Structure of Brazil twin boundaries in amethyst showing brewster fringes

Optical investigations of Brewster fringes in natural amethyst crystals have indicated that the fringes are Brazil twin boundaries consisting of McLaren and Pitkethly type zig-zag structures and closed parallelograms. The two types of Brazil twin boundaries appear alternately in the successive fringes and in different orientations in one fringe. The Brewster fringes develop by a process of incorporation of many Brazil twin lamellae, when growth conditions are stabilized after this process. If perturbation occurs in growth conditions, distinct growth banding with and without Brazil twin lamellae will develop, and not Brewster fringes.     

X-ray topographic study of quartz crystals twinned according to japan twin law

Quartz crystals twinned according to Japan twin law were investigated by means of X-ray topography in order to understand the origin of characteristic morphology of twin crystals. It is demonstrated that the flattened and elongated morphology characteristic of quartz twins is due to preferential growth at twin junctions where dislocations with the Burgers vector direction 〈11 \(\overline {\text{2}} \) 1〉 concentrate, and that such preferential growth operates only when {10 \(\overline {\text{1}} \) 1} faces meet at the twin junction. Once {10 \(\overline {\text{1}} \) 0} faces appear at the twin junction due to the change of growth conditions, the effect diminishes sharply and the characteristic morphology becomes less pronounced. This leads to the conclusion that the characteristic morphology of quartz crystals twinned according to Japan twin law is formed at the earlier stage of growth and becomes less pronounced at the later stage of growth.     

Deformation and transformation in experimentally shock-loaded quartz

Single crystals of quartz, shock-loaded along the a axis to pressures of 22 Gpa, 24 GPa, 26 GPa and 30 GPa were examined by high-voltage transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction. Asymmetric broadenings of X-ray lines indicate spatial inhomogeneity of shock effects. X-ray streaking angles in the reciprocal lattice planes h0 \(\bar h\) l, 0k \(\bar k\) l and hki0 indicate a slight tilting deformation by rotation about [00.1] in (0001). TEM reveals glass lamellae which are mostly in (01 \(\bar 1\) 2) orientation, and are correlated with optical planar elements and with surface steps seen in SEM. No dislocations are found. There are (0001) lamellar features, probably Brazil twins. The (01 \(\bar 1\) 2) glass lamellae develop directly from bands of quartz in which intense deformation has produced a fine-scale lamellar to blocky structure, possibly also originating by twinning. Relics of crystalline structure are found in almost completely vitrified lamellae. Stishovite occurs in heavily deformed parts of the 22 GPa and 24 GPa specimens, in patches of densified glass distinct from the sharply bounded lamellae. The nucleationless, pervasive transformation of lamellae to glass, with preservation of their sharp boundaries, is attributed to defect coalescence analogous to vitrification by radiation damage (metamictization). Some patchy glass may be due to melting.     

Growth defects in metamorphic biotite

Metamorphic biotites examined by transmission electron microscopy contain planar defects on the (001) plane, superlattices, twins and a microstructure causing streaking of k≠3n rows. Analysis of the fringe contrast shows that the fault vectors associated with the planar defects are either R ₁=±1/3 [010], R ₂=±1/6 [310] or R ₃=±1/6 [3 \(\bar 1\) 0]. Structural considerations indicate that a stacking fault R ₁, R ₂ or R ₃ is most likely to exist in the octahedral layer rather than the potassium layer. The result of such a fault on a unit layer of mica is effectively to rotate it through ±120° about c* (equivalent to the common mica twin law). These stacking faults can provide the mechanism for producing the ±120° rotations associated with the common mica polytypes. Furthermore, many of the observed microstructures can be generated by these stacking faults.     

Experimental delineation of the “e” ⇌ I\bar 1 and “e” ⇌ C\bar 1 transformations in intermediate plagioclase feldspars

Approximately 125 hydrothermal annealing experiments have been carried out in an attempt to bracket the stability fields of different ordered structures within the plagioclase feldspar solid solution. Natural crystals were used for the experiments and were subjected to temperatures of ～650°C to ～1,000°C for times of up to 370 days at \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) =600 bars, or \(P_{{\text{H}}_{\text{2}} {\text{O}}} \) =1,200 bars. The structural states of both parent and product materials were characterised by electron diffraction, with special attention being paid to the nature of type e and type b reflections (at h+k=(2n+1), l=(2n+1) positions). Structural changes of the type C \(\bar 1\) → I \(\bar 1\) , C \(\bar 1\) → “e” structure, I \(\bar 1\) → “e” and “e” structure → I \(\bar 1\) have been followed. There are marked differences between the ordering behaviour of crystals with compositions on either side of the C \(\bar 1\) ? I \(\bar 1\) transition line. In the composition range ～ An₅₀ to ～ An₇₀ the e structure appears to have a true field of stability relative to I \(\bar 1\) ordering, and a transformation of the type I \(\bar 1\) ? e has been reversed. It is suggested that the e structure is the more stable ordered state at temperatures of ～ 800°C and below. For compositions more albite-rich than ～ An₅₀ the upper temperature limit for long range e ordering is lower than ～ 750°C, and there is no evidence for any I \(\bar 1\) ordering. The evidence for a true stability field for “e” plagioclase, which is also consistent with calorimetric data, necessitates reanalysis both of the ordering behaviour of plagioclase crystals in nature and of the equilibrium phase diagram for the albite-anorthite system. Igneous crystals with compositions of ～ An₆₅, for example, probably follow a sequence of structural states C \(\bar 1\) → I \(\bar 1\) → e during cooling. The peristerite, Bøggild and Huttenlocher miscibility gaps are clearly associated with breaks in the albite, e and I \(\bar 1\) ordering behaviour but their exact topologies will depend on the thermodynamic character of the order/disorder transformations.     

Transmission electron microscopy of experimentally deformed chalcopyrite single crystals

Two crystals of natural chalcopyrite, CuFeS₂, experimentally deformed at 200° C have been studied by means of transmission electron microscopy (TEM). The activated glide planes are (001) and {112}. The dislocations in (001) have the Burgers vector [110] and a predominating edge character. They are split into two colinear partials b=1/2[110] and can cross split into {112}. The dislocations in {112} consist of straight segments along low index lattice lines. They are often arranged in dipoles generating trails of loops. Few dislocations with b=1/2[ \(\overline {11} \) 1] and [1 \(\bar 1\) 0] are present and dislocations with b=[0 \(\bar 2\) 1] occur in low angle subgrain boundaries. From weak beam contrasts it is presumed that most of the dislocations gliding in {112} have b=1/2〈3 \(\overline {11} \) 〉. They are dissociated into up to four partials. Microtwins and different types of stacking faults in {112} also occur. Models of the dissociation of dislocations are discussed.     

Standing stock and estimated production rates of phytoplankton and zooplankton in Narragansett Bay,Rhode Island

Seasonal changes in phytoplankton biomass and production, total zooplankton biomass, and biomass and potential production rates of the two dominant copepods, Acartia hudsonica (formerly called Acartia clausi) and Acartia tonsa are described for several stations in Narragansett Bay, R.I. Plankton in the bay behaved as a single population with simultaneous changes occurring at the upper bay (Station 5) and the lower bay (Station 1). Phytoplankton biomass was higher in the upper bay ( \(\bar x\) =16.95 mg chl a·m^?3) than in the lower bay ( \(\bar x\) =6.37 mg chl a·m^?3) and these 0269 0101 V differences in biomass were reflected in the phytoplankton production rates. The zooplankton, which was dominated by A. hudsonica in the spring and early summer and A. tonsa during summer and fall, showed no such consistent differences between the stations. Mean A. hudsonica biomass (St 1, \(\bar x\) ;=82.7 mg dry wt·m^?3; St 5, _ \(\bar x\) ;=95.2 mg dry wt·m^?3) exceeded that of A. tonsa (St 1, \(\bar x\) ;=56.7 mg dry wt·m^?3; St 5, \(\bar x\) ;=60.0 mg dry wt·m^?3). Potential production rates of the two Acartia 0269 0101 V spp. were strongly temperature dependent. Despite the higher biomass levels of A. hudsonica, low temperatures resulted in lower potential production rates ( \(\bar x\) ; St 1=7.25 mg C·m^?3 day^?1; \(\bar x\) ; St 5=10.77mg C·m^?3 day^?1) and biomass doubling times of up to 9.6 days. Potential production rates of A. tonsa at summer temperatures were high ( \(\bar x\) ; St 1=19.0 mg C·m^?3 day^?1; \(\bar x\) ; St 5=22.9 mg C·m^?3 day^?1) and biomass doubling times were generally less than one day.     

Interdiffusion of NaSi—CaAl in peristerite

The ‘average’ interdiffusion coefficient ( \(\bar D\) ) for NaSi—CaAl exchange in plagioclase for the interval from An₀ to An₂₆ was estimated from experimentally determined homogenization times for peristerite exsolution lamellae. The average spacing between adjacent (unlike) lamellae is 554±77 Å. Dry heating in air at 1,100°C for 98 days produced no change in the exsolution microstructure; thus \(\bar D\) (dry)<10^?17 cm²/s. This limit is consistent with the recently reported ‘average’ \(\bar D\) (dry) values for the Huttenlocher interval (An_70–90) at this temperature. At 1.5 GPa with about 0.2 weight percent water added the ‘average’ diffusion coefficient from 1,100°C to 900°C is given by: \(\bar D\) (wet)=18 _?15 ⁺¹⁰⁸ (cm²/s) exp (?97±5 (kcal/mol)/RT), where R is the gas constant, and T is °K. This \(\bar D\) (wet) at 1,100°C is more than three orders of magnitude greater than \(\bar D\) (dry) for Na- and Ca-rich plagioclases.     

Transmission electron microscopy on meteoritic troilite

Phase transitions and associated domains of meteoritic troilite (FeS) have been studied by means of transmission electron microscopy (TEM). Three polymorphs have been found, two of which can be described by superstructures of the NiAs-type structure (A, C subcell). The P \(\overline 6\) 2c (√3A, 2C) polymorph, stable at room temperature, displays antiphase domains with the displacement vector 1/3< \(\overline {\text{1}}\) 10>. In situ heating experiments showed that the P \(\overline 6\) 2c polymorph changes at temperatures of 115°–150° C into an orthorhombic pseudohexagonal transitional phase with the probable space group Pmcn (A,√3A, C). It contains antiphase domains with the displacement vector 1/2 [110] and twins with a threefold twin-axis parallel c. When heated above 210° C the transitional phase transforms into the high-temperature modification with NiAs structure (P6 ₃/mmc). All observed phase transitions are reversible. The occurrence of antiphase and twin domains, respectively, agrees with the symmetry reductions involved in the subsolidus phase transitions. This is demonstrated by group-subgroup relationships among the space groups P6 ₃/mmc, Pmcn, and P \(\overline 6\) 2c.     

A quantitative study of deformation mechanisms and finite strain in quartzites

The Weverton quartzites in the Maryland Blue Ridge are deformed by one major period of greenschist-grade deformation. The components of finite strain due to different independent mechanisms have been measured for these rocks. The total strain is split up into two major components: $$\varepsilon ^t = \varepsilon ^p + \varepsilon ^d .$$ The finite natural strain caused by dislocation creep (? ^d ) is measured by a new technique using folded and stretched rutile needles which are good strain markers within the quartz crystals. Pressure solution strain (? ^p ) is measured from the ratio of the area of new crystals and fibers to the whole rock area in principal sections. Grain boundary sliding is a dependent process which accompanies both mechanisms. Pressure solution obeys a linear Newtonian flow law, \(\left| {\dot \gamma _0^p } \right| = A_p \left| {\tau _0 } \right|\) , while dislocation creep obeys a power law of the form \(\left| {\dot \gamma _0^d } \right| = A_d \left| {\tau _0 } \right|^n \) where \(\dot \gamma _0^p ,\dot \gamma _0^d \) are octahedral shear strain rates, τ₀ is the octahedral shear stress and A _p , A _p and n are constants. A direct correlation between finite strain measurements and the operating flow laws can be made. Application of these methods and principles to a few field examples indicates that the rocks obey a flow law partly governed by each mechanism. Any set of physical conditions defines a unique flow law and there is a transition in creep behavior from dominantly Newtonian to a power law with increasing strain rate.     

Twin laws of whewellite,CaC2O4·H2O. A structural and growth approach

Both natural and synthetic crystals of whewellite (CaC₂O₄·H₂O, sp.g. P2₁/c) occur commonly as twins. The geometrical, reticular and structural features of the most important twin law, twin plane (100), and the dubious law ( \(\overline {\text{1}} \) 01) were investigated. The strict application of the periodic bond chain (PBC) theory (Hartman and Perdok 1955) makes possible the identification of original composition planes (OCP). Following the OCP analysis it has been deduced that the energy necessary to produce a (100) twin is very low. This is evidenced by the easy reciprocal arrangement of the two crystal structures which can be shown in two slightly different ways, i.e., by a (100) reflection with a mean [0, 1/2, ?a ₀cosβ] translation and a weak relaxation (from 0.1 to 0.25 Å) of the common interface. In OCP an important role is played by water molecules. With regard to ( \(\overline {\text{1}} \) 01) twinning, the geometrical and reticular agreement is not matched by the structural situation. The structural and growth approach to the twin laws of whewellite therefore justifies the very common occurrence only of (100) twinned crystals.     

Transmission electron microscope study of experimentally deformed K-Feldspar single crystals

Single crystals of sanidine which were experimentally deformed so as to introduce the (010)[100] slip system were examined by transmission electron microscopy (tem). Dislocation glide is mainly manifested in the samples deformed at 700° C, with a strain rate \(\dot \varepsilon = 1 - 2 \times 10^{ - 6} s^{ - 1} \) . In addition to the expected slip system another more important one, (12 \(\bar 1\) )[101], was found. The dislocations lying in (010) present a glissile dissociation. These observations have been discussed in term of the feldspar structure. Models for glissile dissociation in (010) are proposed: [100]=1/2[100]+1/2[100] or 1/2[101]+1/2[10 \(\bar 1\) ] and [101]=1/2[101]+1/2[101].     

Schiller effects and exsolution in sodium-rich plagioclases

Plagioclase feldspars with mean compositions Ab_91,3Or_4,7An_4,0 and Ab_88,7An_10,1Or_1,2 have been studied by transmission electron microscopy and electron diffraction. The substructure consists of thin lamellae of albite and oligoclase. Two types of orientations of the lamellar planes were observed. The orientation of the more common type was found to change from (08 \(\bar 1\) ) to about ( \(\bar 1\) , 21, \(\bar 2\) ) as a function of the mean potassium content. The plane of the other type was found to be near ( \(\bar 7\) 12). Only the first type of lamellae produces visible Schiller colours.     

Transmission electron microscopic study of experimentally deformed k-feldspar single crystals

A set of sanidine single crystals were previously deformed at 700° C in a Griggs triaxial press with different crystallographic orientations of the core so as to induce dislocation glide of different slip systems respectively. Deformed crystals have been studied by transmission electron microscopy (TEM) and the activated slip systems have been characterized for two orientations. (010)[001] and (001)1/2[ \(\overline 1 \) 10] systems expected for one orientation (main stress nearly parallel to [012]) are observed, whereas the (001)[100] system expected for the other orientation (main stress nearly parallel to [101]) is never observed. In the latter specimen the deformation is rather difficult and occurs through unexpected systems characterized as (110)1/2[1 \(\overline 1 \) 2] and (1 \(\overline 1 \) 1)1/2[110]. In all the samples studied the deformation is heterogeneous, exhibiting dislocation configurations related to temperature variations.     

A dynamical model for the I\bar 1 - P\bar 1 phase transition in anorthite,CaAl2Si2O8

The non-ferroic triclinic to triclinic \(I\bar 1 - P\bar 1\) phase transition in anorthite is described in terms of the spontaneous onset of an order parameter η. A triclinic to triclinic phase transition can be driven by order parameters (representations) arising from the Γ, Z, X, U, V, R, Y, and T points of symmetry of the Brillouin zone. Each point leads to a set of two inequivalent representations and thus there is a total of sixteen inequivalent order parameters. However, only the R ₁ ⁺ representation is consistent with the change from the body-centered to primitive cell (increase of primitive cell size of two) and also with the origin of the two space groups (inversion center) being at the same position. The R ₁ ⁺ order parameter of the high symmetry triclinic phase \(P\bar 1_0\) (or equivalently \(I\bar 1\) ) causes a reciprocal lattice change and, in terms of the lower symmetry reciprocal lattice, the order parameter corresponds to the b^* point. This is consistent with experimentally observed x-ray diffuse scattering. Using induced representation theory, microscopic distortions compatible with the R ₁ ⁺ order parameter are obtained. Assuming a distortion in an arbitrary direction at the general 2(i) Wyckoff position (x₀,y₀,z₀) of \(P\bar 1_0\) (the higher symmetry phase) induced representation theory demands an opposite displacement at the position (x₀, y₀, z₀), an opposite displacement at (x₀+1,y₀+1,z₀+1), and the same displacement at ( \(\bar x\) ₀+1, \(\bar y\) ₀+1, \(\bar z\) ₀+1) of \(P\bar 1_0\) . This is also consistent with experiment. The presence of the weak c-type reflections above the transition is attributed to the fluctuating lower symmetry antiphase domains related by the translation (1/2, 1/2, 1/2).     

Elastic constant systematics

This papers reviews elastic constant systematics. The bulk modulus of oxides and silicates is generally predictable on the basis of density and mean atomic weight. For constant mean atomic weight, \(\bar M\) , the bulk modulus is inversely proportional to the fourth power of the molar volume, regardless of whether molar volume changes due to temperature,T, pressure,P, or crystal structure. This iso- \(\bar M\) trend has the explanation that the Grüneisen parameter, (?K/?P) _T , and ?(1/αK)(?K/?T) _P , whereK is bulk modulus and α is volumetric thermal expansion, are approximately constant for most materials. For isostructural compounds, the bulk modulus is inversely proportional to the molar volume. This isostructural trend has the explanation that a certain combination of interatomic force parameters are the same for isostructural compounds. Equivalent iso- \(\bar M\) and isostructural trends are discussed for velocity versus density. Exceptions to the systematics exist.     

Geological similarity byQ analysis

Mineral resource assessment using areal, or unit regional, value approach estimates the variety of commodities likely to occur in a region by comparing its geology with that of regions already well developed and known to produce a variety of commodities. It is, therefore, essential to be able to discern regions which are similar in geology. The data used for this purpose are derived by point-counting geological maps of the regions to be evaluated; one aspect of this data is the presence-absence (i.e., 0–1 data)of 65 standardized rocktypes. It is then necessary to compare geological data from both the developed and undeveloped regions to determine which of the regions are geologically similar. The initial data consists of a matrix (A)of Cregions by Rrock types and all relations (λ)among Cand Rmay be expressed as λ ? C × R.We may then use Atkin's Qanalysis to determine the structure of these relations. Postmultiplying Aby its transpose and subtracting a suitably dimensioned unit matrix yields an output matrix K_C (R; λ)which expresses the relations among regions in terms of their rock types. This output matrix comprises qconnectivities among regions; its diagonal elements (denoted \(\hat q\) )are the number of rock types, minus one, in each region. The offdiagonal elements ( \(\check{q} \) )are the number of rock types (minus one)which are common to each pair. Similarities of regions in terms of their rock types are then found by using tables of equivalences in which the values of \(\hat q\) are the dimensions of simplicies representing each region; the rock types are the apicies of the simplicies and similarities are the shared edges and faces of the simplicies. The largest number of shared apicies equals \(\check{q} \) .Examples of the application of Qanalysis to a comparison of the geology of the 10 counties in New Hampshire and the 50 states of the U.S.A. and Puerto Rico illustrate the procedure. Qanalysis supplies an algebraic language and an equivalent geometry to express the relations among regions in terms of their rock types.     

Cation stacking faults in magnesium germanate spinel

Widely extended, cation stacking faults in experimentally deformed Mg₂GeO₄ spinel have been studied using transmission electron microscopy (TEM). The faults lie on {110} planes. The displacement vector is of the form \(\frac{1}{4}\left\langle {1\bar 10} \right\rangle \) and is normal to the fault plane. The partial dislocations which bound the stacking fault have colinear Burgers vectors of the form \(\frac{1}{4}\left\langle {1\bar 10} \right\rangle \) which are normal to the fault plane.     

High-pressure phase transition in MnTiO3 from the ilmenite to the LiNbO3 structure

Crystals of a high-pressure phase of MnTiO₃ have been synthesized at pressures of 60 kbar using the SAM-85 cubic-anvil high pressure apparatus. Although all crystals examined were twinned on (10 \(\bar 1\) \(\bar 2\) ), a set of diffraction intensities that are essentially unaffected by the twinning were obtained. Three possible structure models were considered: (1) the corundum (completely disordered Mn and Ti), (2) the partially-disordered ilmenite, and (3) the LiNbO₃ structures. The R factors of the corundum and the disordered ilmenite models were much larger than that of LiNbO₃. Using structure factors unaffected by twinning, the final LiNbO₃-type refinement gave R _w=0.037 and R=0.034. The averaged bond lengths for Mn-O and Ti-O were consistent with ones calculated using Shannon and Prewitt's (1969) radii. The study concludes that MnTiO₃ II actually has an ordered LiNbO₃-type structure rather than the disordered one as reported previously. From the analysis of the two MnTiO₃ structures, the transition can be related to a cation reordering process, in which half of the cations participate, accompanied by the rotation of oxygens to accommodate the cations.     

Coherent elastic energies of exsolution boundaries in peristerite and böggild intergrowths

Elastic energy calculations based upon the coherent model of Willaime and Brown [Acta Cryst. A30, 316–331 (1974)] have been carried out for some exsolution textures in peristerite and Böggild intergrowths. For peristerites it is demonstrated that substitution of K for Na moves the orientation of the exsolution lamellae from (08 \(\overline {\text{1}} \) ) to about ( \(\overline {\text{1}} \) , 21, \(\overline {\text{2}} \) ) in agreement with observations. An electron microscope study of exsolution textures in labradorite and andesine plagioclases has been carried out and information about small structural differences in these lamellar structures has been obtained from elastic energy calculations.