SERPENTINE 



51 



extensive literature on serpentine and serpentine min- 

 erals reveals that dozens of varietal names have been 

 used, but many of these have been discarded. Some 

 now used as mineral names were originally intended 

 merely as varietal rock names. Other names conno- 

 tated textures, some of which were inherited from the 

 replaced minerals, whereas others resulted from the 

 crystallization of the serpentine mineral itself. Recent 

 X-ray and thermal studies made of serpentine are 

 still not reconciled, and there remains considerable 

 uncertainty, and some disagreement, as to just how 

 many serpentine minerals there are, and just what are 

 the limitations to their variable physical and optical 

 properties. 



Because it is not possible to differentiate precisely 

 the various serpentine minerals, they will be divided 

 in this report, according to the simplest optical tests, 

 as follows : 



Fibers Length-slow 



Length-fast 



Plates Length-slow 



Length-fast 



Amorphous, or nearly so_ 



Chrysotile 

 Fibrous antigorite 

 Antigorite 

 Not found 

 Serpophite 



This grouping follows for the most part accepted 

 usage, except that fibrous antigorite, the most com- 

 mon of the serpentine minerals in the district, has 

 often been misidentified as normal antigorite because 

 of its low birefringence and the difficulty of ascertain- 

 ing whether it is fibrous or platy. In addition, we 

 include under the term "chrysotile" material with 

 lower birefringence than is customary. "Serpophite" 

 is used for designating the structureless nearly iso- 

 tropic mineral that generally occurs as a pseudomorph 

 after olivine, following usage that is generally ac- 

 cepted by English-speaking geologists even though it 

 differs somewhat from the original intent of Lodoch- 

 nikov (1936), who proposed the term. The senior au- 

 thor is indebted to Mr. V. P. Sokoloff for translating 

 lengthy sections of this ponderous volume for him. 

 Lodochnikov's best definition of serpophite seems to be 

 the one given on pages 34 and 35, but it leaves much to 

 be desired. The term is loosely used for "macroscopi- 

 cally dense, structureless, varieties of serpentine, having 

 waxy or enamellike luster, and light to dark color." 

 According to Lodochnikov (p. 34), serpophite occurs 

 chiefly as veinlets, and apparently he intended the term 

 to be used in megascopic, rather than microscopic, de- 

 scriptions. "Bastite" is used as a varietal term for any 

 serpentine mineral that forms pseudomorphs after 

 either an orthorhombic or monoclinic pyroxene with 

 coincidence of c axes. 



The study of thin sections of serpentine derived 

 from dunite, which consists almost entirely of olivine, 

 provides the logical starting point for the study of the 

 process of serpentinization because of the small num- 

 ber of minerals involved. These sections will therefore 

 be described first, even though in the New Almaden 

 district serpentine derived from dunite is much less 

 common than serpentine derived from rocks contain- 

 ing pyroxenes as well as olivine. 



The least-serpentinized dunite found in the area con- 

 tains less than 30 percent of residual olivine, as shown 



- 





rxr" 



f^aJ-A // 



FIGURE 39. Photomicrographs of dunite partly replaced by serpentine 

 minerals, showing development of typical meshwork. Olivine (ol), 

 serpentine minerals (S), and magnetite (M). Upper, Plane light. 

 Lower, Section oriented so that olivine residuals are near the ex- 

 tinction position. Note that all of these show the same illumina- 

 tion, indicating that they are all parts of a single crystal and un- 

 rotated. Mineral bordering olivine residuals is fibrous antigorite ; 

 narrow veinlets are chrysotile. Crossed nlcols. 



