Some writers held that this mineral has resulted from the decomposition of nepheline; others regard it as a primary mineral.
These last consist of an intermixture of nepheline or sodalite and alkali-felspar.
nepheline appears to decrease in amount as leucite increases.
The pyroxene is morphic, yellow or green, and is surrounded by formless areas of nepheline.
Typical nepheline-dolerites are scarce, and consist of idiomorphic augite, surrounded by nepheline.
Sodalite or hauyne may also be present, but nepheline is typically absent.
Leucite and nepheline rocks have been found in various parts of the island, especially in the south-west.
They are chiefly leucite and nepheline rocks, such as leucitite, leucitophyre and nephelinite, but basalt and trachyte also occur.
The nepheline is in small six-sized prisms, and usually cannot be detected with the unaided eye.
(c.) Phonolite (Clinkstone) is a trachytic rock, composed essentially of sanidine, nepheline, and augite or hornblende.