Nesosilicates, frequently metamict due to radioactive impurities
Data for gadolinite-(Y) unless otherwise stated
Specific gravity: 4.36 to 4.77 (gadolinite-(Ce): 4.20 gadolinite(Nd): 4.86 (calculated))
Hardness: 6½ to 7 (all)
Streak: Greenish grey
Solubility: Gelatinises in acids, especially if metamict, dissolves with difficulty on heating in hydrochloric acid (Dana)
Plutonic igneous environments
Gadolinite-(Ce) occurs in alkaline pegmatites and alkaline granite (Dana ).
Gadolinite-(Y) occurs most often in alkaline granitic pegmatites or granite, rich in alkaline earth elements, and also in alpine-type veins (Dana, Webmin), associated with allanite, fluorite, fergusonite, yttrialite-(Y), chevkinite and zircon (HOM, Mindat).
At Mount Tanakami, just east Kyoto, Shiga Prefecture, Japan, gadolinite-(Y) has been reported from pegmatites (R&M 84.6.523).
At its type locality, Buer, Bjørkedalen, Porsgrunn, Telemark, Norway, gadolinite-(Ce) occurs in a syenite pegmatite along a contact between basalt and monzonite (HOM, Webmin), associated with aegirine, pyrochlore, zircon, apatite, titanite, pyrophanite, magnetite, loparite, chevkinite, biotite, microcline, helvine, molybdenite, albite, apophyllite, quartz and calcite (HOM, Mindat).
The primary sequence of crystallisation in this pegmatite is (1 - earliest) pyrochlore, aegirine, zircon and minor apatite and titanite; (2) pyrophanite and minor magnetite; (3) loparite, chevkinite, gadolinite-(Ce) and zircon; (4) biotite; (5) microcline. The later hydrothermal sequence of crystallisation is: (l) albite and minor helvine, galena and molybdenite; (2) minor apophyllite, quartz and calcite (AM 63.188-195).
At its type locality, the Malmkärra Mine, Norberg, Västmanland, Sweden, gadolinite-(Nd) is associated with fluorbritholite-(Ce), västmanlandite-(Ce), dollaseite-(Ce), bastnäsite-(Ce) and tremolite (Mindat).
The type locality for gadolinite-(Y) is Ytterby, Resarö, Vaxholm, Stockholm, Sweden.
In the UK gadolinite-(Y) and fergusonite-(Y) have been reported from the granitic Mourne mountains in north east Ireland, and in granite in Arran, Strathclyde, Scotland (JRS 14.23-25).
At Mount Rosa, El Paso county, Colorado, USA, beryl is absent, but late-stage to secondary beryllium-bearing minerals are present. Crystals of gadolinite to 1 mm have been collected from a miarolitic cavity associated with smoky quartz, microcline, zircon and kainosite-(Y) (R&M 95.3.271-272).
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