P-T Stability of Phlogopite, K-Richterite and Phengite, as a Source of Potassium in the Mantle - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 369

In these assemblages, K is partitioned into hydrous phases or liquid, rather than
into the clinopyroxene. By inference, phlogopite (or its higher-pressure breakdown
products) is the primary host or K in the mantle (if H 2 O is present), and any
coexisting clinopyroxene has very low concentrations of K. Conversely, the natural
occurrence of clinopyroxene with

1 wt% K 2 O requires that phlogopite, potassium
richterite, or phase X is not stable in the local source environment of such samples.
Luth (1997) observed that a pyroxene- and phlogopite-bearing assemblage
yields phlogopite + diopside + garnet at 6 + 1 GPa. Potassium-richterite joins this
assemblage at 8 + 1 GPa and at still higher pressure, phlogopite is completely
replaced by potassium-richterite and the stable assemblage is diopside + gar-
net + forsterite + potassium-richterite above 11.5 GPa and 1,000

≫

° C and 8 GPa and

1,400 ° C (Fig. 12.16 ).

12.8 K-Richterite as a Source Mineral of Potassium
in the Upper Mantle

12.8.1 P-T Stability of K-Richterite

Scott Smith and Skinner (1984) classi ed potassium-rich lamproites on the basis of

the presence of following minerals: phlogopite, potassium-richterite, olivine,
diopside and sanidine. In case of some of the diamond-bearing ultrama

c rocks
from India, which are described as kimberlites, Scott Smith classified them as
lamproites because of the presence of potassium-richterite. In many lamproites
potassium-richterite is quite common. This mineral is particularly an essential
mineral in the mantle. It is also present as a groundmass mineral in wyomingite,
cedricite, orendite and jumillite. Bergman (1987) also emphasised the presence of
K Ti- richterite, K-riebeckite and K-arfvedsonite, and, particularly the potassium-

richterite as an important mineral constituent of the lamproites. The possible
presence of potassium-richterite in the upper mantle and importance of this mineral
as a source of potassium has been discussed by Kushiro and Erlank (1970) and
Varne (1968). The P-T stability of potassium-richterite was therefore, determined
by Gupta and Venkatesh. Which is shown in Fig. 12.17 . The data at 0.1 GPa and
below are from Charles (1974, 1975). Extension of the sodium-richterite break-
down curve as determined in Gupta and Venkatesh (1993), agrees well with low
pressure stability data of Charles.
In Fig. 12.17 the univariant curves of Gilbert and Briggs (1974) related to the
stabilityofpotassium-richterite

-

fluor-potassium
richterite [KRi(F 100 )] are also included to compare the relative stabilities of the
sodium- and potassium-richterites. It may be noted that although below 1.2 GPa,
hydroxy potassium richterite breaks down at lower temperatures than richterite,

[KNaCaMg 5 Si 8 O 22 (OH) 2 ] and