Nepheline is a feldspathoid, a solid-solution mineral, that does not coexist with quartz; rather, nepheline would react with quartz to produce alkali feldspar.
12.
A dioritic rock containing feldspathoid mineral / s and no quartz is termed " foid-bearing diorite " or " foid diorite " according to content.
13.
It is a feldspathoid and a member of the sodalite group . and was named in 1807 by Brunn-Neergard for the French crystallographer Ren?Just Ha�y ( 1743 1822 ).
14.
At very low degrees of partial melting a silica undersaturated melt is produced, forming a nepheline syenite, where orthoclase is replaced by a feldspathoid such as leucite, nepheline or analcime.
15.
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.
16.
The most important mineral component of lapis lazuli is lazurite ( 25 % to 40 % ), a feldspathoid silicate mineral with the formula ( Na, Ca ) 8 ( AlSiO4 ) 6 ( S, SO4, Cl ) 1-2
17.
Essexites represent gabbros whose parent magma was under-saturated with silica, resulting in the formation of the feldspathoid minerals nepheline, cancrinite, and sodalite as accessory minerals rather than quartz . ( Silica saturation of a rock can be evaluated by normative mineralogy ).
18.
Unusually, phonolite forms from magma with a relatively low silica content, generated by low degrees of partial melting ( less than 10 % ) of highly aluminous rocks of the lower other silica crystals, and is dominated by low-silica feldspathoid minerals more than feldspar minerals.
19.
We might in consequence regard them as varieties of syenite, diorite, gabbro, etc ., in which feldspathoid minerals occur, and indeed there are many transitions between syenites of ordinary type and nepheline or leucite syenite, and between gabbro or dolerite and theralite or essexite.
20.
High-alumina basalts have aluminium contents of 17 19 wt % Al 2 O 3; boninites have magnesium contents of up to 15 percent MgO . Rare feldspathoid-rich mafic rocks, akin to alkali basalts, may have Na 2 O + K 2 O contents of 12 % or more.
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