Mineralogy - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 64

It is stable under following P-T T conditions: 1.2 GPa (800, 1,000 ° C), 1.4 GPa

-

(1,000 ° C), 1.5 GPa (1,000 1,200 ° C), 1.75 1.88 GPa, (1,200 ° C), 2.0 2.2 GPa

- - -

(1,200 ° C) and 2.5 GPa (1,200 and 1,250 ° C) (Arima and Edgar 1980).

2.17 Roedderite-Like Mineral [(Na, K) 2 (Mg, Fe) 5 Si 12 O 30 )]

Roedderite-like minerals have been described from the lamproitic rocks of Can-
carix, Albacete (Spain) and Moon Canyon, Utah (U.S.A.). This is a blue mineral
(Table 2.16 ), belonging to roedderite-eifelite series (Abraham et al. 1983). The
mineral from Cancarix is a sodium-free but iron-rich phase relative to those found
in Eifel, Germany. This is a rare mineral, which is crystallised from the melt. The
same phase is quite abundant in the Moon Canyon lamproite (Wagner and Velde
1986), but is less iron-rich but more enriched in Mg and Na than that from

Cancarix.
Wagner and Valde (1986) suggested a substitution of the type, Fe 3+ Fe 2+ +Na +

but entry of Fe 3+ in the structure creates a vacant site (an alkali site). Wagner and
Valde suggested that the deep blue colour in the mineral may be due to the presence
of Fe 3+ . A similar phase (eifelite, Abraham et al. 1983) has been described from the
K-rich volcanic ejecta of Eifel, of Germany.

⃌

2.18 Pseudo-Brookite

This mineral is a solid solution of the end members Fe 2 TiO 5 (Pseudo-brookite),
FeTi 2 O 5 (ferro-pseudo-brookite), MgTi 2 O 5 (karooite) and Al 2 TiO 5 . Pseudobrookite
is present as an euhedral plate, hexagonal in shape. They are green or purple brown,
usually opaque, but sometimes translucent (e.g. Smoky Butte lamproite, Wagner
and Velde 1986). They exhibit reddish internal re

ection under re ected light.

Table 2.16 Analyses of roedderite-like mineral
SiO 2

FeO a

Al 2 O 3 MgO MnO TiO2 Na 2 O 2 O Total

1 70.29 12.60 11.44 0.23 0.02 0.04 4.48 99.10

2 69.86 0.17 9.83 13.90 0.21 0.21 0.25 5.14 99.57

3 70.60 0.50 5.80 15.70 0.21 0.07 1.80 4.20 98.88

4 71.32 0.36 0.49 17.86 0.31 0.07 5.29 4.16 99.86

a FeO indicates total FeO + Fe 2 O 3
1 Representative electron microprobe analyses of the roedderite-like phase, from Cancarix
lamproite, Albacete, Spain (Wagner and Velde 1986)
2 Representative electron microprobe analyses of the roedderite-like phase, from Moon Canyon
lamproite, Utah, U.S.A. (Wagner and Velde 1986)
3 - 4 Roedderite in Volcanic ejecta, Eifel region Germany (Hentschel et al. 1980)