36-1 



be readily distinguished from apatite by treating the slide with nitric 

 acid and adding a solution of ammonium molybdate. If the doubtful 

 mineral be apatite the characteristic yellow precipitate will be formed. 



Sodalite group. Minerals of the sodalite group are very common in 

 certain rocks containing nepheline and leucite. The three typical mole- 

 cules in this group are represented by the following formulae. Sodalite, 

 2(Na 2 A1 2 3 2 Si0 2 ) + NaCl ; Nosean, 2(Na 2 A1 2 3 2 Si0 2 ) + 

 Na^OSOs; Hauyn, 2(Na 2 A1 2 8 2 SiO a ) + CaOS0 3 . The minerals are 

 rarely if ever found in an ideal state of purity in consequence of the 

 fact that the typical molecules are capable of replacing each other. 

 They crystallize in the cubic system, and the rhombic dodecahedron 

 is the dominant form. The faces of the cube and octahedron may, 

 however, be occasionally observed in combination with those of the 

 dominant form. 



Sodalite occurs abundantly in the elaeolite-syenites. It may be 

 present as irregular grains or more or less idiomorphic crystals. It 

 is colourless or grey and often possesses a greasy lustre. WERVEKE 

 found both sodalite and nosean in the elseolite-syenite of Southern 

 Portugal, the two minerals being intergrown so that the former 

 surrounded a kernel of the latter. Sodalite is readily etched by 

 hydrochloric acid in the thin section, and crystals of common salt 

 are formed as the acid evaporates. The refractive index of the 

 mineral is low and it is of course isotropic. Sections of sodalite may 

 be distinguished from basal sections of nepheline by the fact that the 

 latter give a broad cross in convergent light. The most perfect 

 crystals of sodalite are found in certain trachytes and phonolites. 



Hauyn. In consequence of the difficulty of distinguishing 

 between hauyn and nosean (in the absence of the blue colour which 

 sometimes characterizes the former) it is convenient to have one 

 name to include both minerals. ROSENBUSCH uses hauyn as the group 

 name. The common form of hauyn is the rhombic dodecahedron. 

 The usual sections are therefore hexagonal or rectangular. The 

 mineral is isotropic or shows only anomalous double-refraction. The 

 inclusions are often highly characteristic. They may be distributed 

 irregularly through the substance of the mineral or they may be 

 heaped up, either in the centre or at the periphery, thus producing 

 a zonal structure. Sometimes they are arranged in lines parallel to 

 the crystallographic axes. The crystals of nosean in the leucitophyres 

 of the Eifel district are usually surrounded by a narrow opaque 

 border. Sometimes crystals of a mineral of the hauyn group are so 

 abundant as to form an essential part of the rock-mass. When this 

 is the case the rocks are termed hauynophyres. 



Hauyn and nosean are easily decomposed by acid. If a slide 

 containing either of these minerals be etched with acid then, if the 

 mineral be hauyn, minute crystals of gypsum will be formed in 

 abundance as the acid evaporates ; W if, on the other hand, the 



(1) If strong acid and a high temperature be employed cube-like crystals of anhydrite will 

 form instead of gypsum (SAUEE). 



