S4 



THOMAS STRRRY HUNT ON A NATURAL SYSTEM IN 



loid species, as indicated in Table V. From the stnictnre of these minerals, Dana has 

 suggested that serpentine may be micaceous in crystallization like talc and chlorite.' This 

 is so far true that a silicate having the centesimal composition of serpentine assumes a 

 phylloid type, as seen in thermophyllite and in marmolite, but it also takes on a prismatic 



V. Tribe 4. Protophylloid. 



fibrous type in chrysotile and in picrolite, silicates of unlike density already mentioned 

 in § 65, and is moreoA^er found as an amorphoias colloid species included in the next 

 tribe, that of the Ophitoids, of which it may be regarded as the type. 



Tribe 5. — Ophitoids. 



§ 67. In considering this tribe we begin by noting certain diiFerences in composition 

 and in specific gravity among the magnesian silicates, which (besides thermophyllite, 

 marmolite, picrolite and chrysotile) have hitherto been grouped under the name of serpen- 

 tine. A density of from 2'60 to 2''70 is generally assigned to this silicate, but bowenite, 

 according to the analysis of J. Lawrence Smith and Brush, is a nearly pure serpentine, 

 with a density of 2'69-2*78, and a hardness of 5'5-60. Retinalite, a clearl)^ marked ophi- 

 toid or amorphous species, which includes much of the serpentine of the Laurentian 

 limestones, is a very pure magnesian silicate, distinguished from ordinary serpentine by 

 its lower density, and its larger proportion of water, which, from several analyses, the 

 writer found to be over fifteen hundredths. The specific gravity of retinalite is 2.36-2.38, 

 or nearly that assigned to the phj^lloid species, marmolite. The name of serpentine may 

 perhaps be retained for the amorphous silicate with density 2'6-2.'7, which must be distin- 

 guished from retinalite, as well as from chrysotile, from picrolite, from thermophyllite, 

 and from marmolite. This last requires further study, as does likewise bowenite, which 

 merits particular notice from its superior density and hardness, and requires optical 

 examination. 



§ 68. Following serpentine and retinalite in Table VI, are deweylite and genthite — 

 the latter a niccoliferous ophitoid, as chrysocolla is a cupric one. With the latter are placed 

 the bisilicates, aphrodite and ccrolite, which last appears to liave the volume of retinalite 

 and of deweylite. After these we have j)laced spadaite, as also renssellaerite or pyrallolite 

 (which is perhaps a compact phylloid rather than an ophitoid), and sepiolite. Alongside 

 of this, a iiosition has been conjecturally assigned to glauconite, as not improbably a ferrous 

 potassic ophitoid, of which a large part of the iron has subsequently passed into the ferric 

 condition. 



' System of Mineralogy, 5th Ed., p. 4G5. 



