Allanite
Allanite-(Ce) (also called orthite) is a rare member of the Epidote Group of minerals that includes Allanite, Clinozoisite, Epidote, Hancockite, Mukhinite, Piedmontite, Tanzanite, Thulite and Zoisite. Allanite is very rarely faceted and is usually metamict due to the presence of Cerium (Ce) in its chemical composition. Metamict is the break down of the internal crystal structure by rare earth and radioactive elements. Allanite is usually opaque black but may also be found as light to dark brown or greenish brown crystals. The black, opaque crystals are usually larger and the transparent brownish crystals are usually too small for faceting. Allanite is found in many locations worldwide.
Widespread; some localities affording good crystals or rich material follow. From Qáqarssuatsiaq, Alluk, south Greenland. At Ytterby and Finbo, Sweden. From Kragerö, Arendal, and on Hitterö Island, Flekkefjord, Norway. From Sillböle, Stansvik, and Vaarala, Finland. At Miass, Ilmen Mountains, Southern Ural Mountains, Russia. From the Trimouns talc deposit, six km northeast of Luzenac, Ariμege, France. In the USA, at Pacoima Canyon, Los Angeles County, California; in the Baringer Hill pegmatite, 26 km west of Burnet, and on the Rode Ranch, Llano County, Texas. In Canada, from Olden Township, Frontenac County, Quebec; in the McDonald mine, Hybla, Ontario; and at Hoidas Lake, near Nisikkatch Lake, Saskatchewan. From near Telixtlahuaca, Oaxaca, Mexico. In the Mary Kathleen mine, Queensland, and at Broken Hill, New South Wales, Australia. From Ribuae, Mozambique. ran.
| Chemical Formula: | (Ca;Ce)2(Al;Fe2+;Fe3+)3(SiO4)(Si2O7)O(OH) |
| Calcium Cerium Yttrium Aluminum Iron Silicate Hydroxide | |
| Molecular Weight: | 614.71 gm |
| Composition: | Calcium | 3.91 % | Ca | 5.47 % | CaO |
| Cerium | 27.35 % | Ce | 32.04 % | Ce2O3 | |
| Yttrium | 4.34 % | Y | 5.51 % | Y2O3 | |
| Aluminum | 9.88 % | Al | 18.66 % | Al2O3 | |
| Iron | 6.81 % | Fe | 9.74 % | Fe2O3 | |
| Silicon | 13.71 % | Si | 29.32 % | SiO2 | |
| Hydrogen | 0.16 % | H | 1.47 % | H2O | |
| Oxygen | 33.84 % | O | |||
| 100.00 % | 100.00 % | = TOTAL OXIDE |
| Crystallography: | Monoclinic – Prismatic |
| Crystal Habit: | Crystals tabular, or prismatic to acicular, to 90 cm; granular, massive. |
| Twinning: | Polysynthetic, common on [100] |
| Cleavage: | Imperfect on [001], Poor on [100], Poor on [110] |
| Fracture: | Irregular/Uneven, Conchoidal |
| Tenacity: | Brittle |
| Moh’s Hardness: | 5.5 – 6.0 |
| Density: | 3.50 – 4.20 (g/cm3) |
| Luminescence: | None |
| Radioactivity: | Weak; GRapi = 27,438.55 (Gamma Ray American Petroleum Institute Units) |
| Color: | Black, dark Brown, dark Reddish Brown, dark Greenish Brown |
| Transparency: | Translucent to Opaque |
| Luster: | Vitreous, Resinous to Sub-Metallic |
| Refractive Index: | 1.715 – 1.822 Biaxial ( – ); Metamict Allanites are isotropic with N as low as 1.61 |
| Birefringence: | 0.018 – 0.031 |
| Dispersion: | Distinct to Strong; r > v |
| Pleochroism: | X = pale olive-green, reddish brown; Y = dark brown, brownish yellow; Z = dark reddish brown, greenish brown |
