Clays. Descrip­tion, vari­eties and dis­tri­b­u­tion

General information

Clay is a weak­ly cement­ed, fine-grained sed­i­men­ta­ry rock that is pow­dery when dry and plas­tic when moist­ened. Clays are very com­mon, com­pris­ing about 50% of all sed­i­men­ta­ry rocks in the Earth­’s crust. Clay-rich rocks are rocks that con­tain more than 50% of the pel­let frac­tion of sed­i­men­ta­ry or aeo­lian ori­gin.

Mineral composition

Clay min­er­als are salts of alu­minum and sil­i­cat. In addi­tion to these, clay also con­tains water mol­e­cules. The ratio of com­po­nents can vary depend­ing on the place of extrac­tion. Sil­i­cat oxide accounts for approx­i­mate­ly 45–50%, alu­minum oxide (alu­mi­na) accounts for about 40–45%, and water accounts for up to 15%. The min­er­al com­po­si­tion is main­ly rep­re­sent­ed by hydrosil­i­cates, kaolin, chlo­rite, and mont­mo­ril­lonite, but oth­er min­er­als such as andalusite, dis­thene, sil­li­man­ite, gal­lu­cite, hydroargillite, corun­dum, mus­covite, dias­pore, pyro­phyl­lite, and oth­ers are also present.

Clays may con­tain pig­ments, chro­mophores, includ­ing iron, tita­ni­um, mag­ne­sium, cop­per, nick­el, and chromi­um oxides. These give the mate­r­i­al dif­fer­ent shades, from yel­low and brown to blue, red, and even black. About 50% of clay par­ti­cles have a diam­e­ter of up to 0.01 mm, and over 25% have a diam­e­ter of up to 0.001 mm. The shape of the grains in the mate­r­i­al is flat, resem­bling scales. The par­ti­cles have a neg­a­tive charge and are attract­ed to pos­i­tive water mol­e­cules. This is one of the rea­sons for the hygro­scop­ic­i­ty of clay, its abil­i­ty to absorb and accu­mu­late liq­uids.

The min­er­al com­po­si­tion large­ly affects the qual­i­ta­tive char­ac­ter­is­tics of clay and, accord­ing­ly, its appli­ca­tion. For exam­ple, hydrosil­i­cate clays are char­ac­ter­ized by low plas­tic­i­ty but can increase in vol­ume by 9 times in water. They con­tain many impu­ri­ties of frag­men­tary rocks. Kaolin vari­eties are main­ly com­posed of kaolin and are char­ac­ter­ized by high plas­tic­i­ty. They are most com­mon­ly used in ceram­ics pro­duc­tion. Mont­mo­ril­lonite vari­eties have pro­nounced sorp­tion prop­er­ties.

Qualitative characteristics of clays

Dis­per­sion is the degree of frag­men­ta­tion of a rock. Based on the degree of dis­per­sion, clays are divid­ed into:
High-dis­per­sion, or fine-dis­per­sion (con­tain 85% par­ti­cles small­er than 10 microns or 60% — up to 1 micron)
Dis­per­sion (par­ti­cles up to 10 microns make up 40–85% or up to 1 micron — 20–60%)
Coarse-dis­per­sion (less than 40% of mate­r­i­al con­sists of par­ti­cles with a diam­e­ter of up to 10 microns or up to 20% with a diam­e­ter of up to 1 micron).

Plas­tic­i­ty is the prop­er­ty of a rock to change its shape and size irre­versibly with­out destruc­tion. Based on plas­tic­i­ty, clays are divid­ed into:
High-plas­tic (above 25) — heavy or fat­ty clays
Medi­um-plas­tic (15–25) — light or lean clays
Mod­er­ate­ly plas­tic (7–15) — loams
Low plas­tic (less than 7) — shales or sands.
Clays with high plas­tic­i­ty are used for the pro­duc­tion of ceram­ics and sculp­tures, while those with medi­um and low plas­tic­i­ty are used for bricks and tiles.

Refrac­tori­ness is the abil­i­ty of rocks to resist high tem­per­a­tures with­out melt­ing. Based on refrac­tori­ness, clays are divid­ed into:
Refrac­to­ry (or chamotte) clays melt at tem­per­a­tures above 1580°C. These clays con­sist main­ly of kaolin with a small amount of sil­i­ca. They are used to pro­duce high-qual­i­ty ceram­ics, refrac­to­ry (chamotte) bricks, and tiles.
Fire­clays with­stand tem­per­a­tures from 1350°C to 1580°C. They con­sist of 20–40% alu­mi­na and may con­tain inclu­sions of met­al oxides and salts in small quan­ti­ties. Fire­clays with high plas­tic­i­ty are used to pro­duce clink­er bricks and tiles, as well as ceram­ics.
Light­weight clays con­tain 60–80% sil­i­ca and 5–20% alu­mi­na with a small amount of tita­ni­um oxide. They melt at tem­per­a­tures of 1350°C. Light­weight mate­ri­als are used in the pro­duc­tion of ordi­nary bricks, ceram­ic tiles, and pot­tery.

Mois­ture absorp­tion is the abil­i­ty of clay to absorb water.
High mois­ture absorp­tion is con­sid­ered to be 25–35%, which is typ­i­cal for loams and clays.
Low mois­ture absorp­tion is 9–13%, which is typ­i­cal for shales.
Swelling is the abil­i­ty of clay to increase in vol­ume when wet­ted. This prop­er­ty is impor­tant for soil sta­bil­i­ty and can be used to deter­mine the qual­i­ty of clay.

Genesis of clay

Clays are formed in the process of weath­er­ing of rocks, rep­re­sent­ing the final stage of their degra­da­tion. Clay is found in sol­id mass­es or thin lay­ers, usu­al­ly rel­a­tive­ly close to the sur­face. The par­ent rocks for clay are gran­ite and gneiss. As a result of weath­er­ing, they break down into quartz, mica, and feldspars. Fur­ther degra­da­tion leads to the for­ma­tion of kaoli­n­ite, mont­mo­ril­lonite, and hydromi­cas. The process takes hun­dreds of thou­sands, and even mil­lions of years.

Clays are clas­si­fied by ori­gin: elu­vial, marine sed­i­men­ta­ry, and con­ti­nen­tal sed­i­men­ta­ry.
Elu­vial or resid­ual clays are the prod­uct of weath­er­ing of par­ent rock, vol­canic lava, and tuff. Mass­es are often not sol­id, form­ing flat lay­ers (lens­es) or lim­it­ed areas (pock­ets, nests). The low­er lay­ers grad­u­al­ly tran­si­tion into the rock, and may be con­t­a­m­i­nat­ed with inclu­sions of stones (mica, feldspars, gran­ite). Deposits often pre­serve the struc­ture typ­i­cal of the par­ent rock.

Marine sed­i­men­ta­ry clays are formed by trans­port­ing weath­ered rocks to the seafloor or coast­line. Three types of clays of this ori­gin are dis­tin­guished:
coastal, char­ac­ter­ized by a high con­tent of sand and car­bon­ates;
lagoon, with a high con­tent of salts;
shelf, form­ing pow­er­ful homo­ge­neous lay­ers up to 100 m thick and cov­er­ing an area of tens of square kilo­me­ters.
Con­ti­nen­tal sed­i­men­ta­ry clays are formed after the trans­porta­tion of weath­ered rocks by the wind, water flows, and after earth­quakes and land­slides. This group includes: delu­vial (char­ac­ter­ized by a sig­nif­i­cant vari­a­tion in gran­u­lo­met­ric com­po­si­tion and low thick­ness), lacus­trine (dis­tin­guished by a homo­ge­neous gran­u­lo­met­ric com­po­si­tion, fine dis­per­sion, and high refrac­tori­ness), allu­vial (formed by tem­po­rary flows), and river­ine (devel­oped in riv­er ter­races, and there­fore poor­ly sort­ed and thin­ly lay­ered).

Applications of clay

Clays are a wide­ly used min­er­al resource. They are used in var­i­ous indus­tries, includ­ing drilling for oil and gas. The qual­i­ty of drilling muds and grouts depends on the type and prop­er­ties of the clay. Clay is also used in the pro­duc­tion of ceram­ics, cement, and glass, as a filler in the paper, paint, and rub­ber indus­tries, and as an absorbent for clean­ing flu­ids and oils. In med­i­cine, it is used in the pro­duc­tion of ban­dages and dress­ings, and as a filler in pills and tablets. Clay is also used in agri­cul­ture as a fer­til­iz­er, a soil con­di­tion­er, and as an ani­mal feed sup­ple­ment.

The widest use of clay is in the con­struc­tion indus­try. Com­mon­ly found light sandy (lean) clays of any col­or are used for pro­duc­ing build­ing bricks through­out the coun­try. By rapid­ly fir­ing these light, eas­i­ly melt­ed clay min­er­als in rotary fur­naces at tem­per­a­tures of 1,000 to 1,200 °C, light­weight con­crete fillers such as expand­ed clay and oth­er ther­mal and sound insu­la­tion mate­ri­als are obtained. Fire-resis­tant types of clay min­er­als are used for mak­ing foundry molds. The clay base is mixed with quartz sand, and a mold is made for cast­ing prod­ucts from black and col­ored met­als.

Kaolin clays are used for man­u­fac­tur­ing fac­ing mate­ri­als, porce­lain and faience dish­es, and var­i­ous tech­ni­cal parts. Mont­mo­ril­lonite clays were for­mer­ly called fuller’s earth because they were used for bleach­ing cloth. Nowa­days, they are used in the petro­le­um, med­ical, and chem­i­cal indus­tries as sor­bents.

Availability in Ukraine

The most valu­able clays in eco­nom­ic terms are locat­ed with­in the Ukrain­ian Shield, in the ter­ri­to­ries of Dnipropetro­vsk, Cherkasy, and Zapor­izhzhia regions, as well as in Donet­sk, Vin­nyt­sia, Kirovohrad, and Khmel­nyt­skyi regions. Ben­tonite clay deposits are exploit­ed in Cherkasy and Zakarpat­tia regions. Poly­metal­lic clay min­er­als are wide­spread in sed­i­men­ta­ry deposits through­out Ukraine, and numer­ous brick and cement fac­to­ries use them.

The largest deposits of refrac­to­ry and kaolin clays are con­cen­trat­ed in the Donet­sk (Chaso­vo-Yarske, Novo­raiske) and Zapor­izhzhia (Polo­givske) regions. There­fore, the major­i­ty of the extrac­tion vol­ume was in these regions: 99.18% in Donet­sk and 0.82% in Zapor­izhzhia. The total extrac­tion of refrac­to­ry and kaolin clays in 2021 amount­ed to 4548 thou­sand tons. Thir­teen com­pa­nies (12 in Donet­sk region and 1 in Zapor­izhzhia) extract­ed refrac­to­ry clays in 2021, with the largest extrac­tion vol­ume by: PAT “Vesko” (1,074.20 thou­sand tons), PrAT “Hlyny Don­ba­su” (827.30 thou­sand tons) — in the Donet­sk region; and LLC “Min­ing Com­pa­ny “Min­er­al” (37.4 thou­sand tons) — in the Zapor­izhzhia region. Ukraine ful­ly meets its own needs for clay raw mate­ri­als and is also one of the lead­ing world sup­pli­ers of high-qual­i­ty ceram­ic clays.

Accord­ing to the US Geo­log­i­cal Sur­vey, in 2021, our coun­try export­ed 64.1% of the extract­ed white clay. Ukraine’s share in the world’s kaolin and oth­er kaolin clays exports was 7.9%. Most­ly, these are thin­ly strat­i­fied clays from the Donet­sk region, which are well-suit­ed for the pro­duc­tion of ceram­ic gran­ite because of their high plas­tic­i­ty, white­ness, and low water absorp­tion after fir­ing.

The largest deposits of ben­tonite clays are uneven­ly dis­trib­uted in Ukraine. For exam­ple, the main resources of zeo­lites and high-qual­i­ty alka­line ben­tonites are found only in the Zakarpat­tia and Cherkasy regions. The largest deposit in Ukraine is the Cherkasy deposit (where 86% of all Ukraine’s reserves are locat­ed). Oth­er deposits, such as Berezhanske, Bilovodske, Velykocher­netchynske, Hor­bkivske, Hry­horivske, Il’nytske, Kudrynske, Kurt­sivske, and oth­ers, have sig­nif­i­cant­ly small­er reserves.

Before Rus­si­a’s full-scale inva­sion of Ukraine, geo­log­i­cal explo­ration was car­ried out at the Hry­horivske deposit (Don­bas), Pavlivs­ka and Shy­manivs­ka sec­tions (Dnipropetro­vsk region), Pil­i­avske deposit (Vin­nyt­sia region), and search and eval­u­a­tion work was car­ried out in the Polta­va, Sumy, and Kharkiv regions.