MAGNETITE GARNET ROCKS COUL80N. 295 



rock. The commonest alteration of the garnet is the production of chlorite, quartz, 

 and magnetite which is especially well displayed in the chlorite magnetite schist 

 No. 527A. The reaction can be illustrated by the equation 



2 MgFe 3 AlSi 3 O, 2 + 4 H 2 ; HgMgaFegAljSigO,,, + 2 Si0 2 + Fe 3 4 . 



Garnet. CUorite. Quartz. Magnetite. 



The two arrows going in opposite directions means that the equation representing 

 a reaction is reversible. 



In this composition of the garnet the ratio of Mg to Fe is assumed to be as 1 to 2, 

 and the ratio of Fe to Al as 1 to 1 . The chlorite is also assumed to be an isomorphous 

 mixture of the two. silicate molecules, H 4 (MgFe) 3 Si 2 9 and H 4 (MgFe) 2 Al 2 Si0 9 in the 

 proportions of 1 to 1. Chemical work has indicated, however, that the garnet 

 contains manganese in many cases where this change is observed. On alteration, 

 it is probable that the manganese enters the composition of magnetite rather than the 

 chlorite, producing manganmagnetite, the composition of which is expressed by 

 (FeMn)O, Fe 2 3 . 



While quartz, chlorite, and magnetite are the usual alteration products of garnet, 

 magnetite was the sole recognisable product, in some cases thus indicating the migration 

 of quartz and chlorite. In the magnetite-garnet-schist No. 889, there is a production 

 of quartz, magnetite, and either biotite or muscovite. This may have been attained 

 by a reaction between garnet and the microcline or orthoclase in the following manner : 



Fe 3 Al2Si 3 O 12 + KAlSi 3 8 + 2 (OH) - H 2 KAl 3 Si 3 12 + 3 Si0 2 + Fe 3 4 . 



Garnet. Orthodox. Muscovite. Quartz. Magnetite. 



Stillwell * gives the following reaction which may be applicable to the case in 

 which biotite is produced : 



2 (MgFe)O, CaO, A1 2 3 , 3Si0 2 + 2 KAlSi 3 8 ^ (KH) 2 (MgFe) 2 Al 2 (Si0 4 ) 3 



Garnet. Ortkodatt. Biotite. 



+ Si0 2 . 



Anortkite. Quartz. 



The anorthite molecule is considered to enter into, or be derived from, the composition 

 of the plagioclase which then becomes relatively more basic or more acid according to 

 the direction of the action. It is doubtful whether there is sufficient lime in the garnet 

 to give rise to anorthite in the present case. 



Grubenmann f illustrates the production of quartz and biotite from garnet and 

 orthoclase by an equation in which sillimanite is produced. The latter mineral does 

 not appear in the present case. 



Tilley J interprets the reaction as follows : 

 H 4 K 2 Mg 2 Fe 4 Al 6 (SiO 4 ) 9 + 3 SiO 2 ; 2 KAlSi 3 O 8 + 2 Fe 2 MgAl 2 (Si0 4 ) 3 + 2 H 2 0. 



Biotite. Quartz. Ortkodate. Garnet. 



Op. cit., p. 168. 



f ' Die KraUllinen Schirfer." U. Grubenmann. Berlin, Vol. I (1904), p. 62. 



J " The Granito-OneUM of Southern Eyre Penuuulm." C. . Tilley, Q.J.G.S., Vol. Ixxvii, Pt. 2 (1921), p. 94. 



"MM < 



