486 Johnston, Merwiit, and Williamson — 



ature our products also transformed rapidly ; but it is an open 

 question how rapidly pure /ti-CaC0 3 , free from admixture of a 

 more stable form, would be changed. On the basis of the 

 chemical tests alone it would be indistinguishable from aragon- 

 ite, though easily differentiated by its density and positive 

 optical character.* In our experiments this form has been 

 more easily recognizable optically than calcite or aragonite 

 when the latter were in aggregates. 



This form has undoubtedly been present in the precipitates 

 of previous investigators, of Peine in particular, but has been 

 confused with aragonitef by reason of the fact that it gives 

 the same color tests ; to this circumstance are to be ascribed 

 statements as to the formation of aragonite under certain con- 

 ditions (e. g., in presence of magnesium salts at low tempera- 

 tures), statements which we have been unable to confirm in 

 spite of repeated trials. 



Other Reputed Forms of Calcium Carbonate. 



1. VaUrite. — Vater^: has described precipitates obtained 

 from series of diffusion experiments, usually in presence of an 

 alkaline solution containing barium ; these products were 

 fibrous aggregates, usually nearly spherical, with a density of 

 2'54, containing, however, some proportion of barium. He 

 considered this material to be a new modification of CaC0 3 ; 

 and it was subsequently called " vaterite " although there is no 

 record of its occurrence as a mineral and no proof that it was 

 a single definite crystalline species. Indeed, Yater observed 

 the presence of occasional lens-shaped aggregates and hexag- 

 onal plates, which, as is now evident from our work, are cer- 

 tainly identical with the/i-form described above. On repeating 

 some of Yater's experiments, we failed to obtain products 

 identical with his ; but we found later that such spherical 

 aggregates can be prepared quite readily by slow precipitation, 

 in an alkaline medium. 



The best practical way, according to our experience, is as 

 follows: dissolved grams KOH in 600 co water, and drop in 

 from the burettes equal volumes of Ol M solutions of CaCL, 

 and K 2 C0 3 at such a rate that about 30 cc of each solution is 

 added in an hour ; the stirring must not be too violent, and 

 care must be taken that no grains of calcite find their way into 



*Lacroix (Compt. rend., cxxvi, 602, 1898) observed positive interference 

 figures in thin sections of certain pisolites. He considered the mineral of 

 the pisolites to be a new form of calcium carbonate (see " Ktypeite"). 



fSee for instance, G. Linck, Neues Jahrb. Min., Beil. Bd., xvi, 495 (1903) ; 

 H. Warth, Centr. Min. Geol., 1902, 492; F. Cornu, Oesterr. Z. Berg. Hut- 

 tenw., lv, 596 ; J. Peine, Inaug. Diss. Jena, 1913. 



X H. Vater, Z. Kryst., xxvii, 477, 1897, and subsequent papers. 



