Domain twinning of Laihunite and refinement of its crystal structure

Domain twinning of laihunite has been investigated based on diffracton phenomena, and its crystal structure has then been refined. Space group with respect to the domain isP2₁/c, and cell parametersa=5.813,b=4,812,c=10.211(A), β=90.87°. Atomic coordinate and bond length have been recalculated. Discussions are made of the Fe²⁺ distribution, lattice distortion, degree of order of laihunite and the relationship of this mineral with fayalite and ferrifayalite. The authors still hold that laih unite should be considered as a new silicate mineral with dominant Fe³⁺ and less amount of Fe²⁺.     

Stacking disorder in wollastonite and its relationship to twinning and the structure of parawollastonite

It has been shown that a unit cell of parawollastonite in a matrix of triclinic wollastonite may be generated by (1) a single stacking fault and (2) by twinning. A wide domain of parawollastite may be produced by stacking faults in every second triclinic unit cell, whereas a twin lamella may be produced by faults in every adjacent triclinic unit cell.     

Switzerite: Its chemical formula and crystal structure

Summary Switzerite has the following schematical chemical formula Mn ₄ ^2+(VI) (Me ^3+,2+,1+,)^(VI) (Me ^3+,2+,1+,)^(V) (PO₄)₄. 8 H₂O, whereMe is mainly iron; the mineral is monoclinic, space groupP2₁/c, Z=4; lattice parameters area=8.496,b=13.173,c=17.214 Å, -96.65°. The atomic arrangement was determined by direct methods and refined by least-squares method. FinalR index is 0.077 for 3038 observed reflections. The crystal structure of switzerite can be described as built up by octahedral sheets parallel to (001), with formula [Mn₄O₁₀(H₂O)₄]₂.Me coordination bipyramidal dimers link these units in thec direction whileMe coordination octahedra stick out from the sheets to which they are connected through a vertex. The atomic arrangement of switzerite is compared with that in ludlamite, Fe₃(PO₄)₂·4H₂O, and in whitmoreite, Fe²⁺Fe ₂ ³⁺ (OH)₂(PO₄)₂·4 H₂O. The only analogy in all these structures is the presence of octahedral slabs exhibiting, however, different shapes.

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Switzerit: Chemische Formel und Kristallstruktur

Zusammenfassung Switzerit hat die schematische chemische Formel Mn ₄ ^2+(VI) (Me ^3+,2+,1+,)^(VI) (Me ^3+,2+,1+,)^(V) (PO₄)₄·8 H₂O, wobeiMe hauptsächlich Eisen ist. Das Mineral ist monoklin, RaumgruppeP2₁/c,Z=4; Gitterkonstanten:a=8,496,b=13,173,c=17,214 Å, =96,65°. Die Atomanordnung wurde mit direkten Methoden bestimmt und nach der Methode der kleinsten Quadrate verfeinert. Es wurde für 3038 beobachtete ReflexeR=0,077 erreicht. Man kann die Kristallstruktur des Switzerits als aus Oktaederschichten der Formel [Mn₄O₁₀(H₂O)₄]₂, die parallel zu (001) liegen, beschreiben. Dimere aus trigonalen Dipyramiden umMe verbinden diese Einheiten in Richtung derc-Achse, während Koordinationsoktaeder umMe aus diesen Schichten, an die sie über eine Ecke verknüpft sind, hervorragen. Die Atomanordnung des Switzerits wird mit denen des Ludlamits, Fe₃(PO₄)₂·4 H₂O und des Whitmoreits, Fe²⁺Fe ₂ ³⁺ (OH)₂(PO₄)₂·4 H₂O verglichen. Die einzige Analogie zwischen allen diesen Strukturen ist die Anwesenheit von Oktaederschichten, die aber verschiedene Gestalt haben.

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With 3 Figures     

Intergrowth and twinning in opal-CT lepispheres

Under conditions of unobstructed diagenetic growth, opal-CT usually forms spherical aggregates, so-called lepispheres. From a scanning electron microscope study of deep-sea porcelanites, the development of these structures is described, from isolated opal-CT platelets via complete lepispheres to large coalescent aggregates. The conspicuous symmetric ultrastructure of lepispheres—reflected by the constancy of angles between interpenetrating crystal blades—is crystallographically determined: The lepispheres consist of groups of opal-CT blades intergrown according to the (30–34) and (10–16) twinning laws of tridymite. The same rules of intergrowth can be applied to the morphology of hydrothermally synthesized opal-CT lepispheres.     

Glide twinning and pseudotwinning in peristerite: twin morphology and propagation

Optically visible Albite glide twins in a peristerite (An₉Or_1.6), identified from their tapering shape and relationship to grain boundaries, were studied by transmission electron microscopy. Near the tips in sections a, the microstructure consists of small (400 nm long) lensshaped Albite twins centred exclusively on the oligoclase lamellae. The lenses extend partly outwards into the two adjacent low albite lamellae and induce strong inhomogeneous strain. Where the lenses are closer together, they form, depending on the sense of shear, nearly linear left or right-stepping en échelon arrays, with overlap of the strain fields. Slightly farther in from the tip, the twin domains coalesce to form continuous pinch-and-swell lamellae, being always thicker in the oligoclase. Because of Si,Al order, only elastic glide pseudotwins are possible in low albite. In oligoclase glide pseudotwins may be mechanically stable (metastable relative to Si,Al order) and may deviate only slightly from true twins. Pseudotwins develop first in the oligoclase, propagate dynamically by jumping across the intervening albite lamellae, extend lengthways and thicken sideways and finally coalesce. They are stabilized by diffusion-controlled inversion of Si,Al order giving rise to true twins described in a companion paper.C.R.P.G. contribution 799     

Investigation of spheroidal weathering and twinning

What determines the geometry of spheroidal weathering patterns? Spheroidal weathering rings appear to arise from the leaching of fresh rock, rather than from reprecipitation of dissolved minerals diffusing out of weathered rock. In previous work the authors developed a mathematical model to explain the geometry of spheroidal weathering structures, and showed how some of the features of weathering patterns, otherwise difficult to explain, were natural consequences of the model. In describing the geometrical aspect of these shapes, we hope to understand how natural forces acted upon the unweathered rock and why and under what conditions spheroidal weathering structures were formed. In this paper we review the main features of our model and examine and discuss twinning of spheroidal weathering structures, concluding that the structures were formed during a distinct period in the past. Twinning is an apparent anomaly in that its natural occurrence, as part of the weathering process, cannot be explained by our model without additional ad hoc assumptions. Careful examination of twins reveals an explanation which does not require revision of the model.     

Polysynthetic feldspar twin-width distributions: Models for inversion twinning

The lamella-width distribution of the transformation polysynthetic twins produced in volcanic anorthoclases during cooling reflects certain aspects of twin nucleation and growth kinetics. An analysis of simple hypothetical statistical models for the kinetics of the transformation process outlines the range and type of twin-width distributions possible. The general characteristics of these distributions, particularly at small twin widths, are sensitive to whether twin boundaries or twin centers nucleate. The rates of twin nucleation and growth during transformation also influence the general form, particularly the skewness, of the final twin-width distribution. The generally exponential nature of the upper tails to the distributions studied is apparently quite insensitive to kinetic details.     

Submicroscopical twinning in lunar and experimentally deformed pyroxenes

During deformation experiments at high pressures (10 kb) and medium temperatures (500–650°) diopside became twinned on {100} in small optically invisible domains. If host and twin occur with equal frequency, the X-ray single-crystal photograph would show orthorhombic symmetry due to the special geometry of the diopside lattice. A crystal of pyroxene from Mare Tranquillitatis has been found to give just such an X-ray photograph with orthorhombic symmetry; this is explained as a possible deformation product.     

Mechanical twinning in small quartz crystals

Quartz is known to be ferrobielastic; that is, quartz crystals have domain states (Dauphiné twins) which differ in their elastic compliance values and which can be switched by an appropriately oriented stress. Polycrystalline quartz has also been reported (Tullis 1970) to show preferential orientation of these domains following application of large uniaxial stresses. These experiments were designed to study twinning of synthetic quartz “grains” (minimum size 0.07×0.07×0.02 cm) in specially-constructed composites and of grains in three natural quartz aggregates — a quartzite, a novaculite, and a jasper. Backreflection X-ray techniques were used to verify twinning in the composite grains, while special electroding and electrical detection allowed the twinning processes to be examined in “real time.” Small synthetic quartz crystals were found to behave identically to the massive samples previously studied. Electrical pulses due to the reversal of piezoelectric coefficient d ₁₁ in twinned quartz were detected from quartzite and from the man-made composites. Novaculite also gave electrical pulses which were probably from twinning (evidenced by the correlation of expected and observed pulse sizes and shapes), while no pulses from the jaspers indicative of twinning were detected. Grain size distribution differences are considered the main structural reason for the different behaviors.     

Mechanical twinning and slip in experimentally deformed plagioclases

Mechanical albite and pericline twins were induced in seven feldspars, (Na, Ca) (Al, Si)₄O₈, in triaxial compression tests at 800° C, 8 to 10 kb confining pressure. Two were initially in a high temperature structural state (disordered) (An₁, An₅₉). Three (An₃₂, An₃₉, and An₅₃) were slightly disordered, and two (An₇₆ and An₉₅) had the transitional and primitive anorthite structure, respectively. No microscopic twinning was detected in comparable tests on ordered albite and oligoclase (An₁, An₂₀), feldspars in the peristerite range (An₁₁ An₁₄) or in a low structural state labradorite (An₅₁). Other deformational features include lamellae 60 to 80° from the c axis and (010) slip in the opposite sense for (010) twinning.Work performed under the auspices of the U. S. Atomic Energy Commission.     

The twinning microstructure and growth of amethyst quartz

The characteristic lamellar-twinning of the right-handed (R) and left-handed (L) structures in the major rhombohedral growth sectors of amethyst quartz has been studied by optical techniques, X-ray topography and transmission electron microscopy (TEM). The TEM observations show that the region of each Brewster fringe consists of fine-scale Brazil twin lamellae parallel to one of the r, z{10 \(\overline {\text{1}} \) 1} planes, and structural considerations suggest that it is one of the r-planes. The twin boundary corresponding to a Brewster fringe has the form of a zig-zag structure consisting of Brazil twin boundaries on two r{10 \(\overline {\text{1}} \) 1} planes, with one predominating. The Brewster fringes appear black between crossed polarizers because light travelling along the optic axis [001] passes through almost equal distances of R and L quartz, giving essentially zero optical rotation. From the visibility of the Brazil twin boundaries in electron micrographs and the visibility of the Brewster fringes in X-ray topographs, the fault vector R and the corresponding composition plane of the major Brazil twin associated with each Brewster fringe has been determined. The streaking of the Brewster fringes observed optically and in the X-ray topographs appears to be due to the stair-rod dislocations at the intersections of the Brazil twin boundaries. Experiments in which synthetic quartz was grown hydrothermally on untwinned seeds and on twinned amethyst seeds showed that the initiation of Brazil twins and the development of Brewster fringes was dependent upon the presence of iron in the growth solution.     

Vertumnite: Its crystal structure and relationship with natural and synthetic phases

Summary Vertumnite, Ca₄Al₄Si₄O₆(OH)₂₄·3H₂O, is metrically monoclinic, strongly pseudohexagonal;a=5.744 (5),b=5.766(5),c=25.12(1) Å, =119.72(5)°; space groupP2 ₁ /m. The crystal structure was determined from X-ray intensities and refined in both the monoclinic and the hexagonal space group [P6 ₃/m; a=(a _mon +b _mon )/2]. The monoclinic refinement did not lead to significant deviations from hexagonal symmetry. The atomic arrangement consists of modified brucite-layers Ca ₂ ^VII Al^VI(OH, H₂O)₈, atz=0 andz=1/2, alternating with tetrahedral double layers and connected only by hydrogen bridges. TheT sites are statistically and only partly occupied by Si and Al. The distances from theT sites to the three basal (O, OH) measure 1.80 Å; this large distance is probably caused by local deformations in connection with the disorder in theT sites. Water molecules occupy statistically the double rings. A comparison with the previously reported powder patterns of gehlenite hydrate and strätlingite is given.

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Vertumnit: Seine Kristallstruktur und seine Beziehungen zu anderen natürlichen und künstlichen Phasen

Zusammenfassung Vertumnit, Ca₄Al₄Si₄O₆(OH)₂₄·3 H₂O, ist metrisch monoklin, ausgeprägt pseudohexagonal;a ₀=5,744(5),b ₀=5,766(5),c ₀=25,12(1) Å, =119,72(5)°; RaumgruppeP2 ₁/m. Die Kristallstruktur wurde aus Röntgenintensitäten bestimmt und sowohl in der monoklinen wie in der hexagonalen Raumgruppe [P6 ₃/m; a ₀=(a _0,mon +b _0,mon )/2] verfeinert. Die monokline Verfeinerung führte auf keine wesentlichen Abweichungen von hexagonaler Symmetrie. Die Atomanordnung besteht aus modifizierten Brucit-Schichten, Ca ₂ ^VII Al^VI(Oh, H₂O)₈, die mit Doppeltetraederschichten abwechseln und mit diesen nur über Wasserstoffbrücken verbunden sind. DieT-Positionen sind durch Si und Al statistisch und nur partiell besetzt. Die Abstände von denT-Positionen zu den drei basalen (O, OH) messen 1,80 Å; dieser große Abstand wird wahrscheinlich durch lokale Verzerrungen im Zusammenhang mit der Unordnung in denT-Lagen verursacht. Wassermoleküle füllen statistisch die Doppelringe. Das Pulverdiagramm wird mit den publizierten Diagrammen von Gehlenit-Hydrat und Strätlingit verglichen.

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With 2 Figures     

Dauphiné twinning and texture memory in polycrystalline quartz

Mechanical twinning in polycrystalline quartz was investigated in situ with time-of-flight neutron diffraction and a strain diffractometer. Dauphiné twinning is highly temperature sensitive. It initiates at a macroscopic differential stress of 50–100 MPa and, at 500°C, saturates at 400 MPa. From normalized diffraction intensities the patterns of preferred orientation (or texture) can be inferred. They indicate a partial reversal of twinning during unloading. The remaining twins impose residual stresses corresponding to elastic strains of 300–400 microstrain. Progressive twinning on loading and reversal during unloading, as well as the temperature dependence, can be reproduced with finite element model simulations.     

New type of shock twinning in potassium feldspar

Peculiar deformations were observed in potassium feldspar grains from the glass-poor suevites of the Zeleni Gay astrobleme on the Ukrainian shield. In cross-polarized light, they resemble kink bands in mica. We investigated the composition of the potassium feldspar and the optical orientation of the deformation features, which appeared to be in a twin relation to the crystal matrix. Twin laws and other characteristics of the deformations were determined. The origin of such textures was attributed to a combination of relatively low-pressure shock loading (possibly, under the conditions of impactor ricochet) of a block of target granites with a subparallel orientation of tabular microcline grains.     

New evidence on secondary twinning in albitic plagioclases

Twinnings in albitic partially disordered plagioclases (An%=0–5; 2V variable between +80° and -87°) are described; they show optical-morphological features very similar to those exhibited by secondary twinning in calcic plagioclases. One of the twinning observed shows strong analogies with a phenomenon induced on a partially disordered albite in laboratory experiments (Borg and Heard, 1969). The data give new evidence on the debated existence of secondary twinning in albitic plagioclases.     

First evidence of secondary glide twinning in K-feldspar

Summary Secondary glide twinning in a partially disordered K-feldspar is described. It is a highly polysynthetic twin according to the Baveno law.

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Erstmaliger Nachweis von sekundärer Gleitverzwillingung in K-Feldspat

Zusammenfassung Es wird sekundäre Gleitverzwillingung in einem teilweise ungeordneten K-Feldspat beschrieben. Es handelt sich um einen hoch polysynthetischen Zwilling nach dem Bavenoer Gesetz.

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With 4 Figures     

Glide twinning and pseudotwinning in peristerite: Si,Al diffusional stabilization and implications for the peristerite solvus

Albite glide pseudotwins related to grain-boundary stresses have been observed in an exsolved peristerite (Brown 1989). The glide operation transposes the pre-existing periodic oligoclase/albite lamellae and interfaces into a position rotated by only 0.5° in the pseudotwins, but transforms the indices from outside to (081) inside the pseudotwin. The pseudotwin is anti-ordered with respect to Al and Si and both it and the transposed interface are unstable. They should revert to the initial state on stress removal. If however the stresses are maintained for a sufficiently long time, the pseudotwins are stabilized by inversion of Si,Al order and re-orientation of the interface by an angle of about 30° into a position close to . The continuous lamellae break up into a series of discs by diffusion of NaSi and CaAl, the minimum diffusion path being about the same as the thickness of the lamellae. On extrapolating available interdiffusion data in Ab-rich plagioclases to low temperatures, possible diffusion times may be calculated. The calculated times are long so that either the peristerite miscibility gap must be at a higher temperature than previously supposed or the low-temperature interdiffusion coefficients must be higher than the extrapolated experimental ones, or both. From recent data on ordering in albite, the crest of the gap is estimated to lie close to 650–625° C at low pressure and it is possible that interdiffusion under natural conditions is facilitated by hydrogen (protons) in feldspars.C.R.P.G. contribution 800     

Deformation twinning in calcite,dolomite, and other rhombohedral carbonates

Transmission electron microscope (tem) observations of single and multiple twins in calcite and dolomite are presented, and the results are analysed by means of selected area diffraction and trace analysis. Simple twinning in rhodochrosite and kutnahorite is also analysed. It is shown that the ordered carbonates, such as dolomite, have a common twinning plane {01 \(\bar 1\) 2} and this appears to be their only mode of deformation twinning. The carbonates with higher symmetry, such as calcite, have {01 \(\bar 1\) 8} as the primary twinning plane but calcite itself has other twinning mechanisms, of which the most important is illustrated. Crossing and stopping twins are also discussed. It is shown that twinning in calcite, which occurs predominantly at low temperatures, is characterized by the generation of large numbers of glide dislocations.     

Acoustic emissions from stress-induced dauphiné twinning in quartz

Dauphiné (electrical) twins have been introduced into quartz crystals by means of suitably oriented mechanical stresses. Stress-induced twinning was observed optically by means of the photoelastic effect. Accompanying acoustic emission, first detected by ear, was quantitatively measured with a piezoelectric transducer and a suitable filtering and amplification system. Synchronized sound cinematography has been used to correlate the acoustic emission with visual observation of twinning. The resultant motion picture has been used to determine the stress levels required for twinning in Brazilian quartz and to set limits on the twin wall velocity. The possible relevance to rock noise and earthquake precursory phenomena is discussed.