Mineralogy - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 49

Wilkinson and Hensel concluded that analcite and silica-undersaturated melt in
the NaAlSiO 4 - H 2 O system, cannot co-exist in equilibrium. Infer-

red solidus temperatures of the various hosts preclude a primary magmatic origin
for the interstitial and groundmass analcites. These are interpreted as subsolidus
phases produced by interaction of nepheline with deuteric and/or hydrothermal

KAlSiO 4 - SiO 2 -

uid
(see also Gupta and Fyfe 1975). Analyses of nephelines and their derivative
analcites indicate that the latter might have been formed from both Si-rich and more
Si-poor nephelines.
Megascopic crystals of analcites are found in the Brown Leucitic Tuffs (Luhr and
Giannetti 1987), where they often replace primary leucite phenocrysts. More basic
leucite-bearing tuffs (CaO > 5.6 wt%) contain a lot of small analcite crystals, which
are 0.02 mm across, but attain a size of 0.25 mm across in some pumices. Analci-
tization of leucite is a common geological phenomenon (Gupta and Fyfe 1975).
Analcites are often found in Tertiary extrusive rocks of north eastern Azerbaijan
(Iran). At Harbab Khandi analcite occurs as well-developed crystals often replacing
leucite in tephritic rocks. They are stoichiometric with respect to SiO 2 , but are silica-
oversaturated with reference to alkali. Analcites from Teic Dam and Razi are SiO 2 -
undersaturated and occur in three forms: (1) euhedral to subhedral or subrounded
analcite in the groundmass, (2) weg-shaped subrounded analcite interfaced with large
plagioclase phenocrysts in the groundmass (Table 2.9 ). Potassic analcite is an
important phase in jumillites, fortunites and varites from southern Spain.
The presence of euhedral analcite has been described from dykes and lava

ows
of alkaline rocks at Pyatistennyl by Bazarova et al. (1981). Abundant micro-in-
trusives occur in this region, comprising leucite, analcite and pyroxene in a glassy
matrix. Here, leucite and analcite coexist in the groundmass. The tuffs consist of
leucite, orthoclase, plagioclase and augite.
Recently Jamtveit et al. (2009) studied replacement of leucite by analcite. They
noted that a 10 % increase in volume is associated with the replacement process,
and this generates stresses that eventually cause fracturing of the reacting leucite.
Experimentally reacted leucite samples display characteristic fracturing patterns that
include both spalling of concentric

-like layers near the reacting
interface and the formation of cross-cutting, often hierarchically arranged, sets of
fractures that divide the remaining leucite into progressively smaller domains.
These structures may explain the

“ onion-skin ”

“ patchy ” alteration patterns observed in natural

leucite samples.

2.11 Melilite

Melilite compositions (Table 2.10 ) plot near the akermanite-sodamelilite join of the
system akermanite-gehlenite-sodamelilite (Schairer and Yoder 1964; Schairer et al.
1965, 1967; Ferguson and Buddington 1920; Ferguson and Merwin 1919; Yoder
1973). Melilites containing small amount of Ca 2 FeSi 2 O 7 have been described from
Capo di Bove Italy by Sahama (1974). He found that in the leucite-melilite