The Several Forms of Calcium Carbonate. 489 



films of the mother liquor in which the material was precip- 

 itated. The aggregate volume of these films, which are mainly 

 water, is about 10 per cent of the whole, and causes a cor- 

 respondingly smaller value of the apparent density and refrac- 

 tive index ; this, together with the fact that well-crystallized 

 calcite does not hold water in solid solution, leads to the con- 

 clusion that the water (and other impurities) is merely en- 

 meshed in the very small interstices among the fibres making 

 up the spherules or other aggregates. The conclusion that these 

 aggregates are essentially calcite can not be considered as estab- 

 lished absolutely, by reason of the fact that the separate fibres 

 even are submicroscopic ; but it accords best with what we 

 know of the properties of these aggregates. There is one 

 aspect, however, of their behavior for which we are unable to 

 account ; namely, that after some of this material had been 

 shaken in a closed bottle for some weeks with water at 25°, it 

 contained a small fraction of one per cent of aragonite needles, 

 although the original aggregates contained no visible admixture 

 of aragonite. The direct inference from this would be that 

 these aggregates are unstable with respect to aragonite, and 

 hence can hardly be calcite since aragonite is unstable with 

 respect to calcite grains ; but this behavior may be only appar- 

 ently anomalous, for too much weight should not be attached 

 to this in view of the circumstances that the aggregates are 

 visibly not homogeneous initially and that the factors which 

 determine the appearance of unstable forms are unknown. 



2. Ktypeite. — The aggregated calcium carbonate of certain 

 pisolites has been considered by Lacroix* to be a distinct form. 

 The single definite difference from other forms upon which an 

 argument for a new form rests is the positive character of in- 

 terference figures obtained from thin sections of the pisolites. 

 If we knew how the minute crystals of the pisolite were 

 oriented we could better interpret the meaning of such inter- 

 ference figures. Lacroix does not consider this structure, but 

 he does suggest that the optical phenomena of the sections are 

 caused in part by strain. If we consider the only plausible 

 theory yet advanced — that of Sorbyf — for the structure of these 

 pisolitic grains, then only sections passing near the center of a 

 grain would give a definite interference figure in parallel 

 light, and that figure would be positive. In convergent light 

 no figure should be obtained from any section, but the positive 

 figure produced by the objective might not be obscured. 



Pisolite grains grown by the addition of plates of yw-CaCO, 

 to the surface would have positive optical character in thin 



* A. Lacroix, Cornpt. rend., cxxvi, 602, 1898. 



f Sorby conceived that the pisolites from Carlsbad — one of Lacroix's local- 

 ities — grow as if minute needles of aragonite were laid on tangential to the 

 surface. Quart. J. Geol. Soc, xxxv, 56, 1879. 



Am. Jour. Sci.— Fourth Series, Vol. XLI, No. 246.— June, 1916. 

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