Summary 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 An 50 the separation is only one-fifth of that at An 0 . 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 An 0 -An 50 . 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.
The new zeolite boggsite occurs as small, colorless-to-white, zoned hemispheres embedded in drusy tschernichite in small vesicles in dense Eocene basalt near Goble, Columbia County, Oregon, where it formed through the action of local hydrothermal activity. The mineral is orthorhombic, space gtoup Imma, with 4 : 20.21(2), b : 23.77(l), and c : 12.80(l)A.ttrestrongestX-raydiffractionlinesare(inA) tt.f(tOO),4.43(70),3.86(80), 3.37(100). The chemical composition is near (CarrNar.rKorMgo')(Feo,Al,83sii76)Ore2' 70H,O, with Z : l, Si/Al : 4.24.Ir is optically biaxial (-), with a : 1.480(l), 0 : 1 .481 ( l ) , and7 :1 .487 ( l ) ,whe reX :c ,Y :a ,Z :b .Thedens i t y i s1 .98 ( l ) g / cm3(obs ) , 1.99 g/cm3 (calc). Tiny blades of boggsite with the forms {01l}, {102}, {010}, {100}, and {001} elongated along the b axis have a morphology similar to that of thomsonite. The name honors Robert M. Boggs of Seattle, Washington, and Russell C. Boggs of Cheney, Washington.
Summary Cell dimensions have been measured from correctly indexed powder patterns of thirty natural and two synthetic orthopyroxenes. The natural specimens (mostly metamorphic) contain exsolved phases largely avoided during microprobe chemical analysis. Linear regression analysis between the cell dimensions and Mg, Al, Ca contents (ignoring minor elements) was quite unsatisfactory until a term in Mg 2 was added. Two separate regression analyses for the ranges Mg 0–0·5 and 0·5–1 using only Mg, Al, and Ca were satisfactory. Olivine yielded satisfactory regressions for the whole range without a term in Mg 2 (Louisnathan and Smith, 1968). The difference between olivine and pyroxene results from absence of site preference by Mg and Fe in olivine compared to strong preference in pyroxene revealed by electron density and Mössbauer studies (Bancroft, Burns, and Howie, 1967; Ghose and Hafner, 1967). The data were recalculated by means of a new best-fit procedure developed by Hey in which the errors in the chemical and physical parameters are considered simultaneously. Earlier measurements of a and b for plutonic pyroxenes are consistent within possible experimental uncertainties with those given here; however a and b data obtained by Kuno and by Hess on volcanic specimens are considerably higher by variable amounts. Although there are uncertainties in the Ca content and its effect on a and b , the larger dimensions probably result from lower site preference as indicated by Mössbauer studies. Prediction of Mg, Ca, and Al from just the cell dimensions is only moderately accurate even for metamorphic orthopyroxenes.
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