Mineralogy - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 41

crystals show overall change in composition from Mg-rich core towards Fe-enri-
ched rim, as bulk rocks become more evolved. In some crystals compositional
range of olivine is found to vary from Fo 93.5 - 87.5 to a larger range Fo 93.4 - 69.7 or
Fo 91.0 - 77.4 , suggesting that olivine could not have reached equilibrium with the host
magma. Macdonald et al. (1992) considered that this diverse olivine population
probably resulted from magma mixing.
Gaussburg olivine leucitites have forsteritic olivines (Fo 91 - 92 ) (Table 2.7 ), and as
expected is high in NiO (0.2

1.0 wt%), consistent with
crystallization from a Mg-rich liquid under low pressure volcanic regime. In the
Lyzhnaya pipe of Aldan (Russia), lamproitic pipes consist of forsteritic olivine
(10

0.6 wt%) and CaO (0.2

- -

fine-grained groundmass comprises
microlites of diopside, phlogopite and K-feldspar. The Ryabinovaya pipe is a olivine-
diopside lamproite, where forsteritic olivine (Fo 92 - 94 ) with up to 0.5 wt% NiO
constitutes 15

20 vol%), diopside and phlogopite. The

-

25 vol% of the rock. Chromite inclusions (Cr 2 O 3 up to 61 vol%) are
present in these rocks.

-

2.8 Nepheline

It has a cubic symmetry at or above 1,254 ° C (Bowen 1912), but hexagonal
symmetry at low temperatures under atmospheric pressure. The cubic phase is
called carnegieite. Nepheline usually contains albite; and at 800 C the maximum

amount of albite incorporated in nepheline is about 33 wt% (Greig and Barth 1938)
under one atmospheric pressure. It has also an extensive solid solution relationship
with kalsilite at 0.5 GPa (Zeng and MacKenzie 1984; Fig. 2.11 ). The system has a
truncated solvus with a maximum temperature at 800

°

C and Ne 30 Ks 70 . Phase
relations of the join nepheline-kalsilite have also being studied by Gupta et al.
(2010). Their isobaric diagram at 2 GPa and variable temperatures is shown in
Fig. 2.12 .
Holmes and Harwood (1937, p. 37) described nepheline-bearing leucitites from
the potassium-rich volcanic

°

fields of following localities in Uganda: Mabunga,

Fig. 2.11 Phase relations in
the system nepheline-kalsilite
at 0.5 GPa (after Zeng and
MacKenzie 1984)

1400

1200

L

1000

Ne +L

ss

Ks +L

ss

800 Ks ss

Ne ss

600

Ne +Ks

ss ss

400

40 20

80 60

NaAlSiO 4

KAlSiO 4

wt. %