Classification - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 80

Olivine, hypersthene, K-feldspar, phlogopite, and glass are the essential minerals
of a fortunite. Its SiO 2 content can go up to 56.68 wt%. Pseudo-brookite is present
in these lamproites as an accessory mineral (Wagner and Velde 1986). It often
incorporates xenocrysts of phlogopite and Al-spinel. Orthopyroxene may be a di-
saggregated product of mantle xenolith in fortunite.
An olivine and phlogopite-bearing lamproite with acmite-rich pyroxene, apatite,
a roedderite-like phase, rutile and calcite is called a cancalite. It contains olivine and
chromian spinel xenocrysts.
Barberi and Innocenti (1967) coined the term selagite to a lamporite, containing
phenocrystal olivine and phlogopite with clinopyroxene, K-feldspar, ilmenite,
richterite, chromian spinel and apatite in the groundmass. The type locality is
Orciatico.
Minettes are characterized by 10

40 vol% phenocrysts with a porphyritic texture
(Macdonald et al. 1992). When the MgO content of the rocks exceeds 17 wt%, the
phenocrystal phases are olivine, chromian spinel, clinopyroxene (diopside or salite)
and phlogopite in a groundmass of phlogopite, K-feldspar and glass. There may be
fresh leucite occurring as inclusions in olivine and clinopyroxene, but pseudoleucite
is present as phenocrysts. In some minettes, analcime constitutes as much as 30 vol%.
Wade and Prider (1940) coined the terms wolgidite,

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fitzroyite. mamilite and
cedricite, to various lamproitic rocks of Western Australia, the de nitions of which

are given below. Wolgidite contains diopsidic pyroxene, large grains of potash
richterite, phlogopite, and somewhat turbid crystals of leucite. Relict olivine crys-
tals are present in a serpentinous groundmass. Amphibole is more common than
phlogopite and zeolite and ilmenite are present. Fitzroyite is a fine-grained rock,

containing phenocrysts of phlogopite and leucite in a brown glassy groundmass.
Rounded to sub-rounded leucite crystals contain characteristic tiny inclusions in the
centers of the grains; this mineral is also present in the groundmass. Phlogopite
(sometimes twinned) and abundant rutile needles are present, and the amount of
leucite exceeds that of phlogopite. A typical fitzroyte is constituted of 47 50 %

-

leucite with 5 10 % phlogopite in a serpentinous base, constituting about 40 %

accessory phases including magnetite, apatite, etc. constituting less than 5 %.
The term mamilite was given to a

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fine-grained rock, consisting essentially of

leucite (about 50 60 %) and potash-richterite with rutile needles and very rare

phlogopite (<5 %) in a serpentinous base (<20 %). Olivine may be present in trace
amount. Mica is present only in the groundmass. Leucite crystals are often altered,
sometimes containing characteristic inclusions.
A lamproite, rich in leucite with small amounts of diopsidic augite embedded in
a brown glassy groundmass containing small to rare amounts of potash richterite,
phlogopite, has been called a cedricite. Cedricites have rutile with serpentines,
pseudomorphous afer olivine. Weed and Pirsson (1996) suggested the term mis-
sourite for a group of rocks consisting essentially of augite (about 35

-

- 40 %), and

leucite (40 5 %), analcite, zeolite, and iron

ore. Washington (1927), however, suggested that olivine is not an essential mineral
in missourite and listed the modal percentages of two missourites which are devoid
of olivine. The compositions of these rocks lie in the

- 50 %) with minor amounts of biotite ( *

field of leucitites.