Geology Reference
In-Depth Information
Fig. 2.2 Coexistence of
calcic plagioclase in
equilibrium with sanidine.
There are two sets of alkali
and plagioclase feldspars as
shown by solid and open
circles. The tie lines joining
them suggest that two
different types of feldspars are
in equilibrium (after Worner
and Schminke 1984)
Ca
80
80
60
60
40
40
20
20
Na
K
20
40
60
80
The leucite-bearing rocks of Central Italy (Francalanci et al. 1987) have sani-
dines with compositions ranging between Or
88
Ab
12
and Or
77
Ab
23.
The single
crystals however, do not show compositional zoning. Such elements as Ba and Sr
along with other trace elements, are present in appreciable amounts in K-feldspars.
Perini et al. (2003) reported the presence of K-feldspar mega crysts hosted in K-rich
rocks at Monte Ciminio volcanic complex Central Italy. These megacrysts are
found in leucite-free olivine latites.
The highly potassic lavas from Latera Caldera (Roccamon
na, Italy) has san-
idinse phenocrysts with compositions (Table
2.2
) ranging from Or
68
to Or
90
. More
potassic and Ba-rich variants are found in the phonolitic pumices. In Trachytic and
phonolitic lavas, sanidine phenocrysts range in composition from Or
68
Ab
32
to
Or
90
Ab
10
. Sanidine crystals are observed also in trachytic pumices of Latera Cal-
dera (Turbeville 1993). They are euhedral, unzoned and untwinned, containing
<0.13 wt% BaO, and they are free from any mineral and glass inclusions. In tuffs of
Latera Caldera sanidine phenocrysts (Or
69
-
82
), are strongly embayed and include
feldspar blebs with a compositional range of Or
63
Ab
37
-
Or
74
Ab
26
. Chemical com-
position of sanidines from K-rich lavas of Vico, are in equilibrium with plagioclase.
Wyoming Orendites and jumillte from Spain contain clear crystals of sanidine of
variable sizes. These feldspars include diopside, leucite and rare apatite. The sa-
nidine crystals from Leucite Hills, contain up to 18 % KFeSi
3
O
8
molecule with very
low amount of anorthite in solid solution (0.01
0.11 wt% CaO).
Lamproites from Mount North (Australia) are constituted of 19
-
48 vol% of
-
K-feldspars containing 95
98 % Or. Those from Shiprock (Montana) contain
-
64
72 % orthoclase molecule, whereas those from Barqueros lamproites, Spain,
-
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