allanite group

{{Infobox mineral | name = Allanite | category = Sorosilicates | image = Allanite-(Ce) - Mary Kathleen Mine, Mount Isa, Queensland, Australia.jpg | imagesize = 260 | alt = | caption = Allanite from the Mt. Isa – Cloncurry area, Queensland, Australia (scale bar 1 inch) | formula = (Ce,Ca,Y,La)2(Al,Fe+3)3(SiO4)3(OH) |IMAsymbol=Aln | molweight = | strunz = 9.BG.05b | dana = | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = P21/m | unit cell = a = 8.927, b = 5.761 c = 10.15 [Å]; β = 114.77°; Z = 2 | color = Brown to black | colour = | habit = Crystals tabular, prismatic to acicular; granular, massive; commonly metamict | twinning = Polysynthetic, common on {100} | cleavage = Imperfect to poor | fracture = Conchoidal to uneven | tenacity = Brittle | mohs = 5.5–6 | luster = Vitreous, resinous to submetallic | streak = Grey | diaphaneity = Translucent to opaque | gravity = 3.5–4.2 | density = | polish = | opticalprop = Biaxial (−) | refractive = nα = 1.715–1.791, nβ = 1.718–1.815, nγ = 1.733–1.822 | birefringence = δ = 0.018–0.031 | pleochroism = X = pale olive-green, reddish brown; Y = dark brown, brownish yellow; Z = dark reddish brown, greenish brown | 2V = Measured: 40° to 80° | dispersion = r > v; strong | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = 25px Radioactive if uranium and/or thorium-rich | prop1 = | prop1text = | references = }} Allanite (also called orthite) is a sorosilicate group of minerals within the broader epidote group that contain a significant amount of rare-earth elements. The mineral occurs mainly in metamorphosed clay-rich sediments and felsic igneous rocks. It has the general formula A2M3Si3O12[OH], where the A sites can contain large cations such as Ca2+, Sr2+, and rare-earth elements, and the M sites admit Al3+, Fe3+, Mn3+, Fe2+, or Mg2+ among others. However, a large amount of additional elements, including Th, U, Be, Zr, P, Ba, Cr and others may be present in the mineral. The International Mineralogical Association lists four minerals in the allanite group, each recognized as a unique mineral: allanite-(Ce), allanite-(La), allanite-(Nd), and allanite-(Y), depending on the dominant rare earth present: cerium, lanthanum, neodymium or yttrium.

thumb|left|upright|Allanite crystals on smoky quartz from the [[White Mountain Wilderness, Lincoln County, New Mexico, USA (size: 2.7 × 1.8 × 1.7 cm)]] Allanite contains up to 20% rare-earth elements and is a valuable source of them. The inclusion of thorium and other radioactive elements in allanite results in some interesting phenomena. Allanite often has a pleochroic halo of radiation damage in the minerals immediately adjacent. Also highly radioactive grains of allanite often have their structure disrupted or are metamict. The age of allanite grains that have not been destroyed by radiation can be determined using different techniques.