﻿Vol. 6 1.] MICROSCOPIC STRUCTURE OF SERPENTINE. 699 



mineral with a higher refractive index than serpentine, and with 

 whitish polarization-tints, which, as will be seen in the next section, 

 has some resemblance to antigorite. A few of the flakes are slightly 

 iron-stained, and they give brighter polarization-tints. These clear 

 spots presumably indicate the former presence of some kind of 

 pyroxene, but retain no trace of its original structure. 



VI. Antigorite-Serpentine. 



Having thus described the forms of serpentine which can be 

 shown to have been produced from olivine, hornblende, augite, and 

 enstatite, we pass on to that called antigorite and the circum- 

 stances of its occurrence. It was named from the Val Antigorio in 

 Piedmont, but it has been described in considerable detail by 

 Dr. Hussak 1 from Sprechenstein on the Brenner. He, however, 

 had not visited the locality, but received his specimens from a 

 friend. The district was examined by Miss Raisin in the summer 

 of 1904, so that, though one of us has seen the Yal-Antigorio rock, 

 we will deal more particularly with that from the Tyrol. Antigorite 

 is defined 2 as a light-green mineral, occurring in scales with a perfect 

 cleavage of the mica-type. Sections at right angles to this perfect 

 cleavage are lath-shaped. They give straight extinction, and are 

 distinctly pleochroic ; leek-green when the short axis of the nicol is 

 parallel to the cleavage-cracks, colourless when the short axis is at 

 right angles to these cracks. The double refraction of this mineral 

 is slight and negative, the negative bisectrix agreeing with the 

 vertical axis. Cleavage-flakes show, in convergent light, the optic 

 picture of a biaxial crystal with small axial angle. The dispersion 

 is well marked. The following analyses are quoted : (I) of the 

 Sprechenstein mineral; (II) of that from the Val Antigorio, in 

 which apparently a little iron-oxide still remained ; (III) of the 

 schistose serpentine (bulk), and (IV) of the massive serpentine 

 (bulk) — both from Sprechenstein. 



I. II. III. IV. 



SiO, 4114 41-58 40-55 4090 



Fe 2 3 301 722 1040 768 



A1 2 3 3-82 2-60 2-70 2-08 



CaO 040 4-40 0-30 



MgO 39-16 36 80 33-59 37'45 



11-85 12-67 932 1215 



99-38 100-87 10096 100-56 



The two mineral analyses, so far as they differ from that of a 

 perfectly-typical serpentine (Si0 2 =44'l, Mg0 = 43-0, H 2 = 12-9) 

 probably do this in consequence of the replacement of magnesia by 



1 Tschermak's Min. & Petrogr. Mitth. n. s. vol. v (18S3) p. 61. 



2 Quoted in substance from the excellent summary of the original paper in 

 Dr. J. J. H. Teall's ' British Petrography ' 1888, pp. 112-15, 



Q. J. G. S. No. 244. 3 d 



