Classification - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 84

The assemblage clinopyroxene + phlogopite + leucite + olivine, similar to that
found in madupite) is formed under certain P-T T conditions (Barton and Hamilton

1978). The product is also similar to a missourite.
When a natural orendite and a wyomingite were heated at high temperature and
low pressure (<300 bar, Barton and Hamilton 1978), an assemblage of clinopy-
roxene + olivine + leucite + K-feldspar was obtained. Yoder also showed that the
join K-feldspar-olivine is broken in presence of H 2 O at lower temperature:

-

þ þ þ ð 3 : 2 Þ

K-feldspar 3 forsterite H 2 O phlogopite 3 hypersthene

The assemblage, biotite + olivine + hypersthene + feldspar, is typical of a
lamproite, called fortunite. Experimental study of Arima and Edgar (1983) showed
that phlogopite clinopyroxenite has a heteromorphic relation to katungite. The
assemblage is formed by the following reaction:

3 phlogopite þ 2 clinopyroxene 5 olivine þ 3 melilite þ 3H 2 O ð 3 : 3 Þ

The rocks called venanzite and melilite ugandite plot in the two new subtetra-
hedra as depicted by Fig. 3.12 . Katungite, vesbite and molibovite also plot in these
two subtetrahedra. Phlogopite breaks down to olivine, leucite, kalsilite and vapour
at high temperatures but low pressures (Yoder and Kushiro 1969). Note the

following reaction:

2 phlogopite 3 forsterite þ leucite þ kalsilite þ H 2 O ð 3 : 4 Þ

Fig. 3.12 Subtetrahedra of
Fig. 3.10 resulting from
reaction of phlogopite (Phl)
and clinopyroxene (Cpx) and
illsutrating the heteromorphic
relations of alnoite

Mel

Mel

italite to

-

Vesbite

Cpx

venanzite melilite ugandite

(after Yoder 1986)

-

Melilite
Ugandite

Lc Lc

Venanzite

Phl