110 A. F. Rogers — Lawsonite from California. 



sheet. At this place Mr. R. Y. Anderson found a large 

 bowlder of a chlorite-lawsonite rock. The lawsonite occurs in 

 well-defined crystals of rhombic tabular habit (fig. 7) with the 

 forms c{001}, mjllOj, and occasionally 5 {010}. The crystals 

 appear almost black on account of the inclusions of dark green 

 chlorite, but on weathered specimens the lawsonite is opaque 

 white and stands out in bold relief. There is distinct cleavage 

 parallel to £{010} and to m{110}. Some of the basal sections 

 show fine polysynthetic twin-lamellse with {110} as twin-plane. 

 In addition to lawsonite "and chlorite this rock contains mus- 

 covite and titanite and also a little albite in veins. An analy- 

 sis of the rock was made by Mr. H. F. Humphrey, formerly 

 assistant in mineralogy at Stanford University, with the fol- 

 lowing results : 







Titanite 



Lawsonite 



Muscovite 



Chlorite 



Si0 2 



38-12 



1*02 



14-41 



10-99 



11-70 



ai 2 o 3 



25-81 







12-54 



9*43 



4-04 



Fe 2 3 



0-13 

















FeO 



9-86 













9-86 



MgO 



6-50 















6-50 



CaO 



7*62 



0-93 



6*69 









Na 2 



0-50 







0-50 







K n O 



2-11 





,_. 



2-11 





H,0- 



0-11 



_ 















H 2 + 



8-16 







4-29 



1-09 



2-78 



Ti0 2 



1-35 



1-35 















100-27 3-36 37-93 2402 34'88 



The percentages of the mineral may be calculated from the 

 chemical analysis. All of the titanium oxide and an equiva- 

 lent amount of lime go to make the titanite. The remainder 

 of the lime goes to the lawsonite. All the potash and soda go 

 to the muscovite, which may be assumed to be H 2 (K,Na)Al 3 

 (Si0 4 ) 3 . All the magnesia and ferrous oxide, and the remain- 

 der of the silica, alumina, and water belong to the chlorite. 



Recalculating to 100 per cent, we have the following per- 

 centage composition of the chlorite : 



Si0 2 _ 33-5 



Al o 3 11-6 



FeO 28-3 



MgO 18-6 



HO 8-0 



100-0 



This is close to an analysis of diabantite from Farmington, 

 Conn., analyzed by Hawes.* The chlorite in this rock, it 



* Dana, System of Mineralogy, 6th edition, p. 659. 



