Phase Relations in the System Leucite-Akermanite-Albite-SiO - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 338

The assemblages at the invariant points i, j and k should be represented by
wollastonite-melilite leucitite, wollastonite-pyroxene phonolite and wollastonite-
pyroxene rhyolite. Once again in presence of FeSiO 3 , the invariant assemblage should
be represented by melilite leucitite (i), pyroxene phonolite (j) and pyroxene rhyolite
(k). As natural rocks contain FeO, the above assumption should be valid.
Lava flows corresponding to a melilite-nepheline leucitite have been reported from

Fort Portal, Uganda (Holmes and Harwood 1932). Nepheline leucitite and nepheline-
melilite leucitite lava

flows have been reported from Colli Albani (Alban Hills,
Fornaseri et al. 1963, Aurisicchio et al. 1988). Pyroxenite, leucite and nepheline-
bearing phonolites from Somma-Vesuvius are found around Naples, Italy. In this
area, igneous activity began with the eruption of trachybasalt and its derivatives such
as trachyandesites and phonolites (Imbo 1965). Highly potassic lavas of Rocca-
mon na (Appleton 1972) are either a nepheline- or leucite-normative series or a

trachybasalt or a latite series, but melilite-bearing rocks are absent in Roccamon na.

Eruption of such contrasting lavas may be controlled by such thermal divides as
albite-K-feldspar- calcic pyroxene or nepheline-K-feldspar-pyroxene joins.
Melluso et al. (2003) described small outcrops of wollastonite- and melilite-
bearing pyrometamorphic rocks (para-lavas), which are found along the Apennine
chain in Central Italy at the localities of Colle Fabbri and Ricetto. These rocks have
coarse- to ne-grained crystalline, spotted, and glassy textures. The Colle Fabbri rocks

have abundant and ubiquitous wollastonite, with plagioclase ± clinopyroxene. Me-
lilite is found only in the most Ca-rich and silica-poor samples. Federico et al. (2002)
studied a suite of lithics (ejecta) collected from the latest erupted pyroclastic products
of the Alban Hills volcano (Central Italy). They studied the rocks to determine their
mineralogical composition and to investigate their genesis. The ejecta commonly
have granular texture and consist of coarse-grained crystals often associated with a

fine- to medium-grained matrix. The mineralogical composition is variable and
consists of both typical igneous minerals and contact metamorphic phases. Garnet,
clinopyroxene K-feldspar are almost ubiquitous, whereas leucite, wollastonite,
sodalite-group minerals, phlogopite, nepheline and phillipsite are present in most of
the ejecta; minor accessory phases include cuspidine, aniphibole, pyrrhotite, mag-
netite, apatite, uranpyrochlore, sphene, kalsilite, and melilite; anorthite, zircon and

fluorine-bearing Ca, Zr silicate phases. Larnite, and baryte are found sporadically

11.4 Experimental Study of the Joins Leucite-Akermanite-
Albite with or Without Anorthite in Air or Under
1 Gpa in Presence of Excess Water

Based on petrological observations of leucite-bearing rocks, Cross et al. (1902)
concluded that leucite and albite do not coexist in equilibrium and they took into
account this incompatibility problem in the norm calculation. Shand (1943)
observed that in leucite-bearing lavas, the coexisting feldspar is either a calcium-
rich plagioclase or potassium-rich alkali feldspar, or both. Yoder and Schairer