Mineral assemblages in phonolite occurrences are usually abundant feldspathoids ( nepheline, sodalite, hauyne, leucite and analcite ) and alkali feldspar ( sanidine, anorthoclase or orthoclase ), and rare sodic plagioclase.
22.
Minerals associated with humite include grossular, wollastonite, forsterite, monticellite, cuspidine, fluoborite, ludwigite, dolomite, calcite, talc, biotite, spinel, vesuvianite, sanidine, meionite and nepheline.
23.
Two types of groundmass are generally recognized : the trachytic, composed mainly of long, narrow, subparallel rods of sanidine, and the orthophyric, consisting of small squarish or rectangular prisms of the same mineral.
24.
Several different dates have been determined on the basis of argon-argon dating, including 5.81?.01 on biotite and 5.65?.01 mya on sanidine, which is the preferred age.
25.
The typical range of phenocrysts in ignimbrites are biotite, quartz, sanidine or other alkali feldspar, occasionally hornblende, rarely pyroxene and in the case of phonolite tuffs, the feldspathoid minerals such as nepheline and leucite.
26.
Hydrothermal alteration of the central lava dome and iron-bearing deposits has generated alunite, anatase, chlorite, copper veinlets, gypsum, illite, jarosite, labradorite, quartz, rutile, sanidine, smectite, and sulfur.
27.
Sanidine, melanite, hauyne and perovskite are frequent accessory minerals in these rocks, and many of them contain melilite in some quantity, The well-known leucitite of the Capo di Bove, near, Rome, is rich in this mineral, which forms irregular plates, yellow in the hand specimen, enclosing many small rounded crystals of leucite.
28.
The sodium-bearing amphiboles and pyroxenes so characteristic of the phonolites may also be found in some trachytes; thus aegirine or aegirine augite forms outgrowths on diopside crystals, and riebeckite may be present in spongy growths among the feldspars of the groundmass ( as in the trachyte of Drachenfels on the Rhine, show this character excellently, having large sanidine crystals of tabular form an inch or two in length scattered through their fine-grained groundmass.
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