Paly­gorskite, also known as atta­pulgite , is a clay min­er­al, an hydrous mag­ne­sium sil­i­cate with a fil­a­men­tous chain-lay­ered struc­ture. It occurs in the con­text of ser­pen­tinized ultra­maf­ic rocks, in mer­les and meta­mor­phic dolomites, and in hydrother­mal veins or granitic peg­matites. It is also found in soils and sed­i­ments result­ing from alter­ation of these rocks, espe­cial­ly in arid and semi-arid regions. The min­er­al has a high spe­cif­ic sur­face area, sig­nif­i­cant sorp­tion capac­i­ty and sta­bil­i­ty in a wide range of physic­o­chem­i­cal con­di­tions. These prop­er­ties deter­mine the indus­tri­al impor­tance of paly­gorskite as a sor­bent, cat­a­lyst, car­ri­er of active sub­stances and com­po­nent in var­i­ous indus­tries. This is one type of fuller clay (clays used as an absorbent, fil­ter or bleach).

Paly­gorskite was first described in 1862 for the Paly­gorsk deposit on the Popov­ka Riv­er, Mid­dle Urals, Rus­sia. The syn­onym atta­pulgite orig­i­nates from the name of the town of Atta­pul­gus in the USA, locat­ed in the extreme south­west­ern cor­ner of the state of Geor­gia, where this min­er­al is wide­spread and mined by open-pit meth­ods.

Paly­gorskite 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 adsorp­tion raw mate­r­i­al.

Conditions for the formation

Paly­gorskite is formed main­ly in sed­i­men­ta­ry basins of arid and semi-arid cli­mat­ic zones. The most favor­able con­di­tions are lagoon­al and marine basins with high water min­er­al­iza­tion, lim­it­ed water exchange and increased con­cen­tra­tion of Mg²⁺, Si⁴⁺ and Al³⁺ ions. The crys­tal­liza­tion process occurs in a weak­ly alka­line or neu­tral envi­ron­ment dur­ing intense evap­o­ra­tion.

In addi­tion to sed­i­men­ta­tion ori­gin, paly­gorskite can be formed under the influ­ence of hydrother­mal solu­tions enriched with sil­i­ca and mag­ne­sium, which deposit the min­er­al in the cracks and pores of sed­i­men­ta­ry and vol­canogenic-sed­i­men­ta­ry rocks. In some cas­es, it is the prod­uct of dia­ge­net­ic changes in mont­mo­ril­lonite or sepi­o­lite, and the result of weath­er­ing of pri­ma­ry sil­i­cates in arid cli­mates.

Properties of palygorskite

The crys­tal struc­ture of the min­er­al is an inter­me­di­ate type between chain and lay­ered sil­i­cates. In the dried state, paly­gorskite clays are usu­al­ly very light, high­ly porous, and do not sink in water. Paly­gorskite is a high­ly dis­persed min­er­al that has a devel­oped spe­cif­ic sur­face area and the abil­i­ty to cation exchange. Zeo­lite chan­nels make it pos­si­ble to absorb small mol­e­cules of such sub­stances as water, ammo­nia, etc. The sec­ondary pores of paly­gorskite exhib­it high adsorp­tion prop­er­ties with respect to hydro­car­bons, par­tic­u­lar­ly alde­hy­des, esters, high­er alco­hols, and acids.

Accord­ing to its phys­i­cal char­ac­ter­is­tics, paly­gorskite has a white, gray­ish, yel­low­ish, or green­ish col­or, a dull earthy lus­ter, and some­times a semi-trans­par­ent appear­ance. Its hard­ness is low—2–2.5 on the Mohs scale, its den­si­ty ranges from 2.0 to 2.3 g/cm³, and the frac­ture is uneven and fibrous.The crys­tals most­ly form nee­dle-like and fibrous aggre­gates, some­times radi­at­ing clus­ters that are clear­ly vis­i­ble under a micro­scope.

The ther­mal nature of this min­er­al is also impor­tant. When heat­ed to 200–300 °C, paly­gorskite los­es adsorbed water; at 600–700 °C dehy­drox­y­la­tion occurs; and at tem­per­a­tures above 800 °C the struc­ture is destroyed, trans­form­ing into amor­phous phas­es. At the same time, it exhibits high thixotropy—the abil­i­ty to form sta­ble col­loidal gels, which deter­mines its wide appli­ca­tion in var­i­ous indus­tries.

Distribution of palygorskite

Among its numer­ous occur­rences, the large Amer­i­can deposit of Atta­pul­gus (Geor­gia) and the Pend Oreille deposit near Met­aline Falls (Wash­ing­ton) should be men­tioned, from which large lay­ers asso­ci­at­ed with cal­cite and barite orig­i­nate. Paly­gorskite is also known from good spec­i­mens in many oth­er coun­tries (Ukraine, Mex­i­co, the Unit­ed King­dom). Among all these occur­rences, spe­cial men­tion should be made of Fich­tel (Bavaria, Ger­many), where impres­sive aggre­gates com­posed of paly­gorskite, mag­netite, and apatite have formed.

In Ukraine, occur­rences of paly­gorskite clays have been found near the city of Korosten in the Zhy­to­myr region, Sim­fer­opol, Cherkasy, and oth­er loca­tions. Visu­al­ly, paly­gorskite clay in a water-sat­u­rat­ed state is dif­fi­cult to dis­tin­guish from clays of oth­er min­er­al com­po­si­tions, so most occur­rences of this min­er­al remain undoc­u­ment­ed. The main min­er­al raw mate­r­i­al base of paly­gorskite in Ukraine is rep­re­sent­ed by the Cherkasy deposit of ben­tonite and paly­gorskite clays.

Applications of palygorskite

It is known from archae­o­log­i­cal research that the min­er­al was used by the pre-Columbian Maya civ­i­liza­tion as a blue pig­ment on ceram­ics, sculp­tures, fres­coes and tex­tiles.

The high adsorp­tion capac­i­ty of these clays makes them suit­able for envi­ron­men­tal purifi­ca­tion from haz­ardous sub­stances and for the absorp­tion of unpleas­ant odors. It is used for the pro­duc­tion of clean­ing agents, drilling flu­ids, as ther­mal and sound insu­la­tion mate­ri­als, as sor­bents and cat­a­lysts in the petro­chem­i­cal, met­al­lur­gi­cal, and nuclear indus­tries, as well as in the pro­duc­tion of tox­ic chem­i­cals, fer­til­iz­ers, fillers in pig­ments, paints, var­nish­es, plas­tics, and for clean­ing food and indus­tri­al oils.