203 



TABLE IV 



MINERALOGY OF GREENSTONES 



Mineral 



Occurrence 



Properties 



Composition 



Albite replacement of calcic plagioclase 



microlites and phenocrysts; and 

 rarely as a vein mineral 



Calcic cores of albitized microlites and 



plagioclase phenocrysts 



Chlorite replacement of glassy matrix, 



pseudomorphs after olivine crystals; 

 abundant vein mineral, partially 

 replaces plagioclase; fills vesicles; 

 commonly intergrown with 

 actinolite 



Actinolite in fibrous aggregates, abundant 



replacement of matrix; commonly 

 occurs in quartz veinlets; 

 pseudomorphs clinopyroxene (?) in 

 metadolerites 



Epidote Veins and in sheared monominerallic 



aggregates (epidosites); does not 

 occur as replacement of pre-existing 

 minerals or matrix; commonly occurs 

 in euhedral crystals large compared 

 to chlorite and actinolite 



Quartz In veins, matrix, and rarely as vesicle 



filling with chlorite (,2-S). Generally 

 present in small amounts. 



Sphene Replacement of Fe-Ti oxides ; occurs 



in all specimens; rare relict Fe-Ti 

 oxides are coated by leucoxene 



Pyrite Small euhedral crystals commonly 



in or near quartz, chlorite and/or 

 epidote veins 



Spinel Rare euhedral crystals unaltered ; 



identical in color to spinels in 

 fresh basalts from dredge 2 



Nontronite Abundant in metatuff"s in dredge 2 

 where it commonly replaces ripido- 

 litic chlorite. In dredge 3 in veins 

 and rarely replaces epidote 



2K2at85°(low 

 albite). Twinning 

 largely destroyed by 

 albitization 



Nj at 1.62 

 anomalous blue to 

 reddish brown 

 colors under crossed 

 nicols 



pleochroic from 

 colorless to light 

 green or brownish 

 green, optics range 

 widely, typically 

 A^^AZat2r, 

 N^= 1.644, and 

 N^= 1.626 



N^ = 1.56, 

 N^ = 1.54; 

 pleochroic: light 

 yellow or light 

 brownish yellow. 

 Co = 12.6A 



Ani-An4 



as high as Anyj 



ripidolitic in all 

 examined greenstones, 

 typical composition 

 MgO = 20%, FeO = 

 19%,Al203=18%. 

 Co = 14.2A 



47-64% tremolite 

 molecule; ranges from 

 1.8to6.9% AbOsin 

 3-3 and 3-6; maximum 

 Al203inJ-7(7.4%). 

 14 analyses of actino- 

 lite in 3-7 give average 

 AI2O3 = 4%. 



weakly zoned, margins 

 may be either higher 

 or lower in Fe203. 

 Average composition 

 (8 analyses) about 75% 

 clinozoisite molecule 



not examined by 

 microprobe 



decomposes under 

 microprobe, no 

 reliable analyses 

 obtained 



