ART. 19. MOEDENITE — ROSS AND SHANNON. 13 



rock or filling gashes in the mordenite which probably result from 

 the solution of plates of calcite as shown in plate 1, upper right. 

 Individual crystals of the analcite reach an extreme diameter of 14 

 millimeters. Where free from inclusions the analcite is clear and 

 colorless, but the crystals commonly inclose fibers of mordenite and 

 spherulites of quartz, giving them a mUky appearance. They are 

 for the most part simple trapezohedrons of the form qill2), which 

 is a common form for analcite as shown in figure 5. Occasionally 

 however, one shows faces of the cube a(lOO) and of the trigonal 

 trisoctahedron w{233) which, though previously known, are rare 

 forms on analcite. A modified crystal is shown in figure 6. Certain 

 very interesting crystals occur in the platy argentine calcite. These 

 are attached to the calcite plates by the equatorial plane and, where 

 space was available, the top halves have developed completely. 

 Where the space between the calcite plates was thin the analcites 



5. 6. 



Figs. 5 and 6.— Analcite crtstals from mordenite mass, Challis, Idaho. 



are very much flattened and are bounded only on their edges by 

 narrow crystal planes. Both undeformed and greatly flattened 

 crystals are shown attached to calcite in plate 3, right. The disk- 

 like crystals may reach 1 centimeter in diameter with a thickness of 

 only 0.2 miUimeter, although most of them are tliicker than this, 

 with a smaller diameter. Selected ones of these are ideal thin 

 sections, and show admirably the birefringence and division into 

 sectors Such a crystal photographed in polarized light between 

 crossed nicols is shown in plate 3, upper. The flattened analcites 

 adhere to the quartz films which coat the- platy calcite and usually 

 show very numerous quartz spicules attached to their surface. This 

 quartz was ground off of the crystal shown in the photograph. Opti- 

 cally the analcite is biaxial and negative with large axial angle and 

 a mean index of refraction of 1.486 The biaxial interference figure 

 is extremely sharp and definite and this, together with the geometri- 

 cal perfection of the division into sectors and the sharpness of the 



