Among accessory minerals, in addition to apatite, zircon, and iron oxides, the following may be mentioned : hornblende ( not common ), riebeckite ( rare ), epidote and zoisite, calcite, sphene, andalusite, sillimanite, kyanite, hercynite ( a green spinel ), rutile, orthite and tourmaline.
22.
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.
