ALTERATIONS OF HORNBLENDE. 289 



form of serpentine by supposing that the number of actinolite molecules is 

 greater than given in the above equations and that such excess of these 

 molecules passes into bastite, according to equation (3). 



While the more common alterations of hornblende are to chlorite, 

 biotite, epidote, and accompanying minerals, as above explained, the change 

 of hornblende into augite, just the reverse of that of augite into hornblende 

 described on pages 274-278, does take place, and probably on a great scale 

 at sufficient depth. The equation for one case may therefore be written: 



(12) Ca 2 Mg 3 Fe 3 Si s 2t . (MgFe),(AlFe) 4 Si,0 12 +2CaC0 3 = 



2 [Ca,MgFeSi 4 12 . ( MgFe ) ( AlFe) ,Si0 6 ] + FeC0 3 + MgC0 3 + k. 



If the Mg : Fe : : 2 : 1, and the Al : Fe : : 2 : 1, the equation is — 



(13 ) Ca 2 Mg 3 Fe 3 Si s 24 .Mg 4 Fe 2 Al 8 Fe 4 Si 6 36 +2CaC0 3 = 



2[Ca 2 MgFeSi 4 12 .Mg 2 FeAl 4 Fe 2 Si 3 18 ]+FeC0 3 +MgC0 3 +k. 



The decrease in volume of the augite as compared with the amphibole is 

 4.13 per cent. 



It is not supposed that the above equations for the alteration of horn- 

 blende necessarily represent the actual facts of specific cases. Doubtless in 

 most instances materials from minerals aside from those given enter into 

 the alterations, and the actual changes are more complex than represented. 

 However, the equations very clearly show why it is that the production of 

 chlorite from hornblende demands also the production of other minerals 

 which Lacroix says so generally accompany chlorite. Also, they show why 

 epidote so frequently accompanies biotite secondary to hornblende. The 

 equations may be considered as average cases, which approximate to the 

 alterations that actually occur in man)- instances. The volume relations 

 calculated from the equations also are probably averages, for the proportions 

 of the elements taken in the equations given are chosen from a considera- 

 tion of analyses of the various minerals. The equations at least make a 

 quantitative estimate of the relations of the original and secondary minerals, 

 and therefore will lead to closer observations as to the minerals which 

 result from the alteration of hornblende, and their relative jjroportions. 



The change of arfvedsonite into acmite is so uncertain in its character 

 that no attempt is made to write out the equations. In order to satisfactorily 

 write equations for this alteration it is necessary to know the composi- 

 tion of the particular arfvedsonite which changes into the particular acmite, 

 and what other minerals aside from the acmite are produced in the change. 

 mon xlvii — 04 19 



