Geology Reference
In-Depth Information
Fig. 8.5 Tie lines showing
bulk compositions and
corresponding feldspar
compositions in equilibrium
with leucite in the system
leucite - albite - anorthite (after
Gupta and Edgar 1975)
KAlSi O
(Leucite)
6
2
Lc ss
B
0
(1413 C)
A
(1068 C)
0
Plag
V
V
V
VVV
VVVVVVVV
V
VVVV
NaAlSi 38
CaAl Si 28
2
(Albite)
(Anorthite)
containing high proportion of anorthite molecule (approximately An 50 Ab
Or 50 ).
The incompatibility of leucite and albitic feldspar in the absence of anorthite
molecule is probably due to the following reaction:
-
2NaAlSi 3 O 8 þ
KAlSi 2 O 6 !
KAlSi 3 O 8
ð
1
X
Þ
leucite
ð
Alkali feldspar
Þ
ð
X
Þ
NaAlSi 3 O 8 þ
2 NaAlSiO 4
X NaAlSi 3 O 8 :
ð
nepheline ss
Þ
Na feldspar and nepheline ss by
the reaction between leucite and albite. In the leucite
The above reaction explains the formation of K
-
anorthite system,
liquids closely representing natural magmas from which leucite and feldspar-
bearing assemblage crystallise, the products of crystallization at low temperatures
near the surface are anorthite-rich ternary feldspars, leucite, and a residual liquid,
enriched in Na 2 O and SiO 2 . Some of the Na 2 O may be incorporated into leucite.
MacKenzie and Rahman (1968) noted that leucite rims in leucite-sodic feldspar
veins in the Massif Central basanite became enriched in Na 2 O with falling tem-
perature. The pressure of a residual liquid might also explain the albitic nature of
the feldspar in these veins; the Na-rich feldspar having formed from an original
K-rich feldspar by a process of alkali ion exchange. Such a mechanism should
explain the maximum orthoclase content of 20-mol% (MacKenzie and Rahman
1968), relative to the albite
albite
-
-
orthoclase content of 50 mol% from the direct primary
crystallization of low temperature liquids in the simpli
-
albite system.
The effect of P(H 2 O) on the incompatibility of leucite and albitic feldspar may
be estimated by comparing the results of this study with that of Fudali (1963) for
ed leucite
-
 
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