Geology Reference
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Because of this, all of the eight subtetrahedra in which phlogopite is a constant
component, is therefore, affected and anhydrous compositions are then represented
by melilite + leucite + forsterite + kalsilite and akermanite + leucite + forste-
rite + clinopyroxene. Melilite ugandite (melilite
olivine leucitite), phlogopite
-
olivine leucititewas studied at one atmospheric pressure by Gupta (1972). Yoder
(1986) pointed out that these two subtetrahedra undergo a reaction as follows:
-
þ
þ
þ
ð 3 : 5 Þ
4 clinopyroxene
3 kalsilite
2 akermanite
forsterite
3 leucite
Holmes (1942) recognized this heteromorphic relationship, and described a lava
comprising clinopyroxene + kalsilite + forsterite and other varieties with melilite
and leucite. Bulk compositions of these different types of rocks are illustrated in
Fig. 3.13 as a melilite mafurite, mafurite and a leucite mafurite.
The above discussion shows that many assemblages, which have heteromorphic
relation have similar chemistry but are products of different pressures and tem-
peratures in presence of excess water. Simple classi
cation based on only chemistry
could therefore, be highly misleading. A mineralogical classi
cation re
ects not
only the chemistry but the P-T
T condition of formation of the rocks. For example,
presence of feldspathoids together with melilite and hydrous minerals in equilib-
rium, should suggest low P-T - T condition of formation. It also indicates low silica-
activity condition of the magmatic
-
fluid responsible for the generation of these
rocks. A knowledge of the presence of oxide minerals (occurring as accessory) is
important, as they indicate oxygen fugacity condition of formation of these rocks.
Fig. 3.13 Subtetrahedra of Fig. 3.10 resulting from reaction of melilite (Mel) and leucite (Lc) and
illustrating the heteromorphic relations of anhydrous assemblages produced from the breakdown
of phlogopite to melilite mafurite
leucite mafurite (after Yoder 1986)
 
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