492 Johnston, Menvin, "and Williamson — 



age and other factors, its properties are to that extent indefi- 

 nite. 



Calcium Carbonate Hexahydrate. 



When calcium carbonate is precipitated at temperatures 

 below about 20°, the product is partly or wholly hydrated ; but 

 the evidence in the literature* is inconclusive since many of 

 the products analyzed were undoubtedly not homogeneous, but 

 merely more or less accidental mixtures. Consequently it did 

 not seem worth while to corroborate or refute all the state- 

 ments made. The hexahydrate is the only one whose exist- 

 ence is undoubted, and the only one which we encountered in 

 our experimental work. 



We prepared this substance by slow precipitation, as already 

 described, in presence of 2 grams KOH, the beaker being kept 

 surrounded by ice ; the product was filtered off rapidly on the 

 pump, washed with alcohol and ether and dried by suction for 

 a few minutes. The product obtained in this way is well crys- 

 tallized and homogeneous and is pure hexahydrate, as the fol- 

 lowing analysis shows : 2 grams when heated to 150° evolved 

 a vapor containing no C0 2 , yielding a residue of calcite (pure 

 in so far as could be told from microscopic examination) which 

 weighed 48-03 per centf of the original, the theoretical propor- 

 tion of CaC0 3 from CaC0 3 .6H 2 being 48-08 per cent. 



The hexahydrate prepared in this way consists of colorless 

 monoclinic crystals, microscopic but well formed ; in habit the 

 crystals vary from tables parallel to the base — similar to those 

 described and figured by Yetter — to stout prisms. A r etter's 

 crystallographic measurements were confirmed, but his orienta- 

 tion of the crystals does not suit the prismatic habit of our 

 preparations. The following orientation, in which the only 

 forms are the prism and the three pinacoids, is satisfactory : 

 a : I : c = 1*02 : 1 : ? ; 2E = 72° ± 3°. The interior angle on the 

 base between the front faces of the prism is 86° ± l c ; this 

 true prism angle is 89°. The refractive indices are : a = 1*460, 

 £ = 1-535, 7 = 1-545. 7 A for yellow = 17° =b 2° in the 

 obtuse angle /3, but there is noticeable dispersion of the bisec- 

 trices ; optic plane parallel to the side pinacoid ; — 2Y = 38°. 



Masses of crystals kept at room temperature under either 

 ether, benzol, or clove oil pass within a few hours into mixtures 

 of calcite and radiating or branching masses of rough crystals. 

 These latter had refractive indices about a = 1-530, 7 = 1*605, 

 if formed under ether, but the somewhat higher values, 



* A summary of which may be found in Doelter's " Handbuch der Miner- 

 alchemie," I, 356. 



f-Cf. F. Vetter, Z. Kryst., xlviii, 71, 1911. 



