Carnegie Institution of Washington Publication, Issue 31

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Carnegie Institution of Washington, 1905 - Science
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Page 56 - Allen 2 on the feldspars, already cited, has direct relation to the above : There is another error to which accurate specific-gravity determinations upon powdered minerals will be subject unless suitable precaution is taken. The exposure to the air during the period of grinding the samples gives opportunity for the condensation of sufficient atmospheric moisture upon the grains to affect the weight in air. The amount varies measurably with the size of the grains, as will be seen from the accompanying...
Page 61 - Kuster perfect isomorphism, and he formulated the "Rule" which has since borne his name, that the solidifying point of an isomorphous mixture lies on a straight line joining the melting points of the components and can be calculated from the percentage composition of the mixture. If this line proved to be slightly concave or convex, as it did in most cases, imperfect isomorphism was assigned as the cause. To this rule an * Lane, Journal of Geology, xn, 2, p.
Page 65 - The components are miscible in all proportions from o to 1oo per cent in both solid and liquid phases. (2) Miscibility is limited to certain concentrations. (3) More than one type of crystal occurs. In the feldspars we are concerned with the first class only, but here also Roozeboom distinguishes three possible types: Type I. — -Melting (or solidifying) points of the mixtures lie on a continuous curve joining the melting points of the components and containing neither maximum nor minimum. Type...
Page 50 - ... at the close. Furthermore the extreme viscosity, of which further evidence will be given directly, and the absence of any disturbance in the orientation of the particles indicating flow, assured us that the lanes of glass represented actual melting and not an inflow of glass from without. Finally, the perfectly homogeneous character of the glass and the unchanged appearance of the crystals as heating progressed, gave no hint of any chemical decomposition. In the hope of obtaining a point of value...
Page 51 - X 1 mm.) of albite and one of microcline were chipped from larger portions, spanned across small empty platinum crucibles, and placed side by side in the furnace. These exposed crystals were heated to 1225° for three hours. When removed they were completely amorphous (melted), but retained their position with hardly a trace of sagging. After this a number of similar slivers were prepared, mounted in the same way, and heated to temperatures of from 1 2oo° to 1 3oo° for a few moments.
Page 57 - AIR. < = finer than. > = coarser than. In the last two groups, note that the moisture in graded portions of the same sample varies with the fineness. We also verified the conclusion of Bunsen...
Page 60 - ... phenomenon is altogether gradual, even with a crystalline feldspar containing only a small percentage of glass. We purpose to extend these observation." to other substances. , CONCLUSIONS. It now remains for us to gather the results together and to draw such conclusions as they appear to justify. (1) If the melting points are now plotted in a system of which they form the ordinates, while the percentage compositions of the different feldspars form the abscissas (fig. 14), we discover, within...
Page 63 - Although the chemical reactions of albite and anorthitc are not of such a character as to prove or disprove a close analogy between them, a comparison of their formulas certainly does not suggest an isomorphous relation. If their formula weights represent true molecules, they possess the same number of atoms to the molecule (NaAl Si3Og, CaAl2 Si2O8)and the group Si2O8 in common, but the remaining atoms taken separately are not mutually replaceable.
Page 43 - CURVES. (In microvolts as observed). The numbers represent the electromotive force of the thermoelements at intervals of one minute, together with a column of differences at the right of each record. The EMF will be seen to approach a minimum as melting progresses and to increase again when it is complete. This minimum rise in the temperature, of course, indicates the maximum absorption of heat. For purposes of rough orientation 1o MV may be considered equivalent to one degree. An.
Page 57 - It is worth noting in this connection that these measured quantities of adsorbed water are of the same order of magnitude as those usually obtained for the water content in feldspar analyses,! where again, of course, the finer the sample is ground for the analysis the greater the possible error from this cause. It may be that a part and occasionally all of the moisture...

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