The structure of a lepidolite-2M1 from Biskupice, Czechoslovakia, has been redetermined. Violations of systematic extinctions and of monoclinic equivalences plus the results of a second harmonic generation test indicate that the true symmetry most likely is C1̄. The deviation of the data set from C2/c symmetry, however, proved to be too small to permit a statistically significant refinement in C1̄. Refinement in C2/c symmetry indicated no ordering of tetrahedral cations but ordering of octahedral cations so that M(1) = Li0.93R2+0.06Fe3+0.01 and M(2) = Al0.58Li0.35□0.07. The tetrahedra are elongated to form trigonal pyramids with a rotation angle of 6.2°. The anomalous orientation of the thermal ellipsoid for the F,OH anion plus the large equivalent isotropic B value of 2.58 for F,OH and of 1.74 for the interlayer K cation, whose position is partly restricted in C2/c symmetry, suggest a lower symmetry than C2/c.

The compositions of this sample and of a second lepidolite-2M1 from Western Australia fall outside the stability field of lepidolite-2M1 in the synthetic system. Structural control of the stacking sequence is discounted on the basis of the structural similarity of the lepidolite unit layers. Crystallization parameters are considered more important than composition or the structure of the unit layer in determining the stability and occurrence of different layer-stacking sequences in lepidolite.

The basic outlines of most of the hydrous layer silicate structures were determined during the 1930’s. Present escalating interest in obtaining additional detail is indicated by (a) publication of over twenty structural refinements (2-D or 3-D) during 1954-64, (b) publication of at least nine structures in 1965 or early 1966, and (c) personal communication that at least fifteen additional refinements are in progress. Points of especial interest in these recent studies follow.

1. 1. Octahedral cation order is common, but tetrahedral cation order is confirmed in only three cases. Because ordering of Si, Al does not significantly affect the statistical tests for centrosymmetry, centrosymmetric space groups that do not permit order should be avoided during refinement.

2. 2. Oversize tetrahedral sheets articulate with smaller octahedral sheets by tetrahedral rotation and, for dioctahedral species, by tetrahedral tilting around vacant octahedra. The latter mechanism influences the type and regularity of layer sequences. Undersize tetrahedral sheets articulate with larger octahedral sheets by tilting plus octahedral contraction or by inversion of some tetrahedra.

3. 3. The amount and direction of tetrahedral rotation and tilt, length of T—O, M—O, and O—O bonds, sheet thicknesses, and relation of cell dimensions to composition can now be predicted with some confidence.

4. 4. Variation in layer stacking (polytypism) is common. In some cases the stabilities of different polytypes can be explained by the relative amounts of repulsion and attraction between the ions in the structures. The stabilities can be correlated with the energy available in the environment of crystallization.

Polytypism in trioctahedral 1 : 1 phyllosilicates results from two variable features in the structure. (1) The octabedral cations may occupy the same set of three positions throughout or may alternate regularly between two different sets of positions in successive layers. (2) Hydrogen bonding between adjacent oxygen and hydroxyl surfaces of successive layers can be obtained by three different relative positions of layers: (a) direct superposition of layers, (b) shift of the second layer by a/3 along any of the three hexagonal X-axes of the initial layer, with a positive or negative sense of shift determined uniquely by the octahedral cation set occupied in the lower layer, and (c) shift of the second layer by ± b/3 along Y1 (normal to X1) of the initial layer regardless of octahedral cation sets occupied. Assuming ideal hexagonal geometry, no cation ordering, and no intermixing in the same crystal of the three possible types of layer superpositions, then twelve standard polytypes (plus four enantiomorphs) with periodicities between one and six layers may be derived. Relative shifts along the three X-axes lead to the same layer sequences derived for the micas, namely 1M, 2M1, 3T, 2M2, 2Or, and 6H. Polytypes 1T and 2H1 result from direct superposition of layers. Layer shifts of b/3 lead to polytypes designated 2T, 3R, 2H2, and 6R. The twelve standard 1 : 1 structures can be divided into four groups (A = 1M, 2M1, 3T; B = 2M2, 2Or, 6H; C = 1T, 2T, 3R; D = 2H1, 2H2, 6R) for identification purposes. The strong X-ray reflections serve to identify each group and the weaker reflections differentiate the three structures within each group. Examples of all four groups and of 9 of the 12 individual structures have been identified in natural specimens. Consideration of the relative amounts of attraction and repulsion between the ions in the structures leads to the predicted stability sequence group C > group D > group A > group B, in moderately good agreement with observed abundances of these structural groups.

The ability of a clay mineral surface to function as an acid is not represented by bulk pH measurements. A method using u.v. analysis and organic indicators has been developed to monitor surface acidity. The u.v. organic indicator method enables sensitive in situ quantification of surface-induced protonation in wet or dry clay systems. The clay preparation procedure used yields reproducible acidic behavior.

Almost four decades of study of Desmoinesian strata of Middle Pennsylvanian age in south-central Iowa and north-central Missouri have provided the stratigraphic control required to test the variation of clay mineralogy vertically and laterally within various paralic clay and shale facies. Local and regional variations in clay mineralogy within Desmoinesian strata are generally predictable and are in agreement with current knowledge of deltaic deposition. A principal environmental variation within a deltaic system is the change from normal marine salinities in deltaic marine environments to brackish- and fresh-water conditions in the marshy delta plains, in upper interdistributary bays, and within flanking interdeltaic embayments. Changes from marine to nonmarine facies coincide with a decrease in illite, and an increase in kaolin, mixed-layer clays, and in the percentage of expansible layers in the mixed-layer clay. The principal clay detritus entering the area was illite, which underwent various degrees of alteration in different aqueous and subaerial environments within deltaic and interdeltaic areas. Clay-mineral composition alone does not provide unique environmental answers. The distribution of clay-mineral suites within these systems, however, both supports the deltaic-interdeltaic depositional model and can be understood within the context of this framework.

The crystal structure of nacrite from Pike’s Peak district, Colorado, has been refined by least squares and electron density difference maps utilizing ten levels of data. Complete refinement was inhibited by thick domains involving a/3 interlayer shifts in the “wrong direction”. The ideal structure is based on a 6R stacking sequence of kaolin layers, in which each successive layer is shifted relative to the layer below by −1/3 of the 8·9 Å lateral repeat. This direction is X in nacrite, contrary to the usual convention for layer silicates, because of the positioning of the (010) symmetry planes normal to the 5·1 Å repeat direction. Alternate layers are also rotated by 180°. The pattern of vacant octahedral sites reduces the symmetry to Cc and permits description of the structure as a 2-layer form with an inclined Z axis.

Adjacent tetrahedra are twisted by 7·3° in opposite directions so that the basal oxygens approach more closely both the Al cations in the same layer and the surface hydroxyls of the layer below. Interlocking corrugations in the oxygen and hydroxyl surfaces of adjacent layers run alternately parallel to the [110] and [11̄0] zones in successive layers. The upper and lower anion triads in each Al-octahedron are rotated by 5·4° and 7·0° in opposite directions as a result of shared edge shortening. Nacrite has a greater interlayer separation and smaller lateral dimensions than dickite and kaolinite, and the observed β angle deviates by 1½° from the ideal value. These features, as well as its overall lesser stability, are believed due to the less favorable positioning in nacrite of the basal oxygens relative to the directed interlayer hydrogen bonds.

Use of a platy internal standard that will orient to the same degree as clay minerals preserves the relative diffraction intensities between the basal reflections from the platy components, regardless of degree of orientation. The method is illustrated with basic zinc chloride and pyrophyllite as the internal standards for quantitative clay mineral analysis in the systems kaolinite-1Md illite and 2M1 muscovite-montmorillonite. Ulite does not orient to the same degree as kaolinite at high illite concentrations. In such nonlinear systems empirical working curves are more reliable than fixed ratios of the scattering powers of the clay minerals present. Random interstratification of 10/15.4Å layers causes a minimum in 001/001 peak height at about 33% of the 15.4Å component. Peak width varies in a similar but inverse pattern, so that the integrated intensity increases in a smooth curve from muscovite to montmorillonite. The major error in application of this quantitative method arises from uncertainty as to the correct allocation of peak areas in cases of overlap of the mixed-layer peak with those of discrete 10 Å and 14 Å clays also present.

Single crystal X-ray diffraction patterns reveal that the structure of selected anauxite crystals is the same as the structure of macroscopic kaolinite crystals. Anauxite and kaolinite crystals are intergrowths on a domain scale of units in pseudotwin orientations. Individual domains in anauxite have the triclinic geometry of kaolinite, and give X-ray reflections that compare closely in intensity with those calculated from the atomic parameters of kaolinite. Large crushed crystals of anauxite give powder patterns identical with that of kaolinite. Because it has been shown recently that the chemical composition of anauxite is also identical with that of kaolinite, it is recommended that the term “anauxite” no longer be used.

The cell dimensions and compositions of four chlorites whose crystal structures have been determined in detail are used to test existing graphs and regression equations designed to give tetrahedral and octahedral compositions. It is found that the thicknesses of the tetrahedral sheet, the 2:1 octahedral sheet, the interlayer sheet, and the space between the 2:1 layer and the interlayer can vary appreciably from specimen to specimen quite independently of tetrahedral composition. Total octahedral composition, the number of octahedral vacancies, cation ordering, and the distribution of trivalent cations and of charge between the two octahedral sheets must have effects on d(001) that are additional to the effect of tetrahedral composition. Nevertheless, Brindley’s d(001) graph and a regression equation by Kepezhinskas both should give tetrahedral compositions with an average error of 10%, or about 0·1 AlIV, for most trioctahedral chlorites. They are not valid for dioctahedral or di, trioctahedral species. Equations derived from the data of von Engelhardt and of Shirozu relating the b parameter to octahedral Fe, Mn content give results with an average error of 10%, or 0·1 Fe, Mn, for the four test chlorites provided Cr is included with the Fe, Mn, as does a regression equation by Kepezhinskas that contains terms for both the b parameter and d(001). Methods using the (00l) intensities or structure amplitudes give less consistent results for heavy atom contents than the spacing methods, but can be used to give approximate values for the asymmetry in distribution of heavy atoms between the 2:1 octahedral sheet and the interlayer.

The crystal structures of a reddish-purple, Mn-bearing muscovite-2M1 (alurgite variety) and a reddish-brown, Mn-bearing phlogopite-1M (manganophyllite variety) were refined to final residuals of 2.7% and 3.1%, respectively. The refinements were carried out in space groups C2/c and C1 for alurgite and C2/m and C2 for manganophyllite. The C1 and C2 subgroup refinements gave atomic coordinates consistent with the parent space group refinements. No cation ordering was found in either specimen, and the structures are very similar to those of muscovite and phlogopite. Residual areas of positive electron densities were found between the tetrahedral cations and neighboring oxygens in difference maps of both minerals. Those of alurgite were examined in detail to show the correlation between the residual densities and covalent bonding in the tetrahedra. The valence of the Fe present was determined by Mössbauer spectra as Fe3+ in both samples and of the Mn by optical spectra as Mn3+ in the alurgite but as Mn2+ in the manganophyllite.

Long-range ordering of tetrahedral cations in micas is favored by phengitic compositions, by the 3T stacking sequence of layers, and by tetrahedral Si:Al ratios near 1:1. Phengites of the 1M, 2M1, and 2M2 polytypes are said to show partial ordering of tetrahedral cations, although the amounts of tetrahedral substitutions are small and the accuracies of determination are not as large as desired. The 3T structures of muscovite, paragonite, lepidolite, and protolithionite show tetrahedral ordering, as do the 2M1 brittle micas margarite and an intermediate between margarite and bityite. Muscovite-3T and margarite-2M1 are also slightly phengitic relative to their ideal compositions. Examples of octahedral cation ordering in micas are more abundant and are to be expected when cations of different size and charge are present. Octahedron M(1) with its OH,F groups in the trans orientation tends to be larger than the mean of the two cis octahedra as a result of the ordering of cations and vacancies. In some samples ordering has reduced the true symmetry to a subgroup of that of the ideal space group. If ordering in subgroup symmetry results in ordered patterns of different geometries but similar energies in very small domains, the average over all unit cells may simulate long-range disorder.

Al-rich di, trioctahedral chlorite exists as the species cookeite and sudoite. Di,dioctahedral chlorite exists as the species donbassite. Cookeite has essential Li in its structure, sudoite has essential Mg, and donbassite has only small amounts of either element. To date, sudoite has been reported to have only IIb structural units and donbassite to have only Ia structural units. Cookeite is based primarily on Ia structural units, but IIb units are present in specimens from two localities. Most Al-rich chlorite species have regular-stacking “r” or “s” 2-layer stacking sequences, but 1-layer Ia-2 and Ia-6 polytypes also are known. The structural units (Ia or IIb) and the specific stacking sequences can be explained by a combination of local charge balance and minimization of cation-cation repulsion involving the interlayer and tetrahedral cations. X-ray powder diffraction data are adequate to differentiate Al-rich chlorite from trioctahedral chlorite and to identify the type of structural unit present, but single crystal study is necessary to identify the 2-layer and 1-layer sequences with certainty.