Phase Relations in the System Leucite-Akermanite-Albite-SiO - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 331

Chapter 11
Phase Relations in the System
Leucite-Akermanite-Albite-SiO 2

In certain ultrapotassic petrographic provinces such as Somma-Vesuvius (Savelli
1967; Cioni et al. 1995), Coli-Albani (Aurisicchio et al. 1988) and Vico, all
localities from Italy (Cundari and Mattias 1974), Bufumbira region of East Africa
(Holmes and Harwood 1937; Rogers et al. 1998) and Highwood Mountains,
Montana, U.S.A. (O`Brien et al. 1991), leucite-bearing SiO 2 -undersaturated rocks
are associated with highly silicic, potassium-rich (K 2 O 3 wt%) feldspathic rocks.

Melilite and clinopyroxene-bearing leucitic rocks occur in the volcanic

≥

fields of
Fort Portal and Birunga petrographic province of Uganda (Holmes 1950; Rogers
et al. 1998). At Birunga, melilite- and leucite-bearing rocks are associated with
more SiO 2 -saturated potash trachybasalt, trachyandesite or latite series. These SiO 2 -
saturated varieties are apparently part of different volcanic cycles and contain alkali
feldspar and clinopyroxene as essential minerals.
The system leucite-akermanite-SiO 2 was investigated to establish petrogenetic
relationship between SiO 2 -saturated and SiO 2 -undersaturated leucite-bearing ma

c

rocks which crystallized under volcanic to subvolcanic conditions.
Albite has an incompatible relationship with leucite (Gupta and Edgar 1975) and
akermanite (Yoder and Schairer 1969). Albite (25 and 40 wt%) was therefore,
added to the leucite-akermanite-SiO 2 join (Fig. 11.1 ) to understand the reaction
relationship inhibiting the coexistence of leucite-albite and akermanite-albite min-
eral pairs in more complex systems. Investigation of the system was carried out also
to learn about the phase relationships involving these three minerals.
The system leucite-akermanite-albite-SiO 2 is bounded by the following joins:
1. akermanite-leucite,
2. leucite-SiO 2 ,
3. leucite-albite,
4. albite-akermanite,
5. albite-SiO 2 and
6. akermanite-SiO 2 .
The join akermanite-leucite was studied by Gupta (1972), who found it to be
pseudobinary, because diopside ss and wollastonite ss crystallize in addition to leu-
cite ss and akermanite ss in the mixtures, Lc 5 Ak 95 and Lc 10 Ak 90 . The pseudoeutectic