Allanite

{{short description|Rare-earth enriched sorosilicate mineral}} {{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<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref> | molweight = | strunz = 9.BG.05b | dana = | system = Monoclinic | class = Prismatic (2/m) <br/><small>(same H-M symbol)</small> | symmetry = ''P''2₁/m | unit cell = ''a'' = 8.927, ''b'' = 5.761 <br/>''c'' = 10.15&nbsp;[Å]; β = 114.77°; ''Z''&nbsp;=&nbsp;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 = ''n''_α = 1.715–1.791, ''n''_β = 1.718–1.815, ''n''_γ = 1.733–1.822 | birefringence = δ = 0.018–0.031 | pleochroism = ''X'' = pale olive-green, reddish brown;<br/> ''Y'' = dark brown, brownish yellow;<br/> ''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 = <ref name=HB>[http://rruff.geo.arizona.edu/doclib/hom/allanitece.pdf Allanite-(Ce)]. Handbook of Mineralogy</ref><ref name=Mindat>[http://www.mindat.org/min-125.html Allanite]. Mindat.org./</ref><ref name=Webmin>[http://webmineral.com/data/Allanite-(Ce).shtml Allanite]. Webmineral.</ref> }} '''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.<ref>{{cite journal|author =Dollase, W. A. |year=1971|title= Refinement of the crystal structure of epidote, allanite, and hancockite|journal=American Mineralogist|volume= 56|pages= 447–464|url=http://rruff.info/uploads/AM56_447.pdf}}</ref> 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.

[[File:Allanite-Quartz-39466.jpg|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.<ref>{{cite journal|author =Catlos, E. J. |author2=Sorensen, S. S. |author3=Harrison, T. M. |year=2000|url=http://sims.ess.ucla.edu/PDF/catlos_et_al_AMMIN_2000.pdf|journal=American Mineralogist|volume= 85|title=Th-Pb ion-microprobe dating of allanite|issue=5–6 |pages= 633–648|doi=10.2138/am-2000-5-601 |bibcode=2000AmMin..85..633C |s2cid=51734682 }}</ref>

Allanite is usually black in color, but can be brown or brown-violet. It is often coated with a yellow-brown alteration product,<ref>Klein, C., Dutrow, B. (2007) Manual of Mineral Science. Wiley Publishers, p. 500.</ref> likely limonite. It crystallizes in the monoclinic system and forms prismatic crystals. It has a Mohs hardness of 5.5–6 and a specific gravity of 3.5–4.2. It is also pyrognomic, meaning that it becomes incandescent at a relatively low temperature of about 95 °C.

It was discovered in 1810 and named for the Scottish mineralogist Thomas Allan (1777–1833).<ref name=HB/> The type locality is Aluk Island, Greenland,<ref name=Mindat/> where it was first discovered by Karl Ludwig Giesecke. {{clear left}}

== See also == * List of minerals * List of minerals named after people * Austrium * Dollaseite-(Ce)

==References== {{Reflist}}

Category:Calcium minerals Category:Lanthanide minerals Category:Cerium minerals Category:Neodymium minerals Category:Yttrium minerals Category:Aluminium minerals Category:Iron(II,III) minerals Category:Epidote group Category:Radioactive minerals Category:Monoclinic minerals Category:Minerals in space group 11 Category:Minerals described in 1810