THE MINERALS OF IDAHO 



351 



in a previous paper. 70 The essential features of the description are 

 given as follows : 



The heulandite occurs in the specimens both as complete fillings of 

 small vesicles of the rock, as drusy linings of cavities, and as thin crusts 

 forming the first lining of cavities later filled with mordenite, chalce- 

 dony, or quartz. The characteristic flesh-red masses in the amygda- 

 loid rock consist of fine transparent heulandite and many of these 

 small masses are hollow and present fine terminations of acute wedge- 

 shaped crystals, the habit being as shown in Figures 109 and 110. 

 These show very well developed cleavage parallel to the pinacoid 

 6(010) as well as some other less perfect cleavages at right angles to 

 this plane. Uusually two or several crystals are grown together in 

 more or less parallel position, but, in 

 the heulandite of this type, the ten- 

 dency to parallel growth is not as 

 conspicuous as usual. A majority of 

 these pebblelike masses are not hollow 

 but are solid masses of heulandite. 

 Those which do show open centers 

 often have a few fine fibers of mor- 

 denite extending across the central 

 cavity. Other heulandite-lined ves- 

 icles in the same rock are completely 

 filled with closely packed cottony mor- 

 denite and the faintly pink to salmon 

 colored masses which fill may of the 

 larger bubble cavities are compact mor- 

 denite with a thin outer skin of heulan- 

 dite. The numerous irregular masses 

 which have weathered free from the 

 inclosing rock and which consist of compact mordenite, of mordenite 

 saturated with quartz and chalcedony, or of quartz and chalcedony, 

 all have a brick red outer coating which resembles a stain of ferric 

 oxide but which, when examined with a lens, is found to consist of a 

 thin outer crust of crystalline heulandite. All of the larger geodal 

 masses of mordenite, of whatever type, have a thin deposit of heul- 

 andite next the inclosing rock. Optical examination of the clear crys- 

 tals from hollow heulandite amygdules showed them to be trans- 

 parent and devoid of any visible pigmenting material to which the 

 red color might be attributed. 



Optically the crystals lining the small cavities or making up the 

 flesh red amygdules of the amygdaloid rock are biaxial positive ( + ) 



Fig. 113.— Heulandite. Drawing in 

 polarized light of crystal 

 lying on 6(010). Seven Devils 

 district, Adams County 



'• Clarence S. Ross and Earl V. Shannon. Proc. U. S. Nat. Museum, vol. 64, Art. 19, 1924. 



