ART. 10 ZPXJLITES FROM OREGON HEWETT, SHANNON, GONYER 9 



Normal hciilandite occurs in the collection under discussion but 

 rarely and as ^ery thin crusts or druses lining amygdules. This 

 heulandite was seen to be overlain b}" stilbite and chabazite in differ- 

 ent specimens. Since the mineral preceded chabazite it was de- 

 posited veiy early in the series and is probably the oldest of the zeo- 

 lites now found in the specimens. The little crystals have the usual 

 form and are colork'«i and glassy witli pearly luster on the (010) 

 face. 



M KSOLITK 



Two minerals which are conspicuous in a considerable number of 

 the specimens of the Ritter Hot Springs collection occur in intimate 

 relation to each other; are very similar in their fibrous structure and 

 appearance; and. as shown by anah'sis of carefully selected and 

 optically proven samples, are practically identical in composition. 

 Nevertheless these two minerals are sharply distinguished from each 

 other optically and it has been necessary' to consider them as two 

 distinct minerals. One seems quite certainly to be mesolite of 

 normal character. The other is described below as pseudomesolite, 

 although it presents some small differences from the material de- 

 scribed by Winchell under that name. 



The analyzed sample of mesoiite is the cast of the interior of a 

 cavity about 5 centimeters licross and similar in character to the 

 specimen illustrated. (PI. lb.) The first deposit in this cavity 

 was calcite in crude, nearly colorless rhombohedra up to 2 cen- 

 timeters across. Upon the calcite crystals lies a layer of compact 

 fibrous zeolite which for the first 3 to 5 millimeters has a silky luster 

 and dense texture. Beyond this zone the fibers diverge, are free, 

 and are stained brownish. Entangled in the free fibers and pene- 

 trated by them are rude crystals of stilbite averaging 3 by 5 mil- 

 limeters in size. None of the minerals completely lined the cavity 

 and in different parts of the specimen either calcite, the fibrous 

 material, or stilbite may rest again.st the wall. 



Optical stud}' proves that there are two distinct minerals in- 

 cluded in the fibrous material. The densest compact-fibrous mate- 

 rial shows a moderately low birefringence and finely fibrous, nearly 

 parallel structure, and though there is a suggestion of slightly 

 inclined extinction, it could not be proven owing to the small in- 

 clination and the low birefringence. The elongation of the fibers is 

 Y and their apparent elongation therefore changes from positive 

 to negative alternately as they are rolled between crossed nicols under 

 the gypsum plate. The mean of refracti(jn index, fi, is about 

 1.517±0.002 (variable). Aggregates of fibers give, when in a posi- 

 tion to show maximum birefringence, a confused figure probably of 

 the obtuse bisectrix with negative elongation, whereas those in posi- 

 tion to give lowest birefringence have positive elongation. These 

 88909— 2&— 2 



