Nesosilicate (insular SiO4 groups), gadolinite group, yttrium-bearing mineral
Specific gravity: 4.36 to 4.77
Hardness: 6½ to 7
Streak: Greenish grey
Colour: Black, greenish black, brown
Solubility: Gelatinises in acids, especially if metamict, dissolves with difficulty on heating in hydrochloric acid (Dana)
Plutonic igneous environments
Gadolinite-(Y) is often slightly radioactive due to minor uranium and/or thorium content, and therefore it is
often metamict. It occurs most often in alkaline granitic
granite, rich in alkaline earth elements, and also in alpine-type
associated with allanite,
chevkinite and zircon
At Mount Tanakami, just east of Kyoto, Shiga Prefecture, Japan, gadolinite-(Y) has been reported from pegmatites (R&M 84.6.523).
The type locality 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).
At the North Sugarloaf Mountain locality, Bethlehem, Grafton County, New Hampshire, USA, gadolinite-(Y) was found with fluorite and bastnäsite-(Ce). The brown crystals are only a few mm in length, and form radiating groups up to 3 cm across (R&M 97.3.220-221).
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