The structures of the plagioclase felspars: VI. Natural intermediate plagioclases

Yoderite, a new hydrous magnesium iron aluminosilicate from Mautia Hill, Tanganyika

Yoderite occurs as a major constituent in a quartz-kyanite-tale schist, and is a high-pressure phase formed by the reaction Ky + Tc → Yd + Qu. The monoclinic unit-cell, a 8·10 Å., b 5·78 Å., c 7·28 Å., β 106°, space-group P21 or P21m, contains approximately (Mg2·0Ca0·2Fe0·5Al5·3Si4·0O17·6(OH)2·4. The structure appears to be related to that of kyanite, with which the mineral is intergrown; it is not an analogue of staurolite. The three strongest powder lines are 3·50 Å. vvs, 3·03 Å. vs, 2·61 Å. s. Specific gravity 3·39. Subsidiary reflections, similar to those of the intermediate plagioclases, have indices with non-simple fractional values of k. Optical properties are α 1·689 pale Prussian blue, β 1·691 indigo, γ 1·715 light olivegreen, absorption β > α > γ, 2Vγ25°, optic axial plane {010}, γ:[001] ≈ 7° in the obtuse angle β. An interpretation of the colour on the electron-exchange hypothesis Fe2+ ⇌ Fe3+ + e−, is suggested. A kinetic study has been made of the disappearance of the subsidiary reflections in the range 700°C. to 820°C.; further heating to 850°C. produces the metastable appearance of mullite, and at temperatures above 1100°C. the equilibrium anhydrous assemblage indialite + sapphirine appears.

A further investigation of tobermorite from Loch Eynort, Scotland

The specimen described by Claringbull and Hey (1952) has been re-examined using electron-microscope, electron-diffraction, X-ray, and dehydration isobar methods. It differs from other specimens in several respects. The basal spacing does not shrink appreciably on dehydration. The principal cleavage is (100) and not (001). The body-centred pseudo-cell of normal dimensions, but the distribution of the weak reflections differs from that found for other specimens. The mechanism of conversion into wollastonite has also been examined.

The powder patterns and lattice parameters of plagioclase felspars. I. The soda-rich plagioclases

Geiger-counter powder records of soda-rich plagioclases have been carefully measured and indexed. The lattice parameters of the synthetic specimens are almost independent of the lime content, the largest variation (that in β*) amounting to only 0·2°. After strong heating the powder patterns of the natural specimens closely approached those of the synthetic specimens.

Eight out of the nine natural plagioclases gave lattice parameters which fall within experimental error on a continuous line. There is a fairly large variation in lattice parameters, especially for γ*, which changes by 2°. As the lime content increases the lattice parameters of the natural plagioclases approach those for the high-temperature synthetic and heated natural specimens until at An50 the separation is only one-fifth of that at An0. The ninth specimen (from a dacite) gives parameters midway between the curves, thus exhibiting a state of partial inversion.

The partially heated natural plagioclases and the plagioclase from the dacite give parameters indistinguishable within experimental error from the parameters of unheated plagioclases with a higher lime content. There is, therefore, no reliable powder X-ray method for the determination of composition in the region An0-An50. If the composition is known the powder method may be used for the determination of the thermal state. If the plagioclase can be judged to be in the low-temperature state from geological evidence the powder method can be used to estimate the composition with an accuracy of 2 % An.

Modulated structure of a plagioclase An52: Theory and structure determination

A modulated structure of an An52(Labrador, Canada) was determined according to an extended theory of satellites. The theory explains the whole extinction rules of satellite reflections of plagioclase feldspars. The superstructure, which is interpreted as the periodic antiphase domains, consists of two centrosymmetric structure elementsϱ1andϱ2with M2T8O16(M: Ca, Na and T: Al, Si),c∼ 7 Å. The structure-factors for main and satellite reflections are expressed asFmain∝F1+F2andFsat∝F1−F2, respectively, whereF1andF2are the Fourier transforms ofϱ1andϱ2. Based on the model,ϱ1andϱ2were refined. TheR-factor attained 8% for 4910 reflections (Ra= 6.6% for 1640areflections andRe= 18.2% for 3270ereflections), including unobserved reflections. Al/Si-distributions in the region of An100–An50are discussed using the results of the refinement.

Investigations on Bøggild intergrowth of intermediate plagioclase by high resolution transmission electron microscopy

Bøggild intergrowth of an intermediate plagioclase from Ylämma, Finland was investigated by high resolution transmission electron microscopy. A quantitative chemical analysis was carried out on the exsolution lamellae by means of an energy dispersive analyzer. The chemical compositions of the two alternating phases were determined by point analysis to Ab38.7An60.1Or1.2 and Ab51.0An46.8Or2.2 for the thicker and the thinner lamellae, respectively. The bulk chemical composition Ab42.0An56.5Or1.5 calculated from the thickness of lamellae was consistent with that determined by X-ray microprobe analysis except for Or contents. Continuous resolution of lattice fringes across exsolution lamellae shows matching of lattices, which is consistent with the compositional difference in the exsolution lamellae.

Diffraction and Mössbauer Studies of Minerals from Lunar Soils and Rocks

Constituents of lunar soils and rocks were studied by powder and singlecrystal x-ray diffraction. In addition to identification of minerals, including rare amphibole, mica, and aragonite, a detailed study of the important rock-forming minerals of the plagioclase, pyroxene, and olivine groups has begun. Mössbauer spectra were recorded from lunar soils, ground rock samples, and separates of iron-bearing minerals. The proportions of iron-bearing minerals were estimated from computer-fitted areas for the bulk samples. The Fe(2+) in the lower-density fraction of pyroxene was ordered, whereas that of the higher-density fraction was disordered.