454 C. E. Tilley — MetaTiiorpliism of 



(iv) Edenite. — This is a clear, colourless inineral, idioblastic, with 

 a moderate refractive index, which occurs sparingly in these rocks, 

 embedded in calcite or dolomite. 



Several sections show the typical amjDhibole cross-section cleavages 

 intersecting at an angle of 124 degrees. The crystals are optically 

 positive, and extinctions of 25 degrees have been observed. These 

 properties do not correspond with those of ordinary tremolite. More- 

 over, a basal section yields only a very low interference colour, 

 and in convergent light such sections are seen to be almost normal 

 to an optic axis. The optic axial angle is not large, and the plane 

 of the optic axes bisects the obtuse intercleavage angle as in normal 

 amphiboles. These characters serve to identify the mineral as a 

 member of the edenite- j)argasite series. If we restrict the name 

 edenite to the colourless members of this group, then this mineral 

 must be classed as edenite. 



Kreutz ^ has described a pargasite, from Pargas, Finland, con- 

 taining 10"83 per cent AlgOg, 1"56 per cent FeO, 0"76 per cent FcjOj. 



The optical properties are given as R.I. = 1"62, D.R. = '0195, 

 optically positive, 2V := 59 degrees, ZaC = 27 degrees. 



W. E. Ford,^ in an optical study of the amphiboles, describes an 

 edenite from Grenville township, Canada, containing 11 "37 per cent 

 AlgOs, FeO =0-42, Fe2O3=:0-42. The double refraction approximated 

 *020, optically positive, 2V = 56 degrees, and the extinction 

 ZaC = 29 degrees. 



Coomaraswamy,^ in describing the crystalline limestones of 

 Ceylon, gives the analysis of an aluminous tremolite, Al.jOg = 13"76. 

 The optical properties are not given, but material from another 

 locality gave extinctions on 110 cleavage flakes of 17 degrees, 

 which would approximate the edenite values on 010. 



In view of these descriptions, and the parallelism of the optical 

 properties of the material in those dolomites, with those described 

 for the edenites by Kreutz and Ford, we can have little doubt in 

 assigning this mineral to the edenite group. 



Unfortunately the material at disposal and the comparative 

 scarcity of the mineral in the rock, has not enabled a chemical 

 analysis to be made. Some sections, cut parallel to the vertical 

 axis showing a single cleavage, are with difficulty distinguishable 

 from colourless diojDside, the optical orientation of which is closely 

 similar. 



(v) Alteration Products of Forsterite. — Forsterite is, as has been 

 noted, usually in part, or wholly replaced by other minerals, amongst 

 which serpentine, calcite, rarely dolomite or magnesite, and opal 

 are the chief. 



Serpentine may be developed to the exclusion of the carbonates 

 or opal. Calcite has been definitely proved in certain cases, and a few 



1 S. Kreutz, Min. Mitt., vol. xxvii, 1908, p. 249. 



^ W. K. Ford, Amer. Journ. Sci., vol, xxxvii, 1914, p. 179. 



^ A. K. Coomaraswam}-, Quart. Journ. Geol. Soc, vol. Iviii, 1902, p. 417. 



