It occurs as a
metasomatic replacement of metamorphosed
limestones in the Mary Kathleen mine, Australia and in alkalic
pegmatites associated with
syenite in an alkaline
massif in
Tajikistan. It occurs in association with
allanite,
garnet,
uraninite in the Australian deposit; with
calcite,
monazite,
bastnasite,
thorite,
uranothorite and
thorianite in the Desmont mine,
Wilberforce, Ontario,
Canada; and with
pyrochlore,
tienshanite,
sogdianite, thorite, caesium
kupletskite,
reedmergnerite,
steacyite,
pectolite and
quartz in the Tajikistan deposit. It has also been reported from
Mont Saint-Hilaire,
Quebec, Canada and from
Mineville,
Essex County, New York. Other occurrences include the Inagli massif,
Yakutia,
Russia, around
Langesundsfjord,
Norway, in the
Ilimaussaq intrusive complex, southern
Greenland and the
Vico volcano,
Lazio,
Italy.
Discovery It was first described in 1955 for an occurrence at the
type locality is the
Mary Kathleen mine, east of
Mount Isa,
Queensland,
Australia. It was named for Australian
mineralogist Frank Leslie Stillwell (1888–1963).
Chemistry Breakdown {| class="wikitable" == SiO2 ==