Mineralogy - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 52

nephelinite lavas from Nyiragongo (Zaire), where it occurs as tiny crystal. These
lavas also contain melilites, with prismatic habit, nearly perpendicular to the cavity
wall. Stout prismatic melilite grains (up to 1 mm in length, averaging a few tens of a
millimetre) occur in this region.
Melilite and leucite-bearing volcanic rocks occur at Mt. Vulture (Lucania, Italy).
The place is located in the easternmost volcanic area of the Roman province. The
most important melilite-bearing localities of this area include Prete Della Scimmia,
whereas melilitite dyke is reported, and the other being Melfi

hauynophyre lava

flow occurs slightly outside a volcanic cone (Melluso et al. 1996).
The Prete Della Scimmia melilites are characterised by large, clear euhedral
phenocrysts grading to microlites in the groundmass. In S. Caterina, melilite-mel-
afoidites have melilites, occurring as a late groundmass phase, inter-grown with
clinopyroxene, magnetite,

ow. The

leucite, nepheline and ha ü yne. At Melfi, , melilite is

present in ha ynophyre as a light yellow-coloured groundmass phase with bire-

fringence up to

ü

first order red and is optically negative.
The three melilite groups plot well within the field of volcanic melilites of Velde

and Yoder (1977). The Pete della Scimmia melilites are very Mg-rich (up to 8 wt%
MgO), alkali- and Al-poor melilites of the data set trend to become alkali-over-
saturated with respect to Al (Fig. 2.14 ). Analysis shows that groundmass melilites
contain 0.65 wt% (Ca, Na)Fe 3+ Si 2 O 7 (Na-ferrimelilite) having SrO contents as high
as (0.5 - 1.5 wt%). In contrast, melilites from S. Caterina melilite-melafoidite lava

flow, are gehlenite-rich (up to 13 mol%). The SrO contents are similar but slightly
higher than melilites of Pete della Scimmia.
Of all the melilites those from Melfi Sr-rich

ones found in Italy. Na-ferrimelilite molecule of these melilites can be as high as
22 mol % and the CaNaAlSi 2 O 7 content is up to 46 mol%; the SrO content is high
and they range from 3 to 1.18 %.
In sodium-rich leucite-bearing ultrama

ha ü ynophyre are the most Na Fe

- -

c rocks melilites were possibly formed
by reaction between nepheline and diopside. It is observed that at 1.7 0.3 GPa,

melilite occurring as a reaction product of diopside and nepheline is eliminated
(Singh et al. 2000). Akermanitic melilite breaks down even at atmospheric pressure
and 690

±

° C to wollastonite and diopside (Yoder 1973), whereas at slightly higher

Fig. 2.14 Compositional plot
of melilite in a
(Na + K)

Fe 2+ Fe 2+

-
(Fe 2+ ) - Al diagram (after
Melluso et al. 1996)

- (Mg + Mn)

Etinde

Oldoinyo
Lengai

Mg + Mn
+ Fe 2+

Na + K

S.Vananzo

Al 3+ Al 3+