Anorthosite is a leu­co­crat­ic igneous rock of the gab­bro group, com­posed almost entire­ly of basic and medi­um pla­gio­clase with a small amount of maf­ic min­er­als (such as olivine, pyrox­ene, mag­netite, or titano­mag­netite). It occurs in large mas­sifs and dikes. It typ­i­cal­ly has a dense, mas­sive tex­ture, and its col­or ranges from light gray to near­ly black.

The first sci­en­tif­ic descrip­tion of an anorthosite deposit was made in 1863 by the chemist and min­er­al­o­gist T. S. Hunt in the Amer­i­can Jour­nal of Sci­ence and Arts. In Ukraine, the devel­op­ment of a vari­ety of anorthosite—labradorite—began in 1835 in Volyn at the quar­ries of Kami­anyi Brid and Slo­bid­ka. The Holovynske labradorite deposit began to be devel­oped much lat­er, in 1900.

Anorthosite is includ­ed in the list of min­er­als of nation­al impor­tance, approved by Res­o­lu­tion of the Cab­i­net of Min­is­ters of Ukraine No. 827 of Decem­ber 12, 1994, as a raw mate­r­i­al for fac­ing mate­ri­als (dec­o­ra­tive stone) and as a raw mate­r­i­al for rub­ble stone and crushed stone..

Origin

On Earth, typ­i­cal near­ly pure anorthosite mas­sifs have been iden­ti­fied only with­in ancient cra­tons (Ukraine, Cana­da, Fin­land), and they are more than 1 bil­lion years old.

Although anorthosites are con­sid­ered igneous rocks, no com­plete ana­logues of vol­canics of this chem­i­cal com­po­si­tion have been found in nature. Some high-alu­mi­na basalts are some­what sim­i­lar to them.

The largest anorthosite mas­sifs (cov­er­ing areas of about or more than 1,000 km²) are spa­tial­ly and, accord­ing to most researchers, genet­i­cal­ly asso­ci­at­ed with large so-called anorthosite–rapakivi gran­ite plutons—as sep­a­rate inde­pen­dent deep mag­mat­ic bod­ies. Anorthosites also occur as lay­ers of vary­ing thick­ness with­in dif­fer­en­ti­at­ed, pre­dom­i­nant­ly gab­broic and gab­bro-peri­dotite mas­sifs, for exam­ple in Bushveld (South Africa), Still­wa­ter (Mon­tana, USA), Skaer­gaard (Green­land), and Monchegorsk (Kola Penin­su­la). Such anorthosites are some­times called autonomous.

In Ukraine, anorthosite deposits are asso­ci­at­ed with the Korosten and Korsun–Novomyrhorod plu­tons. There are a num­ber of quar­ries in the Zhy­to­myr and Cherkasy regions where anorthosites capa­ble of iri­des­cence—labradorites—are extract­ed.

Chemical composition

The rock is formed by more than 90% pla­gio­clase (most­ly the main com­po­si­tion). Sec­ondary and acces­so­ry min­er­als of anorthosites ― orthopy­rox­ene, less often olivine, clinopy­rox­ene, biotite, ilmenite, mag­netite, sul­fides, quartz and its sub­graph­ic growth with cal­ispate (gra­nophyre), apatite, zir­con.

The name comes from anor­thite ― cal­ci­um pla­gio­clase CaAl2Si2O8. But in nat­ur­al rocks called anorthosites, pla­gio­clase most often has an inter­me­di­ate com­po­si­tion, in which the con­tent of the anor­thite com­po­nent is 40–60 %, less often 70–80 %. Only on the Moon are anorthosites com­posed of anor­thite itself.

Physical properties

Anorthosites are typ­i­cal­ly char­ac­ter­ized by a coarse-grained (0.5–3.0 cm) or peg­matitic struc­ture, with pla­gio­clase crys­tals that may reach sizes of 10 cm or more. Indi­vid­ual pla­gio­clase crys­tals are black, white, blue or gray, often show­ing iri­des­cence — a change in sur­face col­or depend­ing on the angle of obser­va­tion or illu­mi­na­tion. Most often, a blue col­or is observed, less often green, some­times yel­low-orange.

In mod­ern lit­er­a­ture, all pre­dom­i­nant­ly pla­gio­clase rocks are clas­si­fied as anorthosites with pos­si­ble addi­tion­al descriptors—labradorite, ande­sine, bytown­ite, etc.

Fea­tures of the rock: extreme reli­a­bil­i­ty, resis­tance to mois­ture, cor­re­sponds to the 1st class of radi­a­tion safe­ty (which indi­cates min­i­mal radioac­tiv­i­ty of the mate­r­i­al), dura­bil­i­ty, resis­tance to high and low tem­per­a­tures, as well as to their dif­fer­ences.

Anorthosite reserves in Ukraine

Ukraine con­tains about 5% of the world’s reserves of dec­o­ra­tive stone, despite occu­py­ing only 0.4% of the Earth’s land area. Near­ly one-third of this ter­ri­to­ry (approx­i­mate­ly 200,000 km²) belongs to the Ukrain­ian Shield, where deposits of unique­ly col­ored gran­ites, dior­ites, labradorites, gab­bro, anorthosites, and oth­er vari­eties of high-qual­i­ty dec­o­ra­tive stone are con­cen­trat­ed.

Among all min­er­al resources in Ukraine, non-metal­lic min­er­als are the most wide­spread. This is due both to their sig­nif­i­cant diver­si­ty in ori­gin and to the large num­ber of known and already devel­oped deposits.

Rocks of the gabbro–anorthosite com­plex are most exten­sive­ly devel­oped in the Zhy­to­myr region and are asso­ci­at­ed with the Korosten Plu­ton, locat­ed in the north­west­ern part of the Ukrain­ian Shield. The area of dis­tri­b­u­tion of these rocks cov­ers about 800 km².

The Korosten Plu­ton is one of the largest intru­sive rock mas­sifs in the north­west­ern Ukrain­ian Shield. The main rocks (occu­py­ing about a quar­ter of the area)—gabbro, gab­bronorites, gab­bro-anorthosites, and labradorites—form two large and sev­er­al small­er mas­sifs. Pri­ma­ry and plac­er deposits of ilmenite and apatite are asso­ci­at­ed with these gab­broids. In Ukraine, accord­ing to the “Geoin­form of Ukraine” DNVP, the bal­ance reserves of explored anorthosite deposits amount to 15,248,446 thou­sand m3. Anorthosite deposits in Ukraine are rep­re­sent­ed by the fol­low­ing deposits: Torchynske, Andri­ivske, Kami­anobridske, Niki­tivske and Lukovetske (Zhy­to­myr region). Cur­rent­ly, anorthosite is extract­ed only at three of these deposits: Torchynske, Niki­tivske, and Lukovetske.

The main eco­nom­ic val­ue of anorthosite bod­ies is tita­ni­um-con­tain­ing ilmenite oxide. How­ev­er, some Pro­tero­zoic anorthosite bod­ies con­tain large amounts of labradorite, which is mined for its val­ue both as a gem­stone and as a build­ing mate­r­i­al. It is used as a fac­ing mate­r­i­al in con­struc­tion inside and out­side premis­es (fire­places, columns, win­dow sills, coun­ter­tops). It can be used to make var­i­ous fig­ures and fakes (vas­es, devices).

Archean anorthosites con­tain high amounts of alu­minum replac­ing sil­i­con; some of these bod­ies are mined as alu­minum ores. Anorthosite was wide­ly rep­re­sent­ed in rock sam­ples brought from the Moon and is impor­tant in stud­ies of Mars, Venus, and mete­orites.