204 



Al,0, 



MgO FeO 



Fig.4. Composition of actinolites (triangles) and ciilorites (circles) in terms of AI2O3, MgO and FeO. 

 Results from partial analyses done on ARL electron microprobe using mineral standards. All Fe 

 assumed to be as FeO; thus, actual FeO less than indicated. Analyses of actinolite and chlorite from 

 other low grade basic rocks included. In these, total Fe was calculated and recast as FeO. Previously 

 analysed actinolite and chlorite are from : (/) Wiseman ( 1 934) ; (2) Tilley ( 1 93 8) ; (i) Hutton (1938); and 

 (4) Seki (1958). Solid circles and triangles indicate 22°N sample 3-6; open circles and triangles, sample 

 3-3. Electron microprobe analytical error probably a maximum of 10% amount present based on 

 intercomparison of mineral standards. 



grade; becoming more aluminous on approach to the amphibolite fades, and more 

 sodic on approach to the lower grades of the "blueschist" facies. The low alumina 

 (Fig.4 and Table III) and soda (< 0.5% Na20) of the actinolites thus further indicate 

 the greenschist facies character of the 22°N rocks. 



The nontronite gives a principal basal reflection on diffractometer traces at 

 12.62A (acetone-duco cement mount) which is quite sharp, indicating a well-crystalli- 

 zed variety. This is in accord with its large crystal size and distinct optical properties 

 in thin-section and in powders (Table IV). The nontronite was found to decompose 

 under the electron microprobe beam even at low voltages, and thus quantitative 

 analyses were not obtained. However, qualitative analyses are in accord with a 

 nontronite composition (abundant Fe and Al). 



Stilpnomelane is common in basic rocks of the greenschist facies (Hutton and 

 Turner, 1936). However, no stilpnomelane was noted in 22°N greenstones. A Dooi 

 of precisely 12.62A helps distinguish the nontronite from stilpnomelane which typi- 

 cally has Dooi equal 12.1 A (Deer et al., 1962b). Furthermore, the basal reflection 

 of the nontronite increases to 17A after glycolation, and the refractive indices of 

 the nontronite (Table IV) are slightly lower than minimum refractive indices of 

 stilpnomelane. 



The 22°N mineral assemblages are compatible with those of the quartz-albite- 

 muscovite-chlorite subfacies. or the quartz-albite-biotite subfacies of Turner and 

 Verhoogen (1960). However, the low-alumina content of the actinolite and the 

 absence of garnet clearly show that they are of lower grade than the quartz-albite- 

 epidote-almandine subfacies. 



