126 Genth and Penfield — Zansfordite, Nesquehonite, etc. 



crystals, and among these the best, giving an angle of nearly 

 66°, which is about that obtained from the natural mineral. 

 One of the largest of the crystals served as a prism for 

 measuring the indices of refraction. The prismatic angle 

 measured 68° 20' ; the minimum deviation for yellow light, 

 soda flame, was found to be 46° 40' for rays vibrating 

 parallel to the vertical axis of the prism and 49° 40' for rays 

 vibrating parallel to the macro axis. Two of the indices of 

 refraction are therefore /?=1*501 and y= 1*526. The relation 

 of the axes of elasticity to the crystallographic is therefore the 



following d=za, b=c and c=h. The brachy-axis being the acute 

 bisectrix the double refraction is negative the same as in the 

 natural mineral. As a further proof of the identity of the 

 natural and artificial salts we find that the two show exactly 

 the same behavior in convergent polarized light ; one of the 

 little crystals lying on a prismatic face shows in convergent 

 polarized light one optical axis almost in the center of the 

 field, the base as the plane of the optical axes and a strong 

 double refraction. Making use of /3= 1*501 for yellow in the 

 determination of the divergence of the optical axes in the 

 natural mineral 2E = 84° 15', we find 2V=53° 5', from which 

 we see that the optical axes vary only about 6° from a normal 

 to the prismatic faces. Also having 2Y=53° 5', /9=1*501 and 

 f= 1*526 we can calculate a— 1*495. 



Owing to the ease with which this simple carbonate of 

 nfagnesia can be produced artificially in crystals and its sta- 

 bility, for the crystals do not lose water in the warm, dry air 

 of a laboratory nor in a desiccator over sulphuric acid, it would 

 seem to be a very natural compound to find in nature ; it is, 

 however, probable that its solubility, especially in water con- 

 taining carbon dioxide, would account for its absence in locali- 

 ties which would seem favorable for its formation. The 

 hydrated carbonates of magnesia which up to this time have 

 been found in nature are basic and are : 



Hydromagnesite 3MgC0 3 , Mg(OH) 2 , 3H 2 ■ 



Lansfordite 3MgCO s Mg(OH) 2 21H 2 



Hydrogiobertite MgC0 3 Mg(OH) 2 2H0 2 or Mg 2 C0 4 .3H 2 



The last of these is a mineral which has recently been de- 

 scribed by E. Scacchi* as occurring in dense spherulitic masses 

 from 2 to 15 mm diameter in a lava at Pollina, Sicily. The 

 formula written Mg 2 C0 4 3H 2 reminds one very much of our 

 mineral but requires 44 94 per cent MgO, while the nesque- 

 honite requires only 28*99 per cent. 



* Rendi. della R. Acad, delle. Sci. di Napoli 12, Die. 1885. 



