rubidium and gallium minerals

By John Betterton
Rubidium and Gallium are among the rarest of metals in the Earth’s crust

Rubidium (Rb) and gallium (Ga) are among the rarest of metals in the Earth’s crust with elemental abundances of about ...

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90ppm and about 18ppm respectively. Between 3 and 4 tonnes of Rb are extracted per year and Canada is the main producer. It has relatively few uses and is mostly used in physics research such as in lasers. Occasionally it has been used as a getter in vacuum tubes, where it mops up traces of oxygen, in purple coloured fireworks, in atomic clocks for GPS standards, positron emission tomography in medical applications and for the possible application as a thermoelectric generator.

Gallium is generally produced as a by-product of Al and Zn production as the few minerals containing it are too scarce. Annual production is estimated at about over 200 tonnes. The main producers are China, Germany, Kazakhstan, Ukraine, Hungary, Japan and the Republic of Korea. The semi-conductor industry dominates its use in numerous ways as in various LED’s, logic chips, diodes, lasers, in mobile phones, satellites, and in various low-melting alloys and pharmaceuticals etc. Because of their great rarity minerals containing these elements are few in number and hard to collect and collectors have seize their change when specimens appear. They are distributed among several different classes as with the oxides and silicates.

Gallite, CuGaS2

is a grey coloured sulphide mineral with a grey-black streak. It possesses a metallic lustre and a low hardness of 3.0 to 3.5. Gallite occurs as massive aggregates and as minute embedded grains, and as exsolution lamellae, up to 1 to 2 mm in size. The mineral crystallises in the tetragonal system. This species is to be found in base-metal vein deposits with a relatively high-gallium content. It is associated with renierite, germanite, bornite, chalcocite, digenite, pyrite, sphalerite, galena and tetrahedrite. Chemistry and x-rays are best for characterisation purposes. This uncommon mineral has been located at Tsumeb mine, Otjikoto Region, Namibia; Kipishi mine, Katanga, Democratic Republic of Congo; Radka mine, Levski, Pazardzhik Oblast and at the Chelopech mine, Chelopech, Sofiya Oblast, Bulgaria; and in the Pinar del Rio Province, Cuba.

Sohngeite, Ga(OH)3

is an alteration product of gallite-bearing germanite in oxidation zones. It is accompanied by germanite, gallite and tsumgallite. The mineral is orthorhombic with pseudocubic crystals forming twinned groups and aggregates.
Trillings with composite flattened to curved crystals also exist. It is white, pale yellow, pale brown or pale greenish yellow in colour and is translucent. The hardness is 4 to 4.5 and the specific gravity is 3.8. Chemistry, x-rays and mode of occurrence are used for evaluation. Specimens have been identified at the Tsumeb mine, Otjikoto Region, Namibia.

Tsumgallite, GaO(OH)

crystallises in the orthorhombic system as irregular scaly platelets and thin platy crystals. It is pale greenish yellow to beige in colour with a pearly lustre, and is translucent. A perfect cleavage is present and the hardness is low at between 1 and 2. It is soft with a white streak. Tsumgallite occurs in vugs in tennantite- germanite ores with sonhgeite and siderite. The Tsumeb mine, Otjikoto Region, Namibia is the only known site for this rare mineral.

Krieselite, (Al,Ga)2(GeO4)(OH)2

is a beige to white mineral with a greasy lustre. Krieselite is found as hemispherical aggregates and crusts. It is translucent with a white streak. The hardness ranges from 5.5 to 6.5, and the specific gravity is 4.0. This mineral crystallises in the orthorhombic system. X-rays and chemistry are needed here. It occurs in small vugs with tennantite, chalcocite, galena and germanite at the single location of Tsumeb mine, Otjikoto Region, Namibia.

Gallbeudantite,PbGa3(AsO4)(SO4) (OH)6

is an uncommon secondary arsenate mineral, developed in cavities in hydrothermal polymetallic ores deposits.
It is found with both common and very rare species like stolzite, silver, otjisumeite, beudantite, hidalgoite, scorodite, hematite, goethite, renierite, gallite, tennantite and chalcocite. It is hexagonal with small isolated rhombohedral crystals that are compositionally zoned. Also found as subparallel aggregates. The mineral is pale yellow, greenish to cream in colour with a vitreous lustre. An even to conchoidal fracture is present. It is brittle with a hardness of 4 and is transparent in character. The specific gravity is 4.8. Chemistry, x-rays and limited distribution aid in its identification.
Tsumeb mine, Otjikoto Region, Namibia is the only existing source of gallbeudantite.

Ramanite-(Rb),Rb[B5O6(OH)4]·2H2O


has been found as part of melt and fluid inclusions in graphic pegmatite quartz. It formed during the final stages of pegmatite formation and has an orthorhombic crystallography. This mineral is associated with sassolite and santite and is colourless and transparent. Traditional properties have not been reported due sample constraints and specialized techniques were needed to identify it. The mineral may be in fact quite common in boron-rich pegmatites. It is included here due to very few rubidium-bearing minerals currently known. Its type location is San Piero in Campo, Elba Island, Livorno Province, Italy.

Rubicline-(Rb), (Rb,K)AlSi3O8

is colourless and transparent with a vitreous lustre. It occurs as rounded grains with microcline, albite, muscovite, quartz and volishinite that cross cuts pollucite in granite pegmatities.
The mineral is triclinic and brittle with prefect and good cleavages. The specific gravity is calculated as 2.81. Rubicline is the first mineral to be discovered with essential rubidium and probably formed by exsolution processes with some later fluid-induced modifications. X-rays and chemical analysis are best for identification.
This rare species has been found the Red Cross Lake pegmatites, Manitoba, Canada; Luolamaki, Somero, Finland. Its type location is San Piero in Campo, Elba Island, Livorno Province, Italy.

Voloshinite Rb (LiAl1.5□1.5)(Al0.5Si3.5)O10F2

is found as rims up to 0.05 mm thick around the lepidolite series mineral. It is closely accompanied by pollucite and commonly muscovite, albite, quartz, rubicline-(Rb), spodumene, montebrasite and elbaite in rare-element granitic pegmatites.

Volishinite is a late-stage mineral formed after pollucite and fills polymineralic veinlets and pods. It’s colourless, transparent with a vitreous lustre. A good cleavage is present and flakes are flexible in character. The specific gravity is estimated as 2.9. X-ray and chemical studies are required for this species characterisation purposes. Specimens have been obtained from Vasin-Mylk Mt, Voroni Tundry, Kola Peninsula, Russia, the type location.

The minerals of indium will be discussed in the next instalment in this series.

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