312 A TREATISE ON METAMORPH1SM. 



As is well known, the scapolite group is analogous to the plagioclase 

 group, both consisting of sodium-aluminum-silicate molecules and calcium- 

 aluminum-silicate molecules in various proportions. Wernerite is a 

 combination of the marialite and meionite molecules in various ratios. 

 Generally the ratios vary between 2 : 1 to 1 : 3. 



occurrence. — Dana summarizes the occurrence of the scapolites as follows: 

 "(1) in volcanic rocks, as in ejected masses on Mte. Somma (meionite); 

 (2) in crystalline limestone, often as the direct result of. contact meta- 

 morphism; (3) crystalline schists, augite-gneiss, etc.; (4) as an alteration 

 product of a plagioclase feldspar, sometimes on an extensive scale, as with 

 amphibole." " 



Alterations. — Dana states that the scapolites are readily alterable. The 

 more common products of alteration are kaolin (monoclinic; sp. gr. 2.6-2.63), 

 talc (orthorhombic or monoclinic; sp. gr. 2.7-2.8), muscovite (hydromusco- 

 vite, pinite) (monoclinic; sp. gr. 2.76-3.0), and epidote (the Al and Fe 

 varying from 6:1 to 3:2; monoclinic; sp. gr. 3.25-3.50). It is also recorded 

 that the scapolites alter into biotite (monoclinic; sp. gr. 2.7-3.1). Accom- 

 panying various of these alteration products quartz (rhombohedral ; sp. gr 

 2.653-2.654) separates. Also, it is probable that in connection with some 

 of them, gibbsite (monoclinic; sp. gr 2.3-2.4) or diaspore (orthorhombic; 

 sp. gr. 3.3-3.5) forms, and very likely also calcite (rhombohedral; sp. gr. 

 2.713-2.714). 



In writing out equations for the alterations to the above minerals, one 

 is handicapped by lack of knowledge as to whether the marialite or the 

 meionite, or a combination of the two, produces a given mineral. In this 

 state of affairs the particular molecule is chosen which is most analogous to 

 the compound produced. It seems probable that kaolin and talc together 

 are produced from marialite, according to the reaction : 



(1) 2Na 4 Al 3 Si 9 24 Cl+9MgC0 3 -T-9H 2 = 



3H 4 Al 2 Si 2 9 +3H 2 Mg 3 Si 4 12 +3Na 2 C0 3 +2NaCl+6C0 2 +k. 



The increase of volume of the kaolin and talc, as compared with the 

 marialite, is 7.69 per cent. 



It may be that kaolin and calcite are also produced from meionite, as 

 follows: 



(2) Ca 4 Al 6 Si 6 2 5+6H 2 0+4C0 2 =3H 1 A] 2 Si 2 O i) -|-4CaC0 3 +k. 



« Dana, J. D., A system of mineralogy; Descriptive mineralogy, by E. S. Dana, Wiley & Sons, New 

 York, 6th ed., 1892, p. 467. 



