Allanite
Encyclopedia
Allanite is a sorosilicate
group of minerals within the broader epidote
group that contain a significant amount of rare earth element
s. 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, Zr, P, Ba, Cr and others may be present in the mineral. The International Mineralogical Association
lists three minerals in the allanite group, each recognized as a unique mineral: allanite-(Ce), allanite-(La) and allanite-(Y), depending on the dominant rare earth present, cerium
, lanthanum
or yttrium
.
Allanite, also called orthite, 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.
Allanite is usually black in color, but can be brown, brown-violet. It is often coated with a yellow-brown alertation product, 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). The type locality is Aluk Island
, Greenland
where it was first discovered by Karl Ludwig Giesecke
.
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
group of minerals within the broader epidote
Epidote
Epidote is a calcium aluminium iron sorosilicate mineral, Ca2Al2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often...
group that contain a significant amount of rare earth element
Rare earth element
As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...
s. The mineral occurs mainly in metamorphosed clay rich sediments and felsic
Felsic
The word "felsic" is a term used in geology to refer to silicate minerals, magma, and rocks which are enriched in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium....
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, Zr, P, Ba, Cr and others may be present in the mineral. The International Mineralogical Association
International Mineralogical Association
The International Mineralogical Association is an international group of 38 national societies. The goal is to promote the science of mineralogy and to standardize the nomenclature of the 4000 plus known mineral species...
lists three minerals in the allanite group, each recognized as a unique mineral: allanite-(Ce), allanite-(La) and allanite-(Y), depending on the dominant rare earth present, cerium
Cerium
Cerium is a chemical element with the symbol Ce and atomic number 58. It is a soft, silvery, ductile metal which easily oxidizes in air. Cerium was named after the dwarf planet . Cerium is the most abundant of the rare earth elements, making up about 0.0046% of the Earth's crust by weight...
, lanthanum
Lanthanum
Lanthanum is a chemical element with the symbol La and atomic number 57.Lanthanum is a silvery white metallic element that belongs to group 3 of the periodic table and is the first element of the lanthanide series. It is found in some rare-earth minerals, usually in combination with cerium and...
or yttrium
Yttrium
Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and it has often been classified as a "rare earth element". Yttrium is almost always found combined with the lanthanides in rare earth minerals and is...
.
Allanite, also called orthite, contains up to 20% rare earth elements and is a valuable source of them. The inclusion of thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
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.
Allanite is usually black in color, but can be brown, brown-violet. It is often coated with a yellow-brown alertation product, likely limonite
Limonite
Limonite is an ore consisting in a mixture of hydrated iron oxide-hydroxide of varying composition. The generic formula is frequently written as FeO·nH2O, although this is not entirely accurate as limonite often contains a varying amount of oxide compared to hydroxide.Together with hematite, it has...
. It crystallizes in the monoclinic system and forms prismatic crystals. It has a Mohs hardness of 5.5–6 and a specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
of 3.5–4.2. It is also pyrognomic
Pyrognomic
Pyrognomic materials are easily made incandescent. Minerals and metals commonly glow when heated, but those that are pyrognomic glow at much lower temperatures. Allanite and gadolinite are examples of pyrognomic minerals...
, meaning that it becomes incandescent at a relatively low temperature of about 95 °C.
It was discovered in 1810 and named for the Scottish
Scottish people
The Scottish people , or Scots, are a nation and ethnic group native to Scotland. Historically they emerged from an amalgamation of the Picts and Gaels, incorporating neighbouring Britons to the south as well as invading Germanic peoples such as the Anglo-Saxons and the Norse.In modern use,...
mineralogist, Thomas Allan
Thomas Allan
The mineralogist Thomas Allan was born in Edinburgh, Scotland, on 17 July 1777 to a family of Scottish merchants and bankers. He took up banking for his profession, but he is remembered today for his contributions to mineral science...
(1777–1833). The type locality is Aluk Island
Aluk Island
Aluk Island, is an island off Prins Christian Sound, in the Kujalleq municipality in southern Greenland. It is the type locality for allanite a member of the epidote mineral group....
, Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
where it was first discovered by Karl Ludwig Giesecke
Karl Ludwig Giesecke
Karl Ludwig Giesecke was a German actor, librettist, polar explorer and mineralogist. In his youth he was called Johann Georg Metzler, in his later career in Ireland he was Sir Charles Lewis Giesecke.-Early life:His father was Johann Georg Metzler, a Protestant who worked as a tailor in Augsburg...
.