The Several Forms of Calcium Carbonate. 477 



considered. Meigerf* himself notes that barium or strontium 

 carbonate and precipitated basic magnesium carbonate give the 

 same indications as aragonite, dolomite the same as calcite. 

 Kreutzf mentions that cerussite (PbC0 3 , orthorhombic) and 

 barytocalcite (BaC0 3 .CaC0 3 , monoclinic) do not color in the 

 solution but alstonite ((Ba,0a)CO 3 , orthorhombic) does. Pane- 

 bianco^: has pointed out that mixtures of aragonite and calcite 

 containing as little as 5 per cent of the former are by this test 

 indistinguishable from aragonite. Hutchinson§ observed that 

 the presence of iron as an impurity masks the color. Skeatsjj 

 states that calcium phosphate gives a blue color resembling 

 that given by aragonite. We have found that the //. form of 

 calcium carbonate and the hydrated magnesium carbonate,^[ 

 nesquehonite, behave like aragonite. It is obvious, therefore, 

 that a considerable knowledge of the material tested is required 

 before the test becomes useful. 



In the case of test II we found that the color of the precip- 

 itate depends upon the concentration and C0 3 content of the 

 solutions ; in support of this a few experiments carried out 

 with equivalent solutions of FeS0 4 and Na 2 C0 3 may be cited : 

 O'Ol JY solutions of FeS0 4 and ]STa 2 C0 3 when mixed give a 

 green precipitate; 0*002 JV solutions give a green solution 

 from which a yellow precipitate slowly settles ; O'OOOl iV 

 solutions give a yellow solution, and in the course of a few 

 minutes a yellow precipitate. If the C0 2 pressure be raised, 

 or if bicarbonate be used instead of carbonate, the yellow pre- 

 cipitate begins to appear at a higher concentration of the 

 reagents ; thus with a much stronger solution of FeS0 4 a green 

 precipitate was not obtained until the concentration of KHCO, 

 was 0'03 JV, and even then it was yellowish-green at first. 



* W. Meigen, Neues Jahrb. Min., 1903, ii, 19. 



f S. Kreutz, Min. petr. Mith., xxviii, 48?, 1909. He considers that crystal 

 symmetry is the dominating factor, carbonates isomorphous with calcite or 

 aragonite giving either the corresponding reaction or none. But this view 

 is not, in our opinion, justified by the facts, for the lilac precipitate is not 

 oriented by the aragonite. 



X G. Panebianco, Rivista Min. Crist, ital. , xxviii, 5, 1902. 



§A. Hutchinson, Min. Mag., xiii, p. xxviii, 1903. 



!| E. W. Skeats, Bull. Mus. Comp. Zool., xlii, 53, 1903. 



IT The magnesium of the calcific hard parts of some (possibly of most) 

 organisms does not give the lilac color reaction and is, therefore, in all 

 probability present as anhydrous carbonate, either as dolomite or rnagnesite. 

 This deduction is based on tests made on the following material analyzed 

 by Clarke and Wheeler (Proc. Nat. Acad. Sci., i, 552, 1915, and later paperB) 

 and selected for us from specimens in the U. S. National Museum by Dr. 

 W. L. Schmitt. Optical examination showed this material, which contains 

 from 7 to 12 per cent magnesia reckoned as anhydrous carbonate, to be 

 fibrous calcite with principal refractions of 1*49 and l'65-l - 66. Slight 

 porosity probably accounts for the fact that the refraction is not significantly 

 changed by the magnesia. The forms tested are : Lepidisis carophyllia, 

 Corallium elatior, Tubipora purpurea, amongst the alcyonoids, and the alga 

 Goniolithon Acropetum, 



