SERPENTINE 



53 



and irregular or embayed outlines where they adjoin 

 the rounded grains of olivine. Some of the enstatite 

 poikilitically encloses small olivine crystals, but this 

 is uncommon. The olivine grains are of about the 

 same size as those in the dunite, and the pyroxene 

 crystals are commonly from 1 mm to several milli- 

 meters long. The alteration of the olivine in the fresh- 

 est rocks has advanced only to the stage at which a 

 few of the cores are replaced by serpophite. The py- 

 roxenes are very slightly replaced along cleavage traces 

 by bastitic chrysotile of low birefringence, forming 

 jagged edges where these intersect crystal boundaries. 

 The only accessory mineral found was a pale-yellow 

 picotite, which is largely anhedral ; this mineral tends 

 to enclose olivine and to be enclosed by pyroxene, and 

 even in the least altered sections it is slightly replaced 

 and rimmed by magnetite. Sections of more altered 

 peridotite show that the orthopyroxene is replaced by 

 a pale-green magnetite-free chrysotile of low bire- 

 fringence at the same stage in which the cores of the 

 olivine grains are being replaced by serpophite. The 

 clinopyroxene generally remains little altered until 

 most of the orthopyroxene is completely serpentinized, 

 and where it forms thin tabular intergrowths with the 

 orthopyroxene, laminae of orthopyroxene may alter- 

 nate with laminae of bastitic chrysotile. With still 

 further serpentinization the clinopyroxene is converted 

 to an intricate intergrowth of needles quite unlike the 

 orderly bastite, but in most sections of completely ser- 

 pentinized peridotite no pseudomorphs of clinopyrox- 

 ene were recognized. Where the serpentinization of 

 the clinopyroxene is complete, the picotite is largely 

 replaced by magnetite, and the iron oxide freed from 

 the olivine has collected into strings of fairly well 

 formed magnetite crystals following the larger frac- 

 tures. Some lenticular and otherwise irregular veins 

 of normal chrysotile also are commonly found in these 

 completely serpentinized rocks. 



The sheared serpentine generally shows in thin sec- 

 tion only scattered fragments of bastitic pseudomorphs 

 and a few grains of residual picotite or magnetite by 

 which one may infer its ultimate origin from an ultra- 

 mafic igneous rock. Because the shearing has de- 

 stroyed all the original textures, one can draw no con- 

 clusions regarding the serpentinization process from 

 examinations of thin sections. The field relations, 

 however, as well as (he chemical composition, are such 

 that there can be no doubt that the sheared serpentine 

 has the same origin as the more massive varieties. 



To this point the serpentinization process is largely 

 one of hydration, and, since the process appears to 

 be a pseudomorphic replacement, some silica and mag- 

 nesia, and perhaps also chromium, must have been re- 



' 



' r : / 



: 





FIGURE 40. Photomicrographs of serpentine derived from peridotite. 

 Upper, Shows complete replacement of olivine and partial replace- 

 ment of large pyroxene on left. Plane light. Lower. Partly re- 

 placed pyroxene consists of parallel growth of orthopyroxene (in 

 extinction position) and cllnopyroxene (narrow light bands) 

 Crossed nicols. 



moved from the rock. Additional changes, which have 

 affected only isolated areas, consist of further veining 

 with chrysotile and recrystallization of the serpentine 

 minerals to platy antigorite accompanied by shearing. 

 The widespread alteration of serpentine to form silica- 

 carbonate rock is a radically different change, believed 

 to have been caused by hydrothermal solutions having 

 their source outside the serpentine. This alteration, 

 which took place at a much later time than the origi- 

 nal serpentinization, is treated at length on pages 

 58-64, after the description of the silica-carbonate 

 rock. 



