Mineralogy - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 29

Ti

20

NATAL

TAL

10

CATS

Na

Al

50

30 10

Fig. 2.8 A plot of bulk composition of pyroxenes from Alban Hills in a Na - Al - Ti diagram. The
dominant core composition of clinopyroxene is plotted here. In this figure NATAL, TAL and
CATAS denotes Na Al Ti-rich, Ti Ti-Al-enriched and Ca-Tschermak ' s molecule respectively (after

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Aurisicchio et al. 1988)

that the green core pyroxenes from the Eifel region is related to chrome content and
their crystallization was associated with polybaric differentiation of alkali basalt
magma.
The dominant core composition of clinopyroxenes from Alban Hills is plotted in
Fig. 2.8 . In this

figure NATAL, TAL and CATS denote enrichment in Na, Al
(NATAL); Ti, Al (TAL); and Ca-Tschermak s molecules (CATS), respectively. In

case of clinopyroxenes from the Alban Hills, Fe 3+ is the dominant cation in the M 1
site. The ferri-Tschermak

'

is molecules, in fassitic pyroxenes, is higher in the Al iv
content in comparison with diopsidic pyroxenes. Aurisicchio et al. found that the Cr
content of clinopyroxenes is highly variable. It is particularly high in cores of the
diopside phenocrysts ranging from 0.31 to 0.92 and sometimes reaching a value as
high as 1.04 % in megacrysts. In the rims, Cr 2 O 3 contents could be as high as 0.61
wt%. The TiO 2 content values in the cores of some of the pyroxenes is as high as
0.98

'

2.58 wt%. In the rim of the same crystal, the TiO 2 content varies between 0.84
and 2.54 wt%, respectively.

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2.5 Mica

Lamproites constitute 15 30 vol% of tri-octahedral mica (Wagner and Valde 1986).

Some of the micas are authigenic, whereas others are xenocrysts derived from the
mantle. Large phlogopites sometimes, occur either as cores of complex micas or
may serve as nuclei for crystallization of other micas. Sometimes, cores show that
the TiO 2 content is around 2 wt%, surrounded by phlogopite having higher Ti, Fe
and lower Al than the cores. Typical lamproitic micas have Fe/(Fe + Mg) ratio
always >0.30, with concentric zoning. They show increase in Ti and Fe contents

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