Gadolinite-(Y)

Chemical formula: Fe<sup>2+</sup>Be<sub>2</sub>Y<sub>2</sub>(SiO<sub>4</sub>)<sub>2</sub>O<sub>2</sub>

Gadolinite-(Y) is a relatively rare yttrium, iron, and beryllium silicate, valued by collectors for its well-formed, black crystals.

## Characteristics Gadolinite-(Y) is a mineral belonging to the gadolinite group, an yttrium, iron, and beryllium silicate. It typically forms prismatic, well-developed crystals with an almost square cross-section, terminated by pyramids. It often occurs as granular or massive aggregates, embedded in the host rock, mainly in granitic pegmatites. Its surface can be dull or covered with a thin, brown layer of weathering products, but a fresh fracture reveals an intense, vitreous or resinous luster. ## Physical Properties This mineral is characterized by significant hardness, ranging from 6.5-7 on the Mohs scale, which makes it scratch-resistant. It is relatively dense, with a specific gravity in the range of 4.0-4.7 g/cm³. It is brittle, and its fracture is conchoidal to uneven. Gadolinite-(Y) is usually opaque, rarely translucent in thin splinters. Some specimens may exhibit weak radioactivity due to the presence of rare earth elements, including thorium and uranium, which substitute for yttrium. ## Colors and Varieties The dominant color of gadolinite-(Y) is black, sometimes with a greenish or brownish tint. In thin splinters, it may appear translucent green or brown. There are no named color or commercial varieties. Color variation is minor and mainly depends on chemical impurities. ## History and Name The mineral's name honors the Finnish chemist and mineralogist Johan Gadolin, who in 1792 was the first to isolate yttrium oxide from a sample of this mineral, originating from a pegmatite in Ytterby, Sweden. The suffix "-(Y)" was added in 1987 by the International Mineralogical Association (IMA) to indicate yttrium (Y) as the dominant rare earth element in its chemical composition, distinguishing it from gadolinite-(Ce). ## Uses Due to its rarity and often small accumulations, gadolinite-(Y) has no industrial significance as a source of yttrium or other rare earth elements. Its primary use is as a collector's specimen and an object of scientific research in the fields of mineralogy and geochemistry.

Properties

Mohs hardness
6.5-7
Luster
Vitreous to Greasy
Streak
Greenish-grey
Density
4.0-4.7
Cleavage
Indistinct on {100}
Fracture
Conchoidal to Uneven
Transparency
Translucent to Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Characteristic features of gadolinite-(Y) include its black color, strong vitreous or resinous luster on a fresh fracture, and typical prismatic crystals. The mineral is relatively heavy and hard. It often occurs in association with other pegmatitic minerals, such as feldspars, quartz, micas, and other rare earth minerals. ## Distinguishing from Similar Minerals Black gadolinite-(Y) crystals can be confused with tourmaline (schorl), allanite, fergusonite, or samarskite. It differs from schorl by its higher specific gravity and the absence of the characteristic striations on prism faces typical of tourmaline. From allanite, which often has a similar habit and occurrence, it is usually distinguished by a more vitreous luster and a greenish-gray streak (allanite has a gray to brown streak). Fergusonite and samarskite typically have a higher specific gravity and are often more strongly radioactive. Definitive distinction requires advanced chemical analysis (EDS/WDS). ## Crystal Forms Gadolinite-(Y) crystals are monoclinic but often exhibit a pseudo-orthorhombic or pseudo-tetragonal appearance. They most commonly form simple, elongated prisms with an almost square cross-section, terminated by simple pyramidal faces. They also occur as granular aggregates, irregular masses, and as embedded, poorly formed crystals within the rock.

Geological environment

## Genesis Gadolinite-(Y) is a typical mineral of granitic and syenitic pegmatites, especially those enriched in rare earth elements. It forms in the late stages of magma crystallization, under conditions of high concentration of incompatible elements such as yttrium, beryllium, and other rare earth metals. It is less commonly found in alpine veins or as an accessory mineral in some granites. ## Mineral Associations This mineral often co-occurs with quartz (especially smoky quartz), microcline, albite, biotite, and muscovite. Its association with other rare earth minerals, such as allanite-(Ce), monazite, xenotime, fergusonite, euxenite, as well as fluorite, zircon, topaz, and beryl, is characteristic. ## Localities Historically, the most important locality is Ytterby in Sweden, where the mineral was discovered. Other significant occurrences worldwide include pegmatites in Norway (Iveland, Evje, Kragerø, Froland), USA (Texas - Baringer Hill; Colorado - White Cloud Pegmatite; Arizona), as well as in Russia (Urals), Kazakhstan, Japan, and Italy (Baveno).

Rarity

Not very common

For collectors

## Quality Criteria The most prized specimens by collectors are sharp, well-formed, freestanding crystals with smooth, lustrous faces. Specimens with a distinct pyramidal termination are highly valued. Crystals embedded in a contrasting rock matrix, such as light-colored feldspar or quartz, are also desirable. Size matters – large, multi-centimeter crystals are rare and sought after. Specimens from classic, historic localities, such as Ytterby or Norwegian pegmatites, have additional value. ## Popular Localities For collectors, the most desirable specimens come from classic Norwegian localities, such as Iveland and Evje, which have yielded many sharp, large crystals. Specimens from Baringer Hill in Texas (now submerged) and from pegmatites in Colorado are also famous. Historic specimens from Sweden (Ytterby) are rare on the market and primarily hold historical value.

Care and storage

## Cleaning Gadolinite-(Y) specimens should be cleaned carefully, using a soft brush and distilled water. Mild soap can be used, but it must be thoroughly rinsed off. For stubborn dirt, an ultrasonic cleaner can be used, but with caution, especially for specimens with cracks or on a fragile matrix. ## What to Avoid Avoid contact with strong acids, which can damage the mineral's surface. Despite its relatively high hardness, gadolinite is brittle and susceptible to mechanical impact. Prolonged exposure to direct sunlight is not recommended, although it does not cause fading. ## Storage Specimens are best stored in separate, padded boxes or display cases to prevent scratches and mechanical damage. Due to potential, though usually low, radioactivity, it is advisable to store larger or numerous specimens in a well-ventilated room and avoid keeping them in direct proximity to a permanent living area (e.g., on a desk or by the bed).

External references

Sources

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