Mr. F. Field on a Variety of the Mineral Cronstedite. 53 



Si0 2 . Fe 2 3 . PeO. MnO. MgO. H 2 0. 

 Steinmann... 22*83 29*08 31*44 3*43 3*25 10*70 



Damour 21*39 29*08 33*52 1*01 4*02 9*76 



Messrs. Maskelyne and Flight, in a valuable paper upon 

 certain Cornish and other minerals (vide ' Journal of the Che- 

 mical Society/ new series, vol. ix. p. 9), gave the results of 

 some analyses of specimens of Cronstedite from Cornwall, 

 handed them by Mr. Tailing ; but these, again, are not very 

 concordant. 



The first analysis of this mineral gave the following num- 

 bers :— Iron protoxide 36*307 



Iron peroxide 36*762 



Silicic acid 17*468 



Water 10*087 



Calcium oxide *087 



100*711 

 A second analysis, with a fresh and more carefully selected 

 material, gave the following percentages : — 



Iron protoxide 38*570 



Iron peroxide 32*752 



Silicic acid 18*546 



Water 10*132 



100*000 

 Mr. Tailing called my attention to an amorphous, dark leek- 

 green mineral, at times associated with Cronstedite, which 

 struck me as interesting, inasmuch as, although differing so 

 widely at first sight from the brilliant black of the latter, yet 

 had exactly the appearance and colour of the streak of Cron- 

 stedite after abrasion with a file or some hard mineral. 



Qualitative examination proved it to consist entirely of fer- 

 ric and ferrous oxides, silicic acid, and water. Its specific 

 gravity was 3, hardness about 2*5. On heating, water was 

 evolved, and the green powder rapidly passed into yellowish 

 brown. No traces of either magnesium- or manganic oxide 

 could be detected (as in the case of the Bohemian mineral) ; 

 and there was no evolution of carbonic acid on the addition of 

 weak hydrochloric acid, by which it is instantly decomposed 

 with separation of silica and solution of the two iron oxides. 

 A quantitative analysis yielded: — 



Ferrous oxide 39*46 



Ferric oxide 18*51 



Silicic acid 31*72 



Water 11*02 



100*71 



