AKT. 28 



MINERALOGY OF TRIASSIC LIMESTONE SHANNON 



19 



It is not believed that there can be any separation into two dis- 

 tinct phases of alteration and deposition of secondary minerals and 

 there is probably every gradation from the so-called high-tempera- 

 ture replacements' to the presumable low-temperature veins. The 

 fact that the low-temperature veins are later in the observed cases 

 than the lime-silicates merely indicates that they were formed by 

 superposition at a period when the environment had become cooler, 

 either by the dying stage of the same current of material or by a 

 new pulse of solutions ascending along new 

 fractures, from a deeper part of the sill. In 

 the earlier stage, the zone characterized by 

 deposition of datolite was well beyond that 

 where the diopside and associated minerals 

 were formed. 



The minerals occurring in the veins are 

 separately described below. 



/ 



Fig. 1. — Diopside. Habit 

 of minute colorless 

 crystals occurring in 

 veins with datolite 



Some of the specimens of crystallized 

 datolite show minute translucent whit© blades 

 which are aggregated into masses, some- 

 times filling a small cavity, and resembling 

 frost crystals. In most cases the mineral 

 rests upon the bare portions of the lime- 

 stone base of the specimens where they are 

 not coated by datolite. Sometimes a com- 

 pletely bounded datolite crystal is impaled 

 upon one of the minute blades. In a few 

 cases they seem to rest definitely on the 

 crusts of datolite crystals as though younger. 

 The amount of the mineral is so small that it was with difficulty that 

 4 milligrams of pure material was obtained for qualitative testing. 

 It is infusible before the blowpipe, insoluble in acids, and suffers no 

 loss on ignition. Its constituents are silica, lime, and magnesia in 

 approximately equal amounts. Optically the laths are biaxial posi- 

 tive with 2V medium, dispersion pronounced r-Cv. In some posi- 

 tions the extinction is parallel with positive elongation, in others the 

 extinction is inclined with ZAc=M°. The refractive indices are 

 a=1.670, p=rl.680, y=1.690, y— am.OSO. All of these properties 

 unite to identify the mineral as diopside, although it looks more like 

 a zeolite and its occurrence and appearance are so unlike those of a 

 pyroxene that the identification was reluctantly accepted. 



