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Laucite. This mineral occurs crystallized in icositetrahedra. It is 

 found in the leucitophyres, leucite-tephrites, leucitites and leucite-basalts. 

 The angles are often somewhat rounded. The smaller crystals appear 

 thoroughly isotropic ; the larger ones often show anomalous double-re- 

 fraction. This double-refraction disappears when the crystal is heated 

 to a certain point. The inclusions in leucite are often arranged in a 

 zonal manner, and this feature becomes of importance in examining the 

 basaltic rocks in which the mineral often occurs without any definite 

 external form. Leucite in thin section is only slightly affected by hot 

 acid. When it is so affected the silica separates in a pulverulent 

 condition. 



Melilite. This mineral is frequently associated with nepheline and 

 leucite. It may occur as more or less idiomorphic crystals or as irregular 

 grains. The common forms are the prism (110) and basal plane (001) ; 

 sometimes the vertical pinacoids are also developed. The crystals are 

 usually tabular. Sections parallel to the vertical axis are therefore lath- 

 shaped, the short side corresponding to the vertical axis ; sections at 

 right angles to the vertical axis are quadratic or octagonal. In many 

 rocks the external form of the melilite is very imperfect, although it 

 approximates to that of the typical crystal. The refraction is somewhat 

 higher than quartz ; the double-refraction is very low, the highest tint 

 in ordinary sections being the grey-blue of the first order. Its character 

 is negative, so that in the lath-shaped sections the major axis of 

 depolarization is at right angles to the length of the section. STELZNER 

 has shown that melilite very commonly possesses a highly characteristic 

 micro-structure. The lath-shaped sections are often crossed transversely 

 by a fine striation or else peculiarly-shaped structures, resembling pegs, 

 spears or spatulse, run out from the basal planes towards the centre 

 of the section for a greater or less distance. This he terms " pflock- 

 structure." 



Melilite is essentially a basic silicate of lime and alumina. It is 

 colourless or brown in thin sections and is very easily acted upon by 

 hydrochloric acid in thin section. By the addition of a little sulphuric 

 acid to the solution gypsum crystals are formed as the liquid evaporates. 



Melanite. This is a lime-iron-garnet. It occurs in idiomorphic 

 crystals which show the forms (110) and (211) according to ROSENBUSCH. 

 It is an accessory mineral very common in some of the leucite- and 

 nepheline-bearing rocks. 



Perowskite, CaO TiO 2 . This is an important accessory in certain 

 rocks of the group now under consideration, especially those rich 

 in melilite. It occurs in minute well-formed octahedra which often 

 show anomalous double-refraction. The refraction is very high. By 

 transmitted light the mineral appears greyish-white, violet-grey, 

 greyish, brown, brownish-yellow to reddish-brown (ROSENBUSCH). It 

 may easily be mistaken for one of the spinels. In doubtful cases a 

 determination can only be effected by isolating the mineral and testing for 

 titanic acid. 



