and interparticle-diffraction effects for illite crystals DnNNrs D. ErBnr, U.S. Geological Survey, Federal Center, Mail Stop 404, Denver, Colorado 80225, U.S.A. J.q.NSnoooN Institute of Geological Sciences,Polish Academy of Sciences,3 I -002 Krak6w, Senacka3, Poland Ansrnncr The Warren-Averbach method, an X-ray ditrraction (xno) method used to measure mean particle thicknessand particle-thicknessdistribution, is usedto restudy sericite from the Silverton caldera. Relative mean particle thicknessesdetermined by this method for the Silverton illites correlate well with cation-exchangecapacity, with fixed interlayer chemistry, with an xRD intensity ratio, with the wave number of a 824-834 cm-' infrared absorption band, with the Kubler index, and with the apparent K-Ar agefor the samples. Apparent particle-thicknessdistributions indicate that the clays may have undergoneOst- wald ripening and that this process has modified the K-Ar ages of the samples. The mechanism of Ostwald ripening can account for many of the features found for the hy- drothermal alteration of illite. Expandabilities measured by the xno peak-position method for illite/smectites (VS) from various locations are smaller than expandabilitiesmeasured by transmissionelectron microscopy (rer',r)and by the Warren-Averbach (W-A) method. This disparity is inter- preted as being related to the presenceofnonswelling basal surfacesthat form the ends of stacks of illite particles (short-stack efect), stacks that, according to the theory of inter- particle diffraction, diffract as coherentX-ray scatteringdomains. Previous determinations of the chargeof the illite interlayer have been based on expandability measurementsthat have not been corrected for the presenceof these nonswelling surfaces.Thus, the value for the illiteJayer charge,previously thought to be about -0.75 equivalentsper Oro(OH)r, needsrevision. By using revr methods for measuringmaximum expandabilities,the fixed cation content of illite layers in I/S is determined to be approximately -0.9 equivalents per O,o(OH)r. This charge is independent of illite origin and expandability. Based on a long extrapolation, the mean charge on exposedbasal surfacesofillite particles from the Silverton caldera is approximately -0.48 equivalents per O'.(OH)'. INrnooucrroN Appr,rclrroN oF THE W,l,nnnN-AvERBACH In a recentpaper, Eberl et al. (1987) discussedthe min- TECHNIQUE eralogy of sericite from the Silverton caldera (Colorado) The Warren-Averbach X-ray diffraction technique was in terms of the theory of interparticle diffraction (Nadeau developed originally for the investigation of cold-work et aI., 1984a, I 984b). This paper was, in part, a searchto distortion of metals. This technique has been used by find a superior technique for studying this commonly oc- metallurgists for more than 35 years, and, in the words curring material. In the present paper, the Warren-Av- ofKIug and Alexander(1974), "A vast amount ofexpe- erbach method (Warren and Averbach, 1950) is used to rience . supportsthe pre-eminenceof the Fourier meth- measureillite-particle thicknesseswith greater precision od of Warren and Averbach in this field." The Warren- than was possible in the previous work. The better cor- Averbach (W-A) method separatesthe effects of X-ray relations found using these measurements,together with diffraction (xno) peak broadening related to particle size additional rervrthickness measurementsof other illites, (or X-ray scattering-domainsize), which is not a function extend our knowledge ofthe relations between structure oftwo-theta angle,from peak broadeningrelated to strain and chemistry for these and similar clays. In addition, in the crystal structure, an effect that is angular depen- apparent particle-thicknessdistributions suggesta mech- dent, by analyzirgdiffraction profiles for two or more 00/ anism for the formation and evolution of illites in hvdro- peaks. thermal systems. The presentapproach, which is empirical, assumesthat 0003404x/88/ll 12-l335$02.00 I 335 r336 EBERL AND SRODON: OSTWALD RIPENING AND ILLITE ARTICULATEDILLITE DISARTICULATEDILLITE CRYSTALS CRYSTAL Non-swsllingsurface 1.0nm illite layerwith fixed Fof lhls partlcle: cations T=8.0nm Fundamentalpartlcle X-rayscattering domain size = 4.0 nm vlewDolnl Expandability= 0% Mineral= illite N=4 T= 4.0nm 1.7nm swelling (wilh glycol)with <- Stackingtaull s=3 exchangeable cations Max.expandability = 25% N=12 MacEwan Crystatllte vlewpolnt Mineral= illite/smectite Ordering= R3 Expandability= 18% Fig. l. A diagram that comparesthe fundamental-particleconcept with the MacEwan crystallite conceptof I/S and that presents other conceptsdiscussed in the text. The c* axis is parallel to the stacks.The crystallographicorigin for the stackingfault is strictly speculative. particle-size xRD peak broadening for illite is related to The assumption that most of the particle interiors are fundamental-particlethicknesses and that, typically, this fault-free is supported for the Silverton sericites by the broadening is influenced minimally by stacking faults data of Eberl and Velde (in prep.). This technique,which within particles that would tend to give a smaller X-ray plots an xno intensity ratio (Srodori, 1984) versus the scattering-domainthickness than the fundamental-parti- Kubler index (Kisch, 1983), indicates that most of these cle thickness.A hypothetical example of a stacking fault sericiteshave xno defect-freedistances parallel to c* that is diagrammed on the right side of Figure 1: the illite are equal to or greaterthan the mean particle thicknesses fundamental-particle thickness in this figure is 8 nm, (defect-freedistance : 20 to >40 illite layers).This tech- whereas the X-ray scattering-domain thickness is ap- nique also indicates that illites from other areas(e.9., il- proximately 4 nm. The interiors of the particles are as- lites from the Niger Delta basin) have defect-free dis- sumed to be strain-free. Strain broadening for basal xno tancesthat are less than particle thicknesses(defect-free reflections is assumedto arise entirely from interparticle distance 5 to l0 illite layers), in which case the W-A swelling (the 1.7-nm spacingsin the left side of Fig. l). techniquemay give spuriously small mean particle thick- This "strain" is kept constant at about 1.7 nm by Sr sat- nesses. uration and glycolation. The assumption of strain-free interiors seemsreason- EBERL AND SRODON: OSTWALD RIPENING AND ILLITE 1337 TaBLE1, Meanparticle thickness (in nm) parallel to d measured TABLE2, Meanparticle thickness parallel to d for sericitesam- for the Silvertonsericites bv the Warren-Averbachand pleRM22 as a functionof interparticlechemistry rEMmethods InterparticlechemistrY Thickness(nm) Sr-saturated,glycolated 10 Warren-Averbach Arithmetic Weighted Ca-saturated,glycolated 11 Smoothed mean mean Mg-saturated,glycolated 11 Na-saturated,glycolated 10 AR1 60 18.6 K-saturated,glycolated 15 AR1R 19 Natural,glycolated 13 LF7 14 sr-saturated,air-dried 13 IQ LF1O 4.4 Ca-saturated,air-dried 20 RM3 23 19 Mg-saturated,air-dried 16 RM4 15 Na-saturated,air-dried v aa RM5 12 K-saturated,air-dried 12 RM6 12 Natural,air-dried I RM8 11 A1 'l 'l RM1.I Nofe: Measurementswere made by the Warren-Averbachmethod' RM12 12 RM13 11 8 RM21 11 RM22 10 RM28 15 7.4 particle mea- RM30 14 11 8 Comparisons between mean thicknesses RM31 17 sured by the xno W-A method (raw data) and by the rnu *arith- RM35A 11 4'l Pt-shadowing method (Table l, column labeled RM35C 12 RM35D 12 1.6 metic mean") indicate that the methods are comparable SG1 13.5 for three samples, although the W-A method generally JU+ 29 20.0 43 28 givesgreater thicknesses. However, it would be more cor- rect to use the weighted means rather than arithmetic means as a measure of mean thicknesses.rru particle- able in applying the W-A method to sericite,because any thickness distributions needed for this calculation were deviation in lattice spacing (from the previously mea- available for only two samples(Table l, column labeled sured value of 9.99 A; ttrat is associatedwith strain with- "weighted mean"). Thus, becausethere are insufficient in particles would be negligible compared with the large reu thicknessmeasurements to calibrate the W-A thick- "strain" related to swelling between particles. The as- nessmeasurements, mean thicknessesand particle-thick- sumption that strain broadeningis equivalent to swelling ness distributions determined by the W-A method are between particles-and, therefore, that its effect on xnp consideredto be correct in a relative senseonly, although peak breadth can be removed for crystallite-sizeanalysis there is some evidence from cation-exchange'capacily by the W-A method-is supported by correlations that measurementsthat the mean thicknessesmay be correct follow. in an absolute sense,as will be discussed.Particle-thick- Weighted mean particle thicknessesfor samples (Sr- ness distributions calculated from the raw data, rather saturated, glycolated, <2-pm size fractions) that were than from the smoothed data (Table l), were used in studied previously by other methods (Eberl et al., 1987) subsequentanalyses because the raw data set is more were determined by the W-A method from the samexRD complete. patternsused previously (Table l, column labeled"raw"). During application of the W-A method, it is important This analysisused the 002 and 005 xno reflections, Na- that the thickness of interparticle expanding layers be tional Bureau of Standards standard