i9o6.] SMITH— PARAGENESIS OF MINERALS. 197 



have been made from other locahties, but the physical properties 

 are so striking that the identification is certain. 



These analyses show that lawsonite is essentially a silicate of 

 alumina and lime with two molecules of water, CaAUSigOg + 2H0O ; 

 the iron, magnesia, alkalies and titanium may be neglected as 

 probably coming from inclusions, and the mineral may be regarded 

 as a metamorphic feldspathoid of the chemical nature of anorthite 

 with the addition of water. 



Lawsonite is known only as a product of metamorphism, and it 

 is doubtful if a silicate with such a high percentage of water, over 

 II per cent., could originate as a product of magmatic crystalliza- 

 tion. It occurs only in rocks that have been more or less recrystal- 

 lized; these are invariably rich in lime, and hence usually basic, 

 though not necessarily so. It was described first from an eclogite 

 rich in garnet, actinolite, glaucophane and margarite, and in nearly 

 all the other rocks where it has been found glaucophane, or some 

 other of the soda-bearing amphiboles, is a common associate, and 

 albite occurs very commonly. On the other hand, garnet and epi- 

 dote, which are abundant in most of the glaucophane-bearing rocks, 

 are usually either lacking or scarce in the rocks with lawsonite. 



The source of the lime for the lawsonite must have been the 

 plagioclases which were abundant in most of the igneous rocks, 

 the arkoses, and tufifs out of which the lawsonite rocks were made. 

 The plagioclases were the first minerals to be disintegrated in the 

 chemical readjustment that accompanied metamorphism. The albite 

 molecule usually joined itself to pyroxenes or hornblende, forming 

 glaucophane or some other member of the glaucophane group, and 

 if there was an excess of sodium-aluminum silicate, it crystallized 

 out as albite. If iron, alumina, and magnesia were in excess, the 

 anorthite portion of the plagioclase molecule joined itself to them, 

 forming garnet and epidote ; but if there was an excess of lime, the 

 anorthite molecule simply took up water and formed lawsonite. 

 These reactions explain clearly why glaucophane is an almost in- 

 variable companion of lawsonite, why albite is a common associate, 

 and why garnet and epidote do not usually occur in considerable 

 quantity in lawsonite rocks. 



Lawsonite occurs in glaucophane-bearing eclogites, in green- 



