JULIEN, DETERMINATION OF MINERAL CONSTITUTION 139 



It has been already intimated that one result of loose and vague mis- 

 apprehension of the essential and non-essential chemical components of a 

 crystallized mineral, or of the predominant mineral in a micro-aggregate, 

 appears to have been that the limitations in the laws of replacement in the 

 composition of a mineral have not always been clearly recognized; inclo- 

 sures have been mistaken for replacements. For example, in the two basic 

 magnesium hydrosilicates, deweylite and antigorite, magnesia may be 

 replaced by ferrous oxide, by manganous oxide, and probably by lime, but 

 never by metallic oxides. 



Recast Analyses of Micro-aggregates. 



A few examples of micro-aggregates, taken from my notes on minerals 

 of the magnesian hydrosilicate group, are presented below. They have 

 been selected to illustrate a variety of mineral constituents, identified in 

 these mixtures by this simple method, in contrast with the chemical formulas 

 on which the present acceptance of these mixtures as possible or certainly 

 independent minerals has been largely founded. 



Fibrous "diabantachronnyn." 



From Grafenwart, Voigtland. Analysis by Liebe, with the formula — 

 RO. SiOj + Mg(OH)2. 



In this calculation the alumina content is taken as the basis for estimating 

 the chlorite (or very likely a mixture of chlorites); the remainder of silica 

 for that of chrysotile, distinguished by Liebe under the microscope; the 

 remainder of the water for that of the fibrous magnesium hydrate, which, 

 as has been pointed out in a previous paper,^ has not been hitherto dis- 

 criminated from chrysotile in optical mineralogy. 



1 Annals N. Y. Acad. Sci., XVI, 410-411. 1906. 



