Question: a rock with equal modal contents of quartz plagioclase and orthoclase is known as

title: Leucitite
text: Those leucite rocks which contain abundant essential plagioclase feldspar are known as leucite tephrites and leucite basanites . The former consist mainly of plagioclase , leucite and augite , while the latter contain olivine in addition . The leucite is often present in two sets of crystals , both porphyritic and as an ingredient of the ground mass . It is always idiomorphic with rounded outlines . The feldspar ranges from bytownite to oligoclase , being usually a variety of labradorite ; orthoclase is scarce . The augite varies a good deal in chemnistry and optical character , being green , brown or violet ( suggesting high Na and Ti content ) , but it is rarely high enough in Na and Fe to qualify as aegirine - augite or aegirine . Among the accessory minerals biotite , brown hornblende , hauyne , iron oxides and apatite are the commonest ; melanite and nepheline may also occur . The ground mass of these rocks is only occasionally rich in glass . The leucite - tephrites and leucite - basanites of Vesuvius and Somma are familiar examples of this class of rocks . They are black or ashy - grey in color , often vesicular , and may contain many large grey phenocysts of leucite . Their black augite and yellow green olivine are also easily observed in hand specimens . From Volcan Ello , Sardinia and Roccamonfina similar rocks are obtained ; they occur also in Bohemia , in Java , Celebes , Kilimanjaro ( Africa ) and near Trebizond in Asia Minor .