Mineralogy - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 16

The Miocene lamproites from the Fitzroy area, West Kimberley (Australia) have
leucite phenocrysts, which are typically euhedral and are unaltered (Jaques and
Foley 1985). They are weakly birefringent, twinned, and commonly contain
inclusions of glass arranged in concentric zone. Most glassy rocks are strongly
resorbed and embayed. Leucite phenocrysts containing aluminous spinel inclusions
are amoeboid-shaped with strongly rounded coalesced aggregates.
Divalent cations replacing K in leucite is rare (Jaques and Foley 1985), but the
CaO content in excess of 1 wt%, has been described from the K-rich volcanic rocks
of the East African Rift Valley.

2.2 K-Feldspar

Sanidine is found in tephritic phonolites, phonolitic tephrites, trachytes and/or
phonolites from different localities of Italy, Manchuria, Tristan da Cunha and
Udsuryo Island. It is also reported from leucite-bearing minettes and lamproites as
an essential mineral. For example, they occur in lamproitic rocks from Navajo,
Highwood Mountains, Smoky Butte (all from U.S.A.), Spain and Damodar Valley
(India). K-feldspar also occurs as minor constituents in K-rich rocks of New South
Wales (Australia) and Birunga (equatorial Africa). Inclusion-free phenocrystal sa-
nidine (up to 2.5 mm long) occurs in Brown Leucitic Tuffs from Roccamonfina
(Luhr and Giannetti 1987).
K-feldspar and plagioclase coexist in leucite-bearing tephritic rocks of the Eifel
region, Germany (Worner and Schminke 1984). Individual sanidine crystals in
these rocks are homogeneous containing 75

90 mol% orthoclase (Table 2.2 ). The
tie lines between plagioclase and K-feldspar (Fig. 2.2 ) suggest equilibrium between
two types of feldspars. In this

-

figure, composition of two types of feldspars (open
and solid circles with tie lines) as determined by Worner and Schminke (1984) from
Laacher See phonolites, is shown. Leucite-bearing minettes from Bearpaw
Mountains, U.S.A., contain invariably sanidine (Or 43 Ab 41 An 16 to Or 93 Ab 5 An 2 ,
Table 2.2 , Macdonald et al. 1992). There is a tendency for the orthoclase content to
increase with decreasing magnesia concentration in the bulk rock. These sanidines
are Ba-rich containing 7 mol% celsian (Cn) molecule. The sanidines (Or 47 A-
b 42 An 6 Cn 5 to Or 2 An 56 Ab 41 Cn 1 ) in Bearpaw latites have variable chemistry.
The Potassic rocks of Highwood Mountains comprise sanidines, which are by
far the dominant feldspars in these rocks (O

' Brien et al. 1991). They occur as

euhedral phenocrysts in ma c phonolites and trachytes and in coarse-grained

shonkinites and malignites. It ranges in composition from K-rich variety
(Or 88 Ab 9 Cn 2 Sl 1 ) to albitic sanidine (Or 64 Ab 34 An 2 ). Sometimes, hyalophane with
up to 11.1 wt% BaO (Or 44 Ab 29 Cn 22 An 3 Sl 2 ), are present (Sl represents slawsonite,
Sr-feldpar). In Highwood Baldy stock, perthitic and antiperthic intergrowths in
feldspars ranging in composition from Or 88 Ab 12 to Ab 100 are observed.