Migmatite is a com­plex rock that forms as a result of the intru­sion of mag­ma into pre-exist­ing rocks. It con­sists of meta­mor­phic rocks and main­ly gran­ite mate­r­i­al. It has a beau­ti­ful con­trast­ing tex­ture with inter­twined, sin­u­ous or lay­ered alter­na­tion of rocks. It is a type of rock that exhibits both sol­id phase and par­tial melt­ing char­ac­ter­is­tics.

Migmatite con­sists of two or more com­po­nents, usu­al­ly repeat­ed­ly lay­ered: one is rep­re­sent­ed by an old­er meta­mor­phic rock trans­formed by par­tial melt­ing (“pale­o­so­ma”), and the oth­er has a peg­matite, aplite, gran­ite or gen­er­al­ly plu­ton­ic char­ac­ter (“neo­so­ma”).

Migmatite 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 rub­ble stone and crushed stone.

The mineral composition

The min­er­al com­po­si­tion of migmatites is char­ac­ter­ized by con­sid­er­able diver­si­ty. In light-col­ored vari­eties of the rock, pla­gio­clase pre­dom­i­nates over potas­si­um feldspar, where­as in micro­cline- or ortho­clase-rich migmatites the sit­u­a­tion is reversed—the main min­er­al becomes potas­si­um feldspar. Despite the high degree of meta­mor­phism, the struc­ture retains lay­er­ing, which allows migmatites to be clear­ly clas­si­fied as meta­mor­phic rocks.

Migmatites occur in the form of veins, some­times as fold­ed lay­ers or irreg­u­lar inclu­sions of granitic nature enriched in sil­i­ca. These light-col­ored lay­ers con­trast with dark­er rocks enriched in iron and mag­ne­sium, cre­at­ing the char­ac­ter­is­tic “var­ie­gat­ed” appear­ance of migmatites with alter­nat­ing dark and light bands.

The min­er­alog­i­cal com­po­si­tion reflects the con­di­tions of amphi­bo­lite-facies meta­mor­phism, which is char­ac­ter­ized by very high tem­per­a­tures and pres­sures, as well as a clear­ly expressed direc­tion­al­i­ty of struc­tur­al ele­ments.

Formation of migmatite

Migmatites form under extreme con­di­tions of tem­per­a­ture and pres­sure dur­ing pro­gres­sive meta­mor­phism, when par­tial melt­ing occurs in the meta­mor­phic pale­o­some. The com­po­nents pro­duced as a result of par­tial melt­ing are called neo­somes (mean­ing “new body”). Migmatites often appear as dense­ly arranged veins. They form clus­ters of leuko­somes, light gran­ite com­po­nents dis­solved inside the melanosome, dark envi­ron­ments rich in amphi­bole and biotite.

For more than a cen­tu­ry, there has been debate among petrol­o­gists about the ori­gin of migmatites. The most wide­spread ver­sion explains their for­ma­tion by par­tial melt­ing of the earth­’s crust dur­ing extreme meta­mor­phism (ana­tex­is). In the process of decom­po­si­tion of hydrous min­er­als (mus­covite, biotite, amphi­bole), water is released, which con­tributes to the for­ma­tion of a gran­ite melt that seeps into meta­mor­phic rocks.

Oth­er researchers believe that migmatites form by injec­tion, when granitic mag­ma pen­e­trates into meta­mor­phic rocks, or as a result of gran­i­ti­za­tion—the replace­ment of min­er­al com­po­si­tion under the influ­ence of flu­ids. In such cas­es, biotite, quartz, and potas­si­um feldspar are formed instead of pla­gio­clase.

Migmatites may form near large intru­sions, where mag­ma is inject­ed into meta­mor­phic stra­ta, or as a result of com­bined par­tial melt­ing and recrys­tal­liza­tion. Their struc­ture varies from clear­ly sep­a­rat­ed lay­ers to indis­tinct bound­aries, and in cas­es of com­plete mix­ing of min­er­als, the rock los­es its tex­ture and resem­bles gran­ite.

Thus, there are sev­er­al mech­a­nisms for the for­ma­tion of migmatites that reflect the tran­si­tion­al nature of these rocks between meta­mor­phism and mag­ma­tism.

Types of migmatites

Migmatites can be clas­si­fied into dif­fer­ent types depend­ing on their min­er­alog­i­cal com­po­si­tion, degree of par­tial melt­ing, and oth­er spe­cif­ic characteristics.Here are some com­mon types of migmatites:

• Gran­ite migmatite: This type of migmatite has a sig­nif­i­cant leu­co­some com­posed of granitic min­er­als such as quartz, feldspar (ortho­clase and/or pla­gio­clase), and mica. The granitic leu­co­some forms char­ac­ter­is­tic veins or lay­ers with­in the dark­er melanosome, which may con­tain maf­ic min­er­als.
• Migmatite gneiss: Migmatite gneiss is char­ac­ter­ized by the pres­ence of both meta­mor­phic gneiss and migmatite com­po­nents. The gneiss part retains a well-devel­oped lay­ered struc­ture, while the migmatite com­po­nent includes bands or veins of the leuko­some.
• Migmatite shale: Sim­i­lar to migmatite gneiss, migmatite shale con­sists of both meta­mor­phic shale and migmatite..
• Maf­ic migmatite: In some migmatites, the melanosome can be rep­re­sent­ed main­ly by maf­ic min­er­als such as biotite and amphi­bole.
• Peg­matite migmatite: Peg­matite migmatites exhib­it a peg­matite tex­ture in the leuko­some char­ac­ter­ized by the pres­ence of large crys­tals in the fine-grained matrix
• Amphi­bo­lite migmatite: Amphi­bo­lite migmatites are char­ac­ter­ized by the pres­ence of an amphi­bole in the melanosome.
• Gar­net-con­tain­ing migmatite: Some migmatites con­tain gar­net in either the melanosome or the leu­co­some.
• Mixed Min­er­al Migmatite: Some migmatites may have a mix­ture of both fel­sitic and maf­ic min­er­als in both the leu­co­some and melanosome.
• Cal­ci­um sil­i­cate migmatite: Under cer­tain geo­log­i­cal con­di­tions, migmatites may con­tain cal­ci­um-sil­i­cate min­er­als such as wol­las­tonite and diop­side, in addi­tion to fel­sitic and maf­ic com­po­nents. These migmatites are often formed in car­bon­ate-rich rocks under­go­ing meta­mor­phism.

Accord­ing to the tex­ture, the fol­low­ing are dis­tin­guished: striped, lentic­u­lar-striped, spec­ta­cled, metablas­tic, por­phy­rob­las­tic and brec­cia-like migmatites.

Distribution of the mineral

Com­mon among meta­mor­phic com­plex­es of the crys­talline base­ment, where they can make up large areas. They form large bod­ies, com­bin­ing with gran­ites, gneiss­es and gran­i­togneiss­es. Migmatites are wide­spread in areas of ancient crys­talline shields and fold­ed struc­tures, where high tem­per­a­tures and pres­sures pre­vail, favor­able for par­tial melt­ing of rocks.

Sig­nif­i­cant mas­sifs of migmatites are found on the ter­ri­to­ry of the Ukrain­ian crys­talline shield, in par­tic­u­lar in the Zhy­to­myr, Kyiv, Kirovohrad and Dnipropetro­vsk regions. In Europe, migmatites are dis­trib­uted with­in the Baltic Shield, which includes Fin­land, Swe­den, Kare­lia, and Esto­nia, as well as in Cen­tral Europe, par­tic­u­lar­ly in the Czech Repub­lic and Ger­many. In North Amer­i­ca, migmatites form exten­sive com­plex­es of the Cana­di­an Shield, which extends across the provinces of Ontario and Que­bec. In addi­tion, migmatites are wide­spread in Africa, par­tic­u­lar­ly with­in the Pre­cam­bri­an shields of East­ern and South­ern Africa, as well as in India and Brazil, where they form large geo­log­i­cal com­plex­es that play an impor­tant role in the for­ma­tion of the con­ti­nen­tal crust.

Uses of migmatite

In prac­ti­cal appli­ca­tions, migmatite is char­ac­ter­ized by high strength, resis­tance to phys­i­cal weath­er­ing, and chem­i­cal cor­ro­sion, which makes it a valu­able con­struc­tion and dec­o­ra­tive mate­r­i­al. In the con­struc­tion indus­try, it is used for facade cladding, the pro­duc­tion of stair ele­ments, mon­u­ments, plinths, and oth­er archi­tec­tur­al ele­ments, where dura­bil­i­ty and aes­thet­ic appear­ance are impor­tant.

In indus­try, migmatite is used in the form of crushed stone as a strong aggre­gate in con­crete, as well as in road con­struc­tion. Due to its char­ac­ter­is­tic striped or mot­tled tex­ture due to the alter­na­tion of leuko­some and melanosome, it is wide­ly used in dec­o­ra­tive mason­ry, in par­tic­u­lar for the man­u­fac­ture of coun­ter­tops, floor slabs, win­dow sills and inte­ri­or dec­o­ra­tion pan­els.