gadolinite

Gadolinite is a rare-earth bearing silicate mineral that typically forms in granitic pegmatites and other felsic igneous rocks. Its general chemical formula is (Ce,La,Nd,Y)2FeBe2Si2O10, indicating that cerium, lanthanum, neodymium, and yttrium commonly occupy the rare-earth element site, with iron and beryllium present along with silicon and oxygen.

Gadolinite forms a solid-solution series, with endmembers commonly described as gadolinite-(Ce) and gadolinite-(Y) depending on which rare-earth element dominates the REE site. The mineral can incorporate various REEs in varying proportions, reflecting the composition of the host rock.

In hand sample, gadolinite typically appears as dark green to olive-brown or black crystals or coarse-grained aggregates. It has a vitreous to resinous luster and is usually opaque, though thin edges may be translucent. The mineral tends to display a relatively high density for silicates and a Mohs hardness around 6, with crystals often prismatic in habit. Gadolinite is commonly associated with other rare-earth minerals such as xenotime, monazite, and zircon within granitic pegmatites.

Gadolinite was first described from the Ytterby district of Sweden, a renowned source of rare-earth minerals. It serves as an ore of several rare earth elements, notably cerium and yttrium, and has historical significance in the early study and extraction of REEs. Today, it is mainly of interest to collectors and researchers as part of the broader rare-earth mineralogy of pegmatites, rather than as a primary commercial source.