Half Unit Cell Research Articles

Iron-sepiolite from the Seikan Tunnel, Japan.

A dark-brown scaly material was found in the pilot tunnel of the Seikan Tunnel. Our mineralogical work has clarified that this material consists largely of iron-sepiolite. The structural formula calculated on the basis of 32 oxygens in the half-unit cell of dehydrated sepiolite is as follows. (Si11.49Ti0.03Al0.38Fe3+0.10)12.00(Fe3+1.25Fe2+0.26Mn0.02Mg6.00)7.53O32•Ca0.03Na0.05K0.04 If the Fe ions are treated as being wholly Fe2+, the following formula will be obtained on the basis of O30(OH)4. (Si11.74Ti0.03Al0.23)12.00(Al0.16Fe2+1.65Mn0.02Mg6.13)7.95O30(OH)4•Ca0.03Na0.05K0.04 This modified formula coincides well with the theoretical one in respect to the numbers of octahedral and tetrahedral cations. Therefore it may be suggested that this sepiolite has been formed primarily as a ferrous-sepiolite and then oxidized in the air-exposed condition. In addition, mineralogical data such as X-ray diffraction, thermal analysis, etc., are presented.

Glauconite and celadonite: two separate mineral species

SummaryAnalyses of glauconites and celadonites from continental sedimentary rocks and sea-floor basalts using X-ray diffraction, electron probe microanalysis, infra-red spectroscopy, and X-ray fluorescence spectroscopy are reported. The minerals are shown to be distinct species; each an isomorphous replacement series, glauconite having an average half unit cell of K0·85(Fe3+, Al3+)1·34 (Mg2+, Fe2+)0·66(Si3·76Al0·24)O10(OH)2 whereas celadonite approaches the ideal half unit cell of K(Fe3+, Al3+)(Mg2+, Fe2+)Si4O10(OH)2. Considerable Fe3+Alvi interchangeability occurs in the octahedral layer in both minerals and considerable substitution of aluminium in the tetrahedral layer of glauconites results in the more disordered 1Md type of structure compared with the more highly ordered 1M structure of celadonites. Some mixed layer glauconite-smectites and celadonites were also examined and could be distinguished from true glauconites and celadonites by chemical analysis, XRD, and IR techniques. It is proposed that the terms ‘glauconite’ and ‘celadonite’ should be used only for those minerals containing less than 5% interlayering.

The elastic accommodation of semicoherent θ′ in Al4wt.%Cu alloy

An electron microscopical investigation of the growth and elastic accommodation of θ′ plates is described for specimens aged above the θ′ solvus. It is concluded that both vacancy and interstitial plates can form as defined by the plates' residual misfit in the C direction. This misfit is not constant during particle thickening but is subject to periodic variations in magnitude and sign, even before accommodation of the residual misfit of the thicker plates by the nucleation or capture of dislocations. It is suggested that in general the system's flexibility in being able to use different multiples of half unit cells of the θ′ structure allows the elastic misfit to be minimised for a given plate thickness.

Unit cell step structures on vapour-grown Cr 2S 3 single crystals

Step structures on the (001) plane of as-grown crystalline Cr 2S 3 that had been prepared by the chemical vapour deposition method have been observed by interference contrast microscopy. Two kinds of steps, with heights of 10.1 and 5.1 Å were found. These are respectively due to the unit cell and a half of the unit cell of the crystal. Unit cell steps were seen to occur successively from a screw dislocation with Burgers vector normal to the surface equal to half of a unit cell. The generated steps with height equal to a half of the unit cell coagulated soon into a step with height of one unit cell. By passing other screw dislocations on the surface the unit cell steps combine with them just as expected from the screw dislocation model for monomolecular steps. The unit cell steps split spontaneously into half unit cell steps forming a fork-like step structure. The successively drawn out forking structure weaves an interlaced step pattern on the surface. However, the two interlaced steps annihilate when they meet.

The crystal chemistry of stilbites and stellerites

A total of 65 samples of stilbite and stellerite have been chemically analyzed and X-rayed. By using F-unit cell also for monoclinic stilbites, the lattice constants ranges are : a = 13.570-13.605 Â, b = 18. 198-18.270 Â, c = 17.823-17.863 Â for stellerites, and a = 13.595-13.657 À, b = 18. 197-18.309 Â, c = 17.775-17.842 Â, β = 90. 06-90.91° for stilbites. Most samples have a half-unit cell content of 4 Ca atoms ; if no other cations are present, the mineral is an orthorhombic stellerite ; if some (Na + K) is present, the mineral is a monoclinic stilbite. The higher the contents in alkalis, the higher the deviation of β from 90°. Four atypical samples have Ca remarkably lower than 4 ; these samples are interpreted as cation-exchanged. The sum of (Si + Al) is constant, and neither cation-deficiency nor cation-excess was detected. There is no gap between sterellites and stilbites. Many samples resulted heterogeneous.

Electron Probe Microanalyses of Chlorites from Low Grade Metamorphic Rocks of Tennant Creek Area Central Australia

Abstract Electron probe microanalyses of 47 chlorites from the Lower Proterozoic rocks, , Porphyroidal' rocks and from magnetite lode structures of Tennant Creek field are presented. Recrystallisation of chlorite, progressive in Warramunga rocks and retrogressive in 'Porphyroidal' rocks was contemporaneous and took place under lower greenschist facies conditions during regional deformation. Chlorite in the magnetite lode structures had formed ahead of the sulphide mineralisation in the Warramunga rocks subsequent to regional metamorphism and was due to metasomatic alteration of the wall rocks. Chlorite occurs with muscovite in all the three rock groups studied. Structurally, the majority of the analysed chlorites have an octahedral cation total short of the stipulated 6.0 per half unit cell. This deficiency is systematically related to the difference between AlvI and AIIV; the former is half the latter and confirms Foster's (1962) conclusion. The generalised relationship is (R3+ - AlIV) : (6.0 - AlIV- R2+) : : 2 : 3. Most of the analysed chlorites have AlvI greater than AlIV and the value of 1.5 reported here for (AlvI - AlIV) may be the limit in natural chlorites. The majority of the analysed chlorites fall on the composition field of ripidolite or brunsvigite or diabantite. The occurrence of diabantite variety in the Tennant Creek field is the first report of its kind from any low grade metamorphic terrain.

Palygorskite from Warren Quarry, Enderby, Leicestershire, England

AbstractSpecimens of palygorskite from Warren Quarry, Enderby, Leicestershire, have been studied in the laboratories. The mineral is found in dioritic rocks in association with angular dolomitic rock fragments as the filling material of a joint, the walls of which are sometimes lined with calcite and dolomite. Palygorskite occurs as 3-10 mm long bundles of fibres, 0·03-0·1μm wide. X-ray diffraction data indicate that the mineral may occur in the monoclinic form. The structural formula for the half-unit cell is Si8(Al1.89Fe0.053+Ti0.01Mg1.99)O20(OH)2(OH2)43·61 H2O.Ca0.07. Data obtained from differential thermal and infrared analyses are comparable with those for palygorskite in the literature, but the six-sided transversal cross-section of the fibres as revealed by electron microscopy is unexpected. The mineral was formed after the deposition of the overlying basal Triassic rocks when ground waters penetrated cracks in the dioritic rocks, causing Si and Mg to be released there and above the unconformity.

A Soil Beidellite

AbstractX‐ray diffraction analysis of the < 2 µ fraction of the main horizons of a Deer Park sand, a Podzol soil in northern Michigan, shows that montmorillonite is the dominant layer silicate in the A2 horizon of this soil. This montmorillonite appears to have weathered from illite and mica and chlorite interstratified with vermiculite and montmorillonite which are present in the underlying horizons. Chemical and X‐ray data indicate that the montmorillonite in the fine fraction (< 0.2 µ) of the A2 horizon contains some randomly interstratified mica. The Greene‐Kelly test, X‐ray powder diffraction, differential thermal, infrared, and chemical data show that the expanding component of this fine fraction is a beidellite of formula [Al1.62Fe0.18Mg0.28][Al0.45Si2.55]O10[OH]2 with exchange capacity = 0.49 M+ per half unit cell. This beidellite has a high layer charge, most of which arises from tetrahedral substitution, and shows a strong tendency to fix potassium.