264 A TREATISE ON METAMORPHISM. 



aluminum is to the iron as 2 to 1 , that the iron is derived from hematite, and 

 that excess of aluminum separates as gibbsite, the reaction for anorthite is: 



(18) 4CaAl 2 Si.,0 8 +Fe 2 3 +6H 2 0=H 2 Ca i Al 4 Fe 2 Si 6 26 +H 4 Al 2 Si 2 9 +2Al(OH) 3 +k, 

 or 



(19) 4CaAl 2 Si,0 8 +7H 2 0+FeA=H 2 Ca 4 Al 4 Fe 2 Si 6 26 +4Al(OH) 3 +2Si0 2 +k. 



The increase of the volume of the epidote, kaolin, and gibbsite as compared 

 with the anorthite and hematite, equation (18), is 3.6 per cent. The 

 increase in volume of the epidote, gibbsite, and quartz as compared with 

 the anorthite and hematite, equation (19), is 6.57 per cent. 



The scapolites include marialite (tetragonal; sp. gr. 2.566), meionite 

 (tetragonal; sp. gr. 2.70-2.74), and various isomorphous mixtures. (See 

 pp. 311-312.) The alterations of the plag'ioclases into these two minerals 

 are given. From these equations those for any definite isomorphous mix- 

 ture of marialite and meionite can easily be formulated. According to 

 Clarke," albite changes into marialite, and anorthite into meionite. The 

 reactions may be written as follows : 



(20) 3NaAlSi 3 8 +NaCl=Na 4 A]3Si 9 24 Cl+k. 



Supposing the NaCI to be in solution, the increase in volume of the solid 

 compound is 10.29 per cent. If the NaCI be supposed to be a solid, the 

 increase in volume of the solid compound is 1.84 per cent, The change 

 from anorthite to meionite is represented by the following reaction: 



(21) 3CaAl 2 Si 2 8 +CaCO.,=Ca 4 Al 6 Si 6 23 4-C0 2 +k. 



If the calcium carbonate be supposed to be present as a solid, the decrease 

 in volume is 3.78 per cent; if it be supposed to be added in solution, the 

 increase in volume is 7.87 per cent, 



Becke records the alteration of orthoclase and plagioclase into albite 

 (triclinic; sp. gr. 2.635), zoisite (orthorhombic ; sp. gr. 3.31), muscovite 

 (monoclinic; sp. gr. 2.88), and quartz (rhombohedral; sp. gr. 2.6535). 

 His equation for this reaction is as follows : b 



( 22 ) x ( NaAlSi 3 O s ) +4 (CaAl 2 Si 2 8 ) +KAlSi 3 8 +2H 2 = 



x ( NaAlSi 3 8 ) +2( HCa 2 Al 3 Si 3 13 ) +H 2 KAl 3 Si 3 1 ,+2Si0 2 . 



It is impracticable at the present state of knowledge to write reactions 

 representing the changes of the less siliceous plagioclases iuto more sili- 

 ceous plagioclases and other minerals, because the exact compositions of 

 the original and resultant minerals are not known. The alterations of the 



« Clarke, F. W., The constitution of the silicates: Bull. U. S. Geol. Survey No. 125, 1895, p. 29. 

 6 Becke, F., Ueber Beziehungen zvrischen Dynamometamorphose und Molecular volumen: Neues 

 Jahrbuch fur Mineralogie, etc., vol. 2, 1896, p. 1S2. 



