Sturges W. Bailey

POLYTYPISM OF TRIOCTAHEDRAL 1" 1 LAYER SILICATES

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.RésuméLe polytypisme des phyllosilicates 1 : 1 trioctahédriques résulte de deux caractéristiques variables de la structure. (1) Les cations octahédriques peuvent occuper le même arrangement de trois positions ou peuvent changer régulièrement entre deux arrangements différents des positions dans des couches successives. (2) La liaison d’hydrogène entre les surfaces adjacentes d’oxygène et d’hydroxyl des couches successives, peut être obtenu en partant de trois positions relatives différentes des couches: (a) superposition directe des couches, (b) déplacement de la deuxième couche par a/3 le long de l’un quelconque des trois axes X hexagonaux de la couche initiale, avec un sens positif ou négatif du déplacement déterminé uniquement par la position du cation octahédrique dans la couche inférieure, et (c) le déplacement de la deuxième couche par ± b/3 le long de Y1 (normale à X1) de la couche initiale, sans égard pour la position du cation octahédrique.En supposant une géométrie hexagonale idéale, sans ordre des cations, et sans interéchange dans le même crystal des trois types possibles de superpositions des couches, on peut alors dériver douze polytypes standards (plus quatre enantimorphes) avec des périodicités variant entre une et six couches. Les déplacement relatifs le long des trois axes X conduisent aux mêmes séquences des couches dérivées pour les micas, soit 1M, 2M1, 3T, 2M2, 2Or et 6H. Les polytypes 1T et 2H1 résultent de la superposition directe des couches. Les déplacements des couches de b/3 conduisent aux polytypes connus sous la désignation 2T, 3R, 2H2 et 6R.Les douze structures standards 1 : 1 peuvent être divisées en quatre groupes (A = 1M, 2M1, 3T; B = 2M2, 2Or, 6H; C = 1T, 2T, 3R; D = 2H1, 2H2, 6R) pour les besoins d’identification. Les fortes réflections des rayon X servent à identifier chaque groupe et les réflections plus faibles différencient les trois structures au sein de chaque groupe. Des exemples des quatre groupes et de 9 des 12 structures individuelles ont été identifiées dans des spécimens naturels. La considération des quantités relatives d’attraction et de répulsion entre les ions dans les structures conduit à prédire la séquence de stabilité groupe C groupe D groupe A groupe B, qui s’accordent modérément aux abondances observées dans ces groupes structurels.KurzreferatDie Polytypie in trioktaedrischen 1 : 1 Phyllosilikaten rührt von zwei veränderlichen Merkmalen im Gefüge her. (1) Die oktaedrischen Kationen können durchwegs in der gleichen Gruppierung von drei Stellungen angeordnet sein oder sie können in aufeinanderfolgenden Schichten jeweils zwischen zwei verschiedenen Gruppierungen der Stellungen abwechseln. (2) Die Wasserstoffbindung zwischen benachbarten Sauerstoff- und Hydroxyloberflächen aufeinanderfolgender Schichten kann durch drei verschiedene Stellungen der Schichten in Beziehung zu einander erhalten werden: (a) unmittelbare Überlagerung der Schichten, (b) Verschiebung der zweiten Schicht um a/3 entlang irgendeiner der drei hexagonalen X-Achsen der Ausgangsschicht, wobei eine positive oder negative Richtung der Verschiebung einzig durch die in der unteren Schicht eingenommene Gruppierung der oktaedrischen Kationen bestimmt wird, und (c) Verschiebung der zweiten Schicht um ± b/3 entlang Y1 (normal zu X1) der Ausgangsschicht ungeachtet der eingenommenen oktaedrischen Kationgruppierungen.Unter der Annahme idealer hexagonaler Geometrie, ohne Kationenregelung und ohne Vermischung innerhalb des gleichen Kristalls der drei möglichen Arten der Schichtüberlagerungen, lassen sich zwölf Normalpolytypen (plus vier Enantiomorphe) mit Periodizitäten zwischen einer und sechs Schichten ableiten. Relative Verschiebungen entlang der drei X-Achsen führen zu den bereits für die Glimmer abgeleiteten Schichtfolgen, nämlich 1M, 2M1, 3T, 2M2, 2Or und 6H. Die Polytypen 1T und 2H1 werden durch direkte Überlagerung von Schichten erhalten. Schichtverschiebungen um b/3 führen zu Polytypen mit der Bezeichnung 2T, 3R, 2H2 und 6R.Die zwölf Normal 1 : 1 Gefüge können für Identifizierungszwecke in vier Gruppen eingeteilt werden (A = 1M, 2M1, 3T; B = 2M2, 2Or, 6H; C = 1T, 2T, 3R; D = 2H1, 2H2, 6R). Die starken Röntgenreflexionen diesen zur Identifizierung der verschiedenen Gruppen, während die schwächeren Reflexionen zwischen den drei Strukturen innerhalb jeder Gruppe unterscheiden. Beispiele für alle vier Gruppen, sowie für 9 der zwölf Einzelgefüge konnten in natürlichen Proben aufgefunden werden. Eine Erwägung der relativen Beträge von Anziehung und Abstossung zwischen den Ionen in den jeweiligen Gefügen führt zu der vorhergesagten Stabilitätsfolge Gruppe C > Gruppe D > Gruppe A > Gruppe B, in ziemlich guter Übereinstimmung mit den beobachteten, mengenmässigen Vorkommen dieser Strukturgruppen.РезюмеПолитипия триоктаэдрических слоистых силикатов 1:1 обусловлена следующими двумя особенностями структуры. (1) Октаэдрические катионы могут занимать либо один и тот же из трех набор положений по всей структуре, либо, упорядоченно чередуясь, могут занимать два набора положений в последовательных структурных слоях. (2) Водородные связи между соседними кислородными и гидроксильными поверхностями последовательных слоев могут реализоваться при трех различных относительных положениях слоев: (а) прямое наложение слоев без смещения; (в) наложение слоев при смещении верхнего слоя на а/3 вдоль любой из трех гексагональных осей X нижнего слоя, причем положительное или отрицательное направление смещения однозначно определяется положением октаэдрических катионов нижнего слоя; (с) наложение слоев при смещении верхнего слоя на ±b/3 вдоль оси У1 (перпендикулярной Х1) нижнего слоя вне зависимости от положения октаэдрических катионов. При предположении гексагональной геометрии, отсутствия упорядочения катионов и невозможности сочетания в одном и том же кристалле 3-х упомянутых типов наложения слоев, могут быть выведены 12 независимых политипов (и 4 энантиоморфных) с периодичностью от одного до щести слоев на элементарную ячейку. Относительные смещения вгонь трех осей X приводят к образованию тех же последовательностей слоев, которые были выведены для слюд, а именно 1М, 2М1, ЗТ, 2М2, 20 и 6Н. Политипы 1Т и 2H1 получаются при прямом наложении слоев без смещения. Смещения слоев на b/3 приводят к образованию политипов, обозначаемых как 2Т, ЗR, 2Н2 и 6R. В целях идентификации 12 полученных независимых структур могут быть разбиты на 4 группы (А = 1М, 2М1, ЗT; В = 2М2, 20, 6Н; С = 1Т, 2Т, ЗR; В = 2Н1, 2Н2, 6R). Каждая группа имеет один и тот же набор сильных рентгеновских рефлексов, по которым может быть проведена идентификация различных групп; слабые рефлексы позволяют различать структуры внутри самих групп. При изучении природных образцов найдены представители всех четырех групп и 9-ти из 12-ти политипных структур. Анализ взаимного притяжения и отталкивания ионов в структурах позволил предсказать относительную устойчивость найденных групп, которая выражается следующей последовательностью: группа С> группа В> группа А> группа В, что находится в относительно хорошем соответствии с наблюдаемой распространенностью этих структурных групп в природе.

Refinement of the crystal structure of a monoclinic ferroan clinochlore

A monoclinic IIb-2 clinochlore from Washington, D.C., contrary to previous studies, is primarily a ferroan rather than a ferrian chlorite. Disorder of tetrahedral Si,Al cations is indicated because of unsuccessful structural refinements in subgroup symmetries. The true space group is C2/m. Slight ordering of Mg, Fe2+, and Fe3+ over octahedra M(l) and M(2) within the 2:1 layer (mean M-O,OH = 2.092 and 2.084 Å, respectively), complete ordering of trivalent Al into the centrosymmetric octahedron M(4) of the interlayer sheet (M-OH = 1.929 Å), and ordering of primarily divalent cations (Mg and Fe) into the two interlayer M(3) octahedra (M-OH = 2.117 Å) exist. The excess of negative charge above unity due to tetrahedral substitution of Al for Si (1.378 atoms) is compensated entirely within the octahedral sheet of the 2:1 layer.Ordering of a trivalent cation into one octahedron in the interlayer should be universal for all stable trioctahedral chlorites. In this specimen the ordering is due to (1) minimization of cation-cation repulsion by layer offsets which provide more space around the trivalent element, and (2) energy minimization by localization of the source of positive charge on the interlayer sheet in one octahedron rather than two. In other structures or for different compositions additional factors can be important also. Most chlorites of the IIb and Ib (β = 90°) types are expected to show disorder of the tetrahedral cations. The b positioning of interlayer and layer provides no preferential driving force for concentration of Si and A1 in any tetrahedron as a consequence of the expected ordering of the interlayer cations. The monoclinic IIb-2 polytype is less abundant in nature than the triclinic IIb-4 and IIb-6 structures, because only half as many possible superpositions of layers exist that will produce monoclinic symmetry. Crystallization factors must also be important, because the IIb-2 chlorite is much less abundant than predicted by this purely geometrical argument.

STRUCTURES, COMPOSITIONS, AND X-RAY DIFFRACTION IDENTIFICATION OF DIOCTAHEDRAL CHLORITES

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.

Formation of regularly interstratified serpentine-chlorite minerals by tetrahedral inversion in long-period serpentine polytypes

Abstract Serpentinite from Lancaster County, Pennsylvania, consists of a variety of fine-grained serpentine minerals, chlorite, randomly interstratified serpentine-chlorite, and a series of phases based on regular interstratification of Serpentine and chlorite (SxCy, where x and y are integers). Within the resolution of the AEM technique, all layer silicates have the same Mg-rich, Al-rich, Cr-rich, and Fe-poor compositions. Regularly interstratified serpentine-chlorite minerals are frequently intimately intergrown with serpentines that have repeat distances identical to those of the regular interstratifications. Thus, dozyite (S1C1, β = 90°) is intimately associated with serpentine with three-layer octahedral order (I,I,II). Longer period polysomes (S2C1, S1C2, S2C2, S1C3, S3C2, and S1C4) all with β = 90°) are each accompanied by serpentines with equivalent c-axis periodicities. SxCy phases apparently form by selective growth of lbb chlorite units from I,II octahedral sequences in long-period serpentines. All microscopic structural evidence is consistent with the formation of regular interstratifications by tetrahedral inversion within existing serpentine. Atomic resolution images reveal that the tetrahedral sheet is displaced by a/3 where it inverts to form the 2:1 layer. A ±a/3 shift is required for hydrogen bonding between OH of the newly formed brucite-like interlayer and O atoms of the 2:1 layer. The sense of the shift is determined by the strong interactions between the octahedral cations in the brucite-like interlayer and the Si in the 2:1 layer (direct superimposition, previously described as a type-a interaction, is strongly unfavorable). Distortion at the inversion point probably lengthens Si-O bonds in the next tetrahedra, facilitating relocation of Si on the other side of the basal O plane. Reversal of the octahedral slant in the 2:1 layer occurs because the +a/3 tetrahedral shift necessitates repositioning of O and OH coordinating octahedral cations, requiring movement of octahedral cations from type-II to type-I positions. Except in the 2:1 layers, the stacking and octahedral slants are inherited. The result is a series of regular interstratifications characterized by a single octahedral slant (specifically, Ibbb,I) and b/3 stacking disorder. This analysis reveals the importance of cation-cation interactions in determining the relative stability of pairs of 1:1 layers and in controlling the detailed structures of layer silicates formed in solid-state serpentine-to-chlorite reactions. Because similar constraints apply to formation of Serpentine from chlorite by direct structural modification, the common 1T lizardite polytype may be produced from both IIbb and Ibb chlorites.

K-AR DATING OF SEDIMENTARY ILLITE POLYTYPES

The illite in several cyclothemic Pennsylvanian shales and clays has been separated on the basis of particle size into 2M 1 and IMd polytypes. K-Ar dates show the 2M 1 component to be considerably older than the Pennsylvanian. The K-Ar age of the IMd component is less than half the age of Pennsylvanian sedimentation. The low age may be due to preferential Ar loss because of the small particle sizes involved or to reorganization and K-fixation in montmorillonite and degraded micas in post-Pennsylvanian time.

REFINEMENT OF THE CRYSTAL STRUCTURE OF NACRITE

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.RésuméLa structure du cristal de nacrite de la région de Pike’s Peak, au Colorado, à été raffinée en carrés minima et en cartes de différence de densite d’électrons en utilisant dix niveaux de données. Le raffinement complet à été gêné par des domaines épais comportant des déplacements de couches intermédiaires a/3 dans la “mauvaise direction”. La structure idéale est basée sur une séquence d’empilement 6R de couches de kaolin, dans lesquelles chaque couche successive est déplacée relativement à la couche inférieure par −1/3 de la répétition latérale 8·9 Å. Dans le nacrite, cette direction est X, contrairement à la convention habituelle des couches des silicates, à cause de la position des plans symétrie (010) normaux par rapport à la répétition de direction 51 Å les couches alternatives sont également basculées a 180°. Le modèle de l’emplacement octaédrique vacant réduit la symétrie à Cc et permet la description de la structure comme étant une formation à 2 couches avec un axe Z incline.Les adjacentes tétraédriques sont détournées de 7·3° en directions opposées si bien que les oxygènes de base se rapprochent des cations Al de la même feuille et des surfaces hydroxiles de la couche inférieure. Les ondulations enchevêtrées des surfaces d’oxigène et cfhyroxiles des couches adjacentes concourent alternativement et parallèlement aux zones (110) et (11̄0) des couches successives. Les anions triades supérieurs et inférieurs de chaque octaèdre Al sont bascules de 5·4° et de 7·0° dans les directions opposées résultant d’un raccoursissement partagé de la bordure. La nacrite possède une séparation de couche supérieure et des dimensions latérales inférieures à la dickite et à la kaolinite, et l’angle β observé dévie de 1½° par rapport à la valeur idéale. Ces caractéristiques, ainsi que sa stabilité générale moindre, paraissent être dues a la position moins favorable des oxygènes de base à l’interieur de la nacrite, comparativement aux chaînes d’hydrogene situées entre chaque couche aux liens d’entre-couches d’hydrogène.KurzreferatDie Kristallstruktur von Nakrit aus dem Pike’s Peak Gebiet in Colorado wurde unter Verwendung von zehn Datenniveaus durch Anwendung der kleinsten Quadrate und Darstellungen der Unterschiede in der Elektronendichte geklärt. Eine vollkommene Klärung wurde durch Verdickungen infolge von a/3 Zwischenschichtverschiebungen in der “falschen Richtung” verhindert. Die Idealstruktur stützt sich auf eine 6R gestapelte Folge von Kaolinschichten, wobei jede der aufeinander folgenden Schichten um −1/3 der 8,9 Å seitlichen Wiederholung in Bezug auf die darunter liegende Schicht verschoben ist. Diese Richtung ist X im Nakrit, im Gegensatz zu den sonst bei geschichteten Silikaten üblichen Verhältnissen, und zwar wegen der Lage der (010) Symmetrieeben normal zu der 5,1 Å Wiederholungsrichtung. Abwechselnde Lagen sind gleichfalls um 180° verdreht. Das Muster leerstehender oktaedrischer Stellen vermindert die Symmetrie auf Cc und erlaubt es, die Struktur als eine zweischichtige Form mit geneigter Z-Achse anzusprechen.Benachbarte Tetraeder sind in entgegengesetzter Richtung um 7,3° verdreht, so dass die Basis-Sauerstoffe näher an die Al Kationen in der gleichen Schicht, sowie an die Oberflächenhydroxyle in der darunter liegenden Schicht, herankommen. Ineinandergreifende Rippen in den Sauerstoff- und Hydroxyloberflächen benachbarter Schichten verlaufen abwechselnd parallel zu den [110] und [11̄0] Zonen in aufeinander folgenden Schichten. Die oberen und unteren Aniontriaden in jedem Al-Oktaeder werden durch gemeinsame Kantenverkürzung um 5,4° und 7,0 in entgegengesetzten Richtungen verdreht. Nakrit hat grössere Zwischenschichttrennung und kleinere seitliche Masse als Dickit und Kaolinit, und der beobachtete β Winkel weicht um 1½° vom Idealwert ab. Diese Merkmale, sowie die geringere Gesamtbeständigkeit des Nakrits, hängen vermutlich mit der weniger günstigen Lage der Basis-Sauerstoffe in Bezug auf die gerichteten Zwischenschicht-Wasserstoffbindungen im Nakrit zusammen.РезюмеКристаллическая структура накрита из района Пайке Пик (шт. Колорадо) была уточнена с помощью метода наименьших квадратов и разностных синтезов электронной плотности (использованы данные для десяти уровней) Полному уточнению структуры препятствовала большая толщина доменов с межслоевыми смещениями a/3 в “неправильном направлении”. Идеальная модель структуры основана на последовательности упакованных по закону 6R каолиновых слоев, в которой каждый последующий слой смещен по отношению к расположенному ниже на — 1/3 от трансляции 8,9 А в плоскости слоя. Это направление в накрите является осью Хвопреки обычному выбору осей слоистых силикатов, так как плоскость симметрии (010) перпендикулярна к периоду повторяемости 5,1 А. Чередующиеся слои повернуты на 180° один по отношению к другому. Распределение вакантных октаэдрических позиций уменьшает симметрию до Сс и позволяет описать структуру как двуслойную с наклонной осью 2.Смежные тетраэдры повернуты на 7,3° в противоположных направлениях так, что базальные атомы кислорода оказываются более близкими как к катионам А1 в том же слое, так и к поверхнос гидроксилов нижнего слоя.Примыкающие друг к другу выступы и впадины соприкасающихся кислородных и гидроксильных слоев располагаются параллельно [110] и [11̄0]. Верхние и нижние анионные треугольные основания в каждом А1-октаэдре повернуты на 5,4° и 7,0° в противоположных направлениях (результат укорочения общего ребра). Накрит имеет больший межслоевой и меньшие латеральные размеры, чем диккит и каолинит; измеренный угол (3 накрита отклоняется на 1½° от идеального значения. Эти особенности, а также меньшая устойчивость накрита, по-видимому, объясняются менее благоприятным положением в его структуре базальных атомов кислорода для образования ориентированных межслоевых водородных

REFINEMENT OF THE NACRITE STRUCTURE

Nacrite crystals from a vug within a matrix of dickite at Red Mountain near Sil verton, Colorado, have a = 8.906(2), b = 5.146(1), c = 15.664(3) Å, β = 113.58(3)°, V = 657.9(3) Å3, and space group Cc. The structure was solved by direct methods to determine phase angles, followed by electron density maps to locate all atoms. Refinement by least-squares ceased at R = 4.5%. Each 7 Å layer has structural detail very similar to those of dickite and kaolinite, although nacrite stacking is based on — a/3 interlayer shifts along the 8.9 Å axis (with octahedral cations alternating between the I and II sites in successive layers), whereas dickite and kaolinite are based on shifts of — a/3 along the 5.1 Å axis (with octahedral cations in the same set of sites in each layer). The angle of tetrahedral rotation is 7.8°, and the octahedral counterrotations are 7.6° and 8.1°. The H+ protons were located on DED maps. The inner 0..H1 vector points exactly toward the vacant octahedron and is depressed — 18.6° away from the level of the octahedral cations. All three surface OH groups have 0...H vectors at 50° to 66° to (001), although OH2 may not participate in interlayer hydrogen bonding. All three interlayer OH-H-O contacts are bent to angles between 132° and 141° and form contacts between 2.94 and 3.12 Å. The interlayer separation of 2.915 Å is slightly larger than in dickite, interpreted as due to a less favorable meshing of the oxygen and hydroxyl surfaces in nacrite—a direct consequence of layer shifts along the 8.9 Å axis.

Complex polytypism; relationships between serpentine structural characteristics and deformation

Abstract Serpentinite from Woods Chrome Mine, Lancaster County, Pennsylvania, consists of planar serpentine, randomly interstratified serpentine-chlorite, a series of phases based on regular interstratification of serpentine and chlorite, minor chlorite, polygonal serpentine, and antigorite. The serpentine mineralogy is complex, including structures with long-range order in (1) the octahedral cation sequence and (2) the sequence of displacements between adjacent 1:1 layers. In addition to previously described (but generally less common) 2T, 2H1, 2H2, 6R1, and 6R2 lizardite polytypes, the assemblage contains planar serpentines with long-range order in octahedral cation sequences but with random b/3 (and possibly a/3) displacements between adjacent layers. By comparison with calculated electron diffraction intensities, we identified three- (I,I,II), four- (I,I,II,II and I,I,I,II), five- (I,II,I,II,II), six- (II,I,I,I,II,II), seven- (I,II,I,II,II,II,II and I,II,I,II,I,II,II), and nine-layer octahedral sequences. In addition, the assemblage includes rare three- and four-layer serpentines with regular stacking; in some cases the stacking is nonstandard, insofar as zero and ±b/3 displacements occur together. By comparison between electron diffraction intensities and calculated patterns, we identified a three-layer regular stacking sequence that involves 0,0,-b/3 displacements between adjacent layers (α = 98°, β = 90°, γ = 90°), a four-layer monoclinic sequence with 0,-b/3,0,+b/3 (α = 90°, β = 90°, γ = 90°; I,I,II,II), and three four-layer triclinic sequences with 0,-b/3,-b/3,-b/3 (α = 90°, β = 90°, γ = 90°), 0,0,0,-b/3 (α = 96°, β = 90°, γ = 90°), and -b/3,-b/3,-b/3,+b/3 (α = 96°, β = 90°, γ = 90°) displacements between adjacent layers. Planar layer silicates exhibit macroscopic preferred orientation. The strong lineation in the foliation defined by the silicate sheets parallels either a or b, suggesting serpentine crystals were rotated, recrystallized, or both during shear deformation. We suggest that for crystals with b parallel to the lineation, deformation induced regular layer displacements. For crystals with a parallel to the lineation, periodic displacements of OH planes may have promoted development of regular octahedral cation sequences. Two-layer (I,II) and three-layer (I,I,II) serpentines and randomly interstratified serpentine-chlorite contain frequent but nonperiodic planar defects perpendicular to a* and the pseudo-a* axes that are interpreted to displace polytypic sequences. These defects predate chloritization and, in some cases, appear to serve as sites for chlorite nucléation. Layer silicates are crosscut by late-stage polygonized two-layer serpentine with disordered or regular stacking. In some cases with regular stacking, enantiomorphic 6R2 segments with a common c-axis direction parallel to their boundary are separated by sectors with 2H2 stacking. Polygonized serpentines are nucleated on steps at the surfaces of layer silicates and may have developed at a late stage by recrystallization of curved serpentine.

Dozyite, a 1:1 regular interstratification of serpentine and chlorite

Abstract Dozyite is a new mineral species involving regular interstratification of trioctahedral serpentine and trioctahedral chlorite units in a 1:1 ratio. It occurs as colorless crystals in an altered skarn adjacent to the Ertsberg East Cu, Au, and Ag mine in central Irian Jaya, Indonesia. The name is after Jean Jacques Dozy, the Dutch geologist who discovered and named the Ertsberg ore province in 1936. Unit-cell parameters are a = 5.323(3), b = 9.214(9), c = 21.45(2) A, β= 94.43(6)°, and V = 1049(2) Å3.It has space group Cm. There is a 21-Å periodicity in the 00l and in most other reflections where k = 3n. The reflections where k ≠3n are continuously streaked. Excellent regularity of alternation of the component serpentine and chlorite units is indicated by the coefficient of variation CV = 0.26 for the 001 reflections. A simplified ideal bulk composition is (Mg7Al2)(Si4Al2)O15- (OH)12with Z = 2, halfway between the compositions of the closely associated discrete chlorite (clinochlore) and the discrete serpentine (amesite). We believe the components in this occurrence of dozyite are clinochlore and amesite and that the interstratification was formed during the conversion of early clinochlore to amesite by Al metasomatism. The structure of dozyite contains a la chlorite unit followed by a serpentine 1:1 layer that is in the same position that the lower tetrahedral sheet of a repeating chlorite unit would occupy in the one-layer monoclinic Iaa-2 chlorite polytype, but rotated 180° so that the octahedral cations alternate I,I,II per 21 Å. The next chlorite unit follows with zero shift of its lower sixfold rings relative to those of the serpentine 1:1 layer. A second occurrence of dozyite has been recognized in a Cr-rich serpentinite from the Wood Chrome mine in Lancaster County, Pennsylvania. It represents a different polytype, with β= 90°, and a different composition relative to the Ertsberg material.

Polysomatism, polytypism, defect microstructures, and reaction mechanisms in regularly and randomly interstratified serpentine and chlorite

High-resolution (HRTEM) and analytical electron (AEM) microscopic evidence for a polysomatic series based on regular interstratifications of serpentine (amesite) and chlorite (clinochlore) are reported from an altered skarn in Irian Jaya. The assemblage includes regular interstratifications of one clinochlore and two (2:1; three structural variants), three (3:1), and four (4:1) amesite composition 1:1 layers as well as randomly interstratified serpentine and chlorite. The order of abundance of regularly interstratified minerals is 1:1>2:1>4:1>3:1. Atomic-resolution images, image simulations, and comparison between calculated and observed diffracted intensities verify the proposed 1:1 and 2:1 structures and reveal details of their defect microstructures. AEM data show that compositions are linear combinations of the associated amesite and clinochlore. The 1:1, 2:1, 3:1, and 4:1 minerals occur both as discrete sub-micron crystals and as domains within serpentine or chlorite. Some crystals of the 2:1 phase were sufficiently large for study by X-ray precession and powder methods. Crystals of the regularly interstratified 2:1, 3:1, and 4:1 phases are usually bent. High-resolution images reveal that, within polygonal segments, the layers commonly exhibit a few degrees of curvature with segments separated by antigorite-type offsets. Deformed chlorite crystals are probably replaced by interstratified minerals during an aluminum metasomatic event. Al may have been deposited from sulfuric acid-rich solutions when they interacted with calcite and dolomite to form the anhydrite-rich corona around the phyllosilicate-rich region of the core. The interstratified chlorite (clinochlore composition) suggests aluminum addition by selective conversion of a sub-set of the chlorite layers to amesite. Defect microstructures suggest that crystals of regularly interstratified material grew by direct structural modification of preexisting chlorite. Regular interstratifications may form in response to thermally controlled limits on Al solubility in chlorite and heterogeneities in the distribution of Al-rich solutions during metasomatism. Regularly interstratified minerals coexist with randomly interstratified serpentine/chlorite, chrysotile, antigorite, lizardite, and several amesite and chlorite polytypes. Tentative chlorite and amesite identifications include one-layer (b=97°, probably IIbb), one-layer (b=90, possibly Ibb), two-, and three-layer chlorites, and 2H1 (but possibly 1M or 1T), rhombohedral (3R or 6R), and twelve-layer (Tc; non standard) serpentine polytypes. The complex phyllosilicates attest to rampant chemical and structural disequilibrium.