ROCK-FORMING MINERALS 15 
colourless glass; and gelatinises with acids. It is a microscopic con- 
stituent of certain trachytes and phonolites, occasionally appearing as 
well-defined crystals in the vesicular cavities and fissures of such rocks. 
Its common alteration-products are fibrous zeolites. A compact blue 
variety, with a greasy lustre is a notable ingredient of elzeolite-syenite. 
Haiiyne and Nosean are isomorphous mixtures of sodium-aluminium 
silicate and calcium-aluminium silicate, both with sulphur. Hardness, 
5°55 specific gravity, 2:28 to 2-5. Both minerals are fusible before the 
blowpipe; and both are decomposed by acids. They crystallise in 
isometric forms. The larger individuals often show crystalline con- 
tours, giving in microscopic sections more or less imperfect hexagonal 
or quadrangular figures. Not infrequently the crystals present the 
appearance of having been corroded. The minerals are hard to 
distinguish from each other, but haiiyne is usually blue or bluish-green, 
while nosean is generally grey, although it may be greenish, brown, 
red, or yellow. Microscopic examination shows that they are usually 
crowded with inclusions. Both minerals are fusible before the blowpipe 
and gelatinise with hydrochloric acid. They occur as macroscopic 
and microscopic constituents of certain igneous rocks that are rich in 
alkalies, such as phonolite. They are thus frequent associates of leucite 
and nepheline. Both are apt to be altered into fibrous zeolites. 
