Feldspar is one of the most impor­tant rock-form­ing min­er­als of the Earth’s crust, belong­ing to the class of sil­i­cates and con­sti­tut­ing about 60% of the crust. They occur in igneous, meta­mor­phic, and sed­i­men­ta­ry rocks in all parts of the world. They belong to potas­si­um, sodi­um, and cal­ci­um alu­mi­nosil­i­cates, form­ing com­plex iso­mor­phous series and vari­eties, among which the most well-known are ortho­clase, micro­cline, albite, and anor­thite.

Most of the Earth’s con­ti­nen­tal crust con­sists of igneous rocks such as gran­ite, dior­ite, and gra­n­odi­or­ite. Feldspars are also impor­tant com­po­nents of gab­bro and basalt, which are the main types of rocks in the ocean­ic crust of the Earth. As igneous rocks weath­er and meta­mor­phose, feldspar min­er­als become com­po­nents of sed­i­men­ta­ry and meta­mor­phic rocks.

The name feldspar is asso­ci­at­ed, on the one hand, with the Greek “spate” plate, and on the oth­er with con­stant finds in the field. The term itself has been used since the mid­dle of the 18th cen­tu­ry.

Feldspar 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 а raw mate­ri­als are glass and for­for-faience.

Physical properties

Feldspar min­er­als have a vari­ety of phys­i­cal prop­er­ties that make them eas­i­ly rec­og­niz­able and use­ful for a vari­ety of appli­ca­tions. Some of the main phys­i­cal char­ac­ter­is­tics are:

• Col­or: Feldspar min­er­als occur in a wide range of col­ors, includ­ing white, pink, red, orange, yel­low, green, blue, and gray. The spe­cif­ic col­or depends on the feldspar type and the pres­ence of impu­ri­ties or inclu­sions.
• Lus­ter: Feldspar typ­i­cal­ly has a vit­re­ous (glassy) or pearly lus­ter, giv­ing it a shiny appear­ance when pol­ished.
• Hard­ness: Feldspar has a hard­ness of 6–6.5 on the Mohs scale, mak­ing it quite hard and durable, suit­able for a vari­ety of indus­tri­al appli­ca­tions.
• Cleav­age: Feldspar min­er­als have two direc­tions of per­fect cleav­age that inter­sect or near­ly inter­sect at an angle of 90 degrees. This char­ac­ter­is­tic cleav­age pat­tern is use­ful for iden­ti­fy­ing feldspar in field con­di­tions and hand spec­i­mens.
• Trans­paren­cy: Feldspar can be trans­par­ent, translu­cent or opaque, depend­ing on the spe­cif­ic min­er­al and the pres­ence of impu­ri­ties or inclu­sions.

One of the most strik­ing fea­tures of some feldspar min­er­als, espe­cial­ly labradorite and moon­stone, is their col­or play, or Schiller effect. This opti­cal phe­nom­e­non, known as labradores­cence or adu­lares­cence, pro­duces a remark­able dis­play of col­ors when light inter­acts with the inter­nal struc­ture of the crys­tal.

Formation of feldspar

Feldspar min­er­als form in var­i­ous geo­log­i­cal envi­ron­ments, includ­ing igneous, meta­mor­phic, and sed­i­men­ta­ry rocks. They crys­tal­lize from mag­ma in both intru­sive and extru­sive igneous rocks and are also impor­tant in meta­mor­phic rocks such as gneiss and shale. Feldspar is also found in sed­i­men­ta­ry rocks such as sand­stones and con­glom­er­ates.

The feldspar group is divid­ed into two main sub­groups: alka­line feldspars and pla­gio­clase feldspars. Alka­line feldspars, which include min­er­als such as ortho­clase, micro­cline, and sani­dine, are rich in potas­si­um and sodi­um. Pla­gio­clase feldspars, which include min­er­als such as albite, anor­thite, and labradorite, are rich in cal­ci­um and sodi­um. Cel­sine (potassium–barium) feldspars are very rare; some of these min­er­als have a del­i­cate cream col­or and there­fore have col­lec­tor val­ue.

Pla­gio­clases, main­ly cal­ci­um, are the main rock-form­ing min­er­als of igneous and many meta­mor­phic rocks. In igneous rocks, pla­gio­clases rich in the anor­thite com­po­nent (An) crys­tal­lize first, and then more acidic (sil­i­ca-rich) vari­eties are formed. In such cas­es, zon­al crys­tals are often formed. Some igneous rocks con­sist almost entire­ly of pla­gio­clase (anorthosite and oth­ers). In peg­matite veins, albite is often found, which is formed due to the recrys­tal­liza­tion of oth­er pla­gio­clases, espe­cial­ly sodi­um-con­tain­ing and potas­si­um feldspars. In hydrother­mal con­di­tions and dur­ing weath­er­ing, pla­gio­clase changes to kaolin min­er­als and sericite. Pla­gio­clases rich in the anor­thite com­po­nent break down faster than acidic vari­eties; albite is more resis­tant.

Potas­si­um feldspars are the main rock-form­ing min­er­als of acidic igneous rocks (gran­ites, syen­ites, gra­n­odi­or­ites, etc.), as well as some wide­spread meta­mor­phic rocks (gneiss­es). Low-tem­per­a­ture micro­cline pre­vails in gneiss­es, while ortho­clase is present in plu­ton­ic-type igneous rocks, and sani­dine in vol­canic rocks.

Ortho­clase and micro­cline, togeth­er with quartz and mus­covite, are the main min­er­als of peg­matites. If beryl is present in them, the micro­cline can be enriched with beryl­li­um, which, like alu­minum, is able to replace sil­i­con atoms. Peg­matites are char­ac­ter­ized by the ger­mi­na­tion of ortho­clase (micro­cline) with quartz, known as “writ­ten gran­ite”, which is a prod­uct of recrys­tal­liza­tion of the eutec­tic mag­mat­ic melt.

Com­pared to pla­gio­clases, potas­si­um feldspars are more resis­tant to destruc­tion, but can be replaced by albite, form­ing “meta­so­mat­ic perthite”. In hydrother­mal con­di­tions and dur­ing weath­er­ing, they change to min­er­als of the kaolin group.

Well-known deposits of potas­si­um feldspars in Nor­way, Swe­den, Mada­gas­car, on the ter­ri­to­ry of the Ilmen Reserve and in many peg­matite man­i­fes­ta­tions of the South­ern Urals. They are also found in Maine (USA) and oth­er regions.

Sources and deposits of feldspar raw materials

The most impor­tant sources of feldspar raw mate­ri­als are granitic peg­matites, gran­ites, nepheline syen­ites, acidic vol­canic rocks, felds­path­ic sands and sand­stones, weath­er­ing crusts, alka­line kaolins, and waste from kaolin pro­cess­ing. Tra­di­tion­al­ly, the great­est impor­tance has been attrib­uted to peg­matites, which have long been exploit­ed due to their large crys­tals that are easy to sep­a­rate. Despite a decrease in their share in min­ing, they remain sig­nif­i­cant, espe­cial­ly the so-called “ceram­ic peg­matites”, known in many regions of the world — in Amer­i­ca, Africa, Asia, Aus­tralia, and Europe.

Alka­line kaolins, which are formed as a result of meta­so­mat­ic trans­for­ma­tions of intru­sive rocks and are con­sid­ered a promis­ing source of high-potas­si­um raw mate­ri­als, also play a major role. In the process of their for­ma­tion, impu­ri­ties harm­ful to ceram­ics are removed, and potas­si­um accu­mu­lates. Such rocks (micro­cline-con­tain­ing kaolins) are of high qual­i­ty and are suit­able for the pro­duc­tion of charges.

In addi­tion to peg­matites and kaolins, var­i­ous gran­i­toids and alka­line rocks, both fresh and altered (albitites, aplites, nepheline syen­ites, etc.), are also of impor­tance. Known deposits include Takob (Tajik­istan), Rezhik (Ural), Asko­ran (Kaza­khstan), Lyan­gar (Uzbek­istan), Kyre-Key (Turkey), Blue Moun­tain (Cana­da), the Kola Penin­su­la (Rus­sia), Sorne Island (Nor­way), and oth­ers. Kaolin­ized intru­sive rocks are also devel­oped in Great Britain, Poland, and Rus­sia. Reserves are clas­si­fied by scale into unique, large, medi­um, small, and very small.

Feldspar reserves in Ukraine

Ukraine has sig­nif­i­cant poten­tial in feldspar raw mate­ri­als; how­ev­er, it ranks only 39th in the world in terms of pro­duc­tion.

The reserves of feldspar raw mate­ri­als in Ukraine amount to 9.1 mil­lion tonnes (based on 8 deposits). Cur­rent­ly, 3 peg­matite deposits (in the Zhy­to­myr, Zapor­izhzhia, and Rivne regions) and 1 micro­gra­n­odi­or­ite deposit (in Tran­scarpathia) are under indus­tri­al devel­op­ment, with an annu­al pro­duc­tion of about 13 thou­sand tonnes.

The main geo­log­i­cal and indus­tri­al types of deposits include granitic peg­matites, kaolins, gran­i­toids, nepheline syen­ites, felds­path­ic sands and sand­stones, as well as non-tra­di­tion­al and techno­genic sources of raw mate­ri­als. In total, about 20,000 peg­matite occur­rences have been iden­ti­fied with­in 67 fields on the Ukrain­ian Shield.

Use of raw materials

Feldspar and peg­matite are wide­ly used in the ceram­ic, porce­lain and faience, glass, elec­tri­cal engi­neer­ing, and abra­sive indus­tries, as well as in the pro­duc­tion of elec­trodes. The largest con­sumers are the ceram­ic indus­try (over 40%) and the glass indus­try (over 50%).

In ceram­ics, feldspar pro­motes the sin­ter­ing of mass­es, while for elec­tro­ce­ram­ics and porce­lain, coarse-grained micro­cline peg­matites are main­ly used. The strictest require­ments are applied to raw mate­ri­als used for insu­la­tors.

In glass­mak­ing, potas­si­um feldspars are required: they increase vis­cos­i­ty and chem­i­cal resis­tance of glass and enhance its lus­ter. Al₂O₃ reduces ther­mal expan­sion and increas­es strength, while iron is con­sid­ered a harm­ful impu­ri­ty.

The raw mate­r­i­al is also used in the pro­duc­tion of enam­els, water­ings, and abra­sives. Potas­si­um feldspars (micro­cline, ortho­clase) are the most suit­able, as they melt well and form dense porce­lain. Unde­sir­able impu­ri­ties include iron-bear­ing min­er­als, which reduce white­ness and insu­lat­ing prop­er­ties.

In gen­er­al, the glass indus­try con­sumes up to 2/3 of glob­al feldspar resources, while the rest is used in ceram­ics. Annu­al glob­al con­sump­tion exceeds 3 mil­lion tonnes, with about half account­ed for by the USA.