JULIEN, DETERMINATION OF MINERAL CONSTITUTION 137 



of chemical components; that process might be almost endless. It is 

 restricted to a careful discrimination of the probable proximate compounds, 

 i. e., simpler existing minerals, consistent with the physical and optical 

 characteristics possessed by the micro-aggregate. Furthermore, when the 

 associations of this aggregate and the probable conditions attending its 

 formation are knowTi, the identification of the constituent minerals may be 

 facilitated by restriction to the class of minerals developed in certain vein 

 or gangue formations or in a particular metamorphic zone: for example, 

 the constituents of the "diabantite" mixture to the series of minerals de- 

 veloped in the belt of weathering and there only. 



In a study of the hydrous silicates, almost completed, to which this 

 paper is a partial introduction, I have prepared a tabulated list to indicate 

 the possible mineral combinations which may logically be sought for in 

 micro-aggregates of this particular class. Taking for present examples in 

 illustration of these views the micro-aggregates of magnesian hydrosilicates 

 of the belt of weathering or decay — one of the groups of amorphous mix- 

 tures of the most difficult resolution — the following are some of the chief 

 indices for detection of the combinations in which the more common com- 

 ponents may occur. 



Silica in three forms : a) colloidal and soluble, in combination with a 

 large proportion of water, e. g., disilicic monohydrate, HjSijOg, containing 

 13.05 per cent, of water, or trisilicic dihydrate, H^SigOg, containing 16.67 

 per cent, of water; h) hyalite or opal, containing 2 to 13 per cent, of water 

 and insoluble, and (c) this, passing through various intermixtures, as semi- 

 opal, chalcedony, etc., into anhydrous and insoluble crystalline quartz. 



Alumina: a) where silica is scanty, as one of the two aluminum hydrates 

 (bauxite, gibbsite); h) with silica abundant, as a residual remnant of an 

 aluminous mineral (pyroxene, mica, feldspar, etc.) or as one of eight alu- 

 minum hydrosilicates (allophane, halloysite, talcosite, etc., but perhaps not 

 kaolinite) ; c) in presence of alkaline and earthy bases, as a newly formed 

 chlorite or zeolite (a restricted list, prochlorite, stilbite, natrolite, etc.). 



Ferric oxide: a) with silica scanty, as anhydrous oxide (hematite, but 

 never magnetite), or as one of the four ferric hydrates (limonite, limnite, 

 turgite, gothite); h) with silica abundant, as one of the three ferric hydro- 

 silicates (hisingerite, chloropal, anthosiderite), or as an aluminum-ferric 

 hydrosilicate (?). 



Ferrous oxide: a) commonly in replacement of magnesia, sometimes as 

 siderite or other carbonate; h) with silica abundant, as one of the two 

 ferrous hydrosilicates (ekmanite, chloropheite of Forchhammer) ; or (c) as 

 the aluminum-ferrous hydrosilicate (aphrosiderite). 



Manganese oxide: a) as manganous oxide (manganosite), sesquioxide 



