16 0. C. Harrington — Chemical Composition of Iolite. 



small percentage of water (0*50 per cent) found by Jackson in 

 one of bis analyses. All otber analyses thus far publisbed 

 sbow amounts of water between 1 and 2 - 5 per cent, the average 

 from six analyses being l - 74 per cent. 



The high temperature required to drive off the water shows 

 that it is practically all constitutional. If present as hydroxyl, 

 it is possible that it combines with Mg to form the univalent 

 radical (MgOH). The recent investigations by Clarke and 

 Schneider* seem to indicate that, if the above molecule is 

 present in a silicate, it can be decomposed by the action of dry 

 HC1 gas, so that an equivalent of MgCl 2 can be dissolved out 

 by water. An experiment, conducted to test this point, gave 

 no satisfactory results. About a gram of the mineral was 

 heated in a current of the dry gas for 8 hours and nearly con- 

 stant weight was attained. On leaching, however, with water 

 and a drop of nitric acid, only 0*14 per cent of MgO went into 

 solution, so that no definite conclusion could be drawn from 

 this result. 



On digesting a sample with strong aqueous HC1 for three 

 days, on the water bath, the following results were obtained : 



Ratio to per cent in 

 complete analysis. 



Undecomposed mineral 23 '20 



Si0 2 36-79 -75 



A1 2 3 with Fe 2 3 ._ 30-50 -78 



MgO with MnO 8-04 -77 



HO„ 1-84 



100-37 



From the above it is seen that about 76 per cent of the min- 

 eral had dissolved and since the different constituents were 

 about equally affected, it seemed probable that by longer treat- 

 ment the mineral could be completely decomposed. Accord- 

 ingly another sample was digested, on the water bath, for 

 fifteen days. The result showed the supposition to be correct, 

 since the insoluble residue was found to be 49"95 per cent or 

 very nearly the .percentage of SiO a in the mineral. The min- 

 eral therefore is completely decomposed by long treatment 

 with hydrochloric acid. 



In conclusion the author wishes to express his especial in- 

 debtedness to Prof. S. L. Penfield, for much valuable assistance 

 and advice rendered during the work. 



Mineralogical Laboratory, Sheffield Scientific School, 

 New Haven, May, 1891. 



* This Journal, III, xl, 303, Dec, 1890. 



