Classification - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 81

Such terms aswyomingite, orendite, andmadupite have been coinedbyCross (1897)
for leucite-bearing rocks from the Leucite Hills. Wyomingite is composed of leucite
(20

80 %) having augite,
phlogopite, apatite, calcite, andmagnetite in order of decreasing abundance. Phlogopite
occurs as phenocrysts, whereas the other minerals are present in the groundmass. Potash
richterite is sometimes present as an accessory phase. Wyomingite often shows parallel

25 %) with trace of olivine in a serpentinous base (

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flow banding, due to the planar arrangements of phlogopite flakes with vesicular cav-

ities, developed parallelly at the flow banding (Yagi and Matsumoto 1966).

According to Cross (1897), Orendite is composed of leucite (14 16 vol%),

olivine (<5 vol%) in a serpentinite base. Sanidine, phlogopite, augite and calcite,
are also present in the glassy groundmass. In olivine orendite, olivine may be
present in small amount. The glass is

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filled with microlites of sanidine and augite.
Sanidine forms euhedral tabular crystals, usually less than 0.1 mm in size, com-
monly twinned on Carlsbad law. The rocks vary in their crystallinity from a highly
glassy variety to a rather crystalline type. In the glassy variety, phlogopite
phenocrysts are set in a glassy groundmass, composed of pale brownish glass,

lled
with abundant acicular crystals of amphibole less than 0.3 mm in length, which
surround the microlite-bearing cavities.
A lamproitic rock comprising small phenocrysts of diopside, enclosed by rare
poikilitic crystals of phlogopite, has been termed as madupite by Carmichael
(1967). The micas are usually set in a turbid groundmass of diopside and chlorite,
together with other accessories. Magnetite is present and olivine is absent
(Carmichael 1967). Pilot Butte at Leucite Hills, is the type locality for madupite.
Mitchell (1985) classi ed lamproites into two categories: (1) madupitic lam-

proite and (2) phlogopite diopside lamproite. Mitchell recom-

mended that in case of phlogopite lamproite, phologopite should occur as
phenocryst and in the case of madupitic lamproite, phlogopite should be present in
the poikilitic groundmass. These two types of lamproites include additional sani-
dine and leucite, and their bulk compositions also plot on the diopside

sanidine leucite

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phlogo-

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pite leucite system. Since there is no contrasting mineralogy or chemistry

in case of the so-called two types of lamproites, one plotting in the limiting join
diopside

sanidine

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phlogopite, and the other inside the tetrahedron, there is no scienti c

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reason for such a classi cation. The accessory minerals in lamproites include,

chromian spinel, priderite [(K, Ba) 2-y (Ti, Fe 3+ ) 8-x O 16 , where the value of y is close
to 1 and that of x is very small], wadeite (K 4 Zr 2 Si 6 O 16 ), sheherbakovite [(Na, K)
(Ba)Ti 2 Si 4 O 14 ], jeppeite [(K, Ba)2(Ti, 2 (Ti, Fe) 12 O 13 ], apatite, perovskite, sphene,
ilmenite (very rare) and armacolite [(Mg, Fe) 2 Ti 4 O 10 ].

3.3.1 Various Lamproitic Assemblages and Their
Heteromorphic Relations to Each Other

The bulk composition of phlogopite, forsterite, akermanite, diopside, sanidine,
leucite, plots in the system larnite forsterite kalsilite SiO 2 - H 2 O (Luth 1967;

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