Gar­net. Char­ac­ter­is­tics, types, dis­tri­b­u­tion

Gar­net is a min­er­al that impress­es with its vari­ety of colours, prop­er­ties and shapes. The study of gar­nets allows us to bet­ter under­stand the con­di­tions of their for­ma­tion and to iden­ti­fy deposits rich in this min­er­al. The unique prop­er­ties of gar­nets pro­vide them with a wide range of appli­ca­tions, from jew­ellery mak­ing to tech­ni­cal process­es.

Varieties

Gar­nets are a group of orthosil­i­cates of vari­able com­po­si­tion that form iso­mor­phic series:

• 1) Pyrope [Mg₃Al₂(SiO₄)₃] — Alman­dine Fe₃Al₂[SiO₄]₃]
• 2) Alman­dine — Spes­sar­tine [Mn₃Al₂[SiO₄]₃]
• 3) Grossu­lar [Ca₃Al₂[SiO₄]₃] — Andra­dite [Ca₃Fe₃⁺²(SiO₄)₃]
• 4) Gro­su­lar — Spe­sar­tine.

Gar­net is includ­ed in the list of min­er­als of nation­al impor­tance approved by the Res­o­lu­tion of the Cab­i­net of Min­is­ters of Ukraine dat­ed Decem­ber 12, 1994, No. 827, as an abra­sive raw mate­r­i­al.

Characteristics and types

Due to their con­sid­er­able hard­ness (6–7 on the Mohs scale), gar­nets of the alman­dine-pyrope and alman­dine-spes­sar­tine series are used as an abra­sive mate­r­i­al: they are used to make var­i­ous grind­ing and grind­ing tools and mate­ri­als, pow­ders, etc.

Gar­net is also used in con­struc­tion as an addi­tive to cement and mor­tar. Trans­par­ent and translu­cent gar­nets are pre­cious stones.

There are no set tech­ni­cal require­ments for raw gar­nets, and high-hard­ness gar­nets such as alman­dine are pre­ferred for pro­duc­tion. The grains should be large enough and free from inclu­sions of oth­er min­er­als. Cracks may be dense, but they should not pen­e­trate the min­er­al. When crushed, gar­nets should pro­duce sharp and cut­ting edges.

The gar­net con­tent in the rock should not be low­er than 10–12%, although some­times deposits with 4% gar­net in the rock are mined. The tech­ni­cal advan­tage is the low mechan­i­cal strength of the rock and the absence of min­er­als with sim­i­lar spe­cif­ic grav­i­ty, which makes it pos­si­ble to reduce costs and sim­pli­fy the extrac­tion of the min­er­al.

In addi­tion to wachovite, all gar­nets dis­solve in HCl after cal­ci­na­tion and usu­al­ly release jel­ly-like sil­i­ca after evap­o­ra­tion. They are dis­tin­guished by their very strong relief and lack of adhe­sion in pol­ished cuts. There is also a cer­tain cor­re­la­tion between the chem­i­cal com­po­si­tion and phys­i­cal prop­er­ties of gar­nets (refrac­tive index and spe­cif­ic grav­i­ty). It allows us to deter­mine the mol­e­c­u­lar com­po­si­tion with a suf­fi­cient approx­i­ma­tion with­out resort­ing to chem­i­cal analy­sis.

The chem­i­cal com­po­si­tion, struc­ture and prop­er­ties of gar­net min­er­als, as well as their con­cen­tra­tion in host rocks, vary depend­ing on the geo­log­i­cal con­di­tions in which they are formed in nature, i.e. depend­ing on the genet­ic type of their deposits, among which two main groups are dis­tin­guished:

• 1. Pri­ma­ry — indige­nous for­ma­tions asso­ci­at­ed with the action of mag­mat­ic, post-mag­mat­ic and meta­mor­phic process­es;
• 2. Sec­ondary — plac­ers formed as a result of mechan­i­cal destruc­tion of pri­ma­ry deposits.

For mag­mat­ic deposits, the most typ­i­cal gar­nets are pyrope and spes­sar­tine.

Pyrope crys­tallis­es in mag­ne­sium-rich and alka­li-poor mag­mas. Its crys­tals are often sur­round­ed by a rim of spinel, pyrox­ene and feldspar.

Spes­sar­tine is typ­i­cal of gran­ites and peg­matites. In addi­tion, alman­dine and andra­dite can be of mag­mat­ic ori­gin.

Deposits at the con­tacts of acidic rocks with iron-bear­ing rocks, con­tain­ing main­ly alman­dine, are small in size and there­fore of lit­tle com­mer­cial impor­tance.

The same pri­ma­ry deposit often con­tains gar­nets formed at dif­fer­ent times and under dif­fer­ent con­di­tions. Changes in these con­di­tions lead to the replace­ment of gar­net with chlo­rite, pyrox­ene, amphi­bole, epi­dote, biotite, feldspar, etc.

The deposits asso­ci­at­ed with the pro­cess­ing of basic rocks are of the great­est indus­tri­al inter­est. Such deposits, being large in size, pro­duce large crys­tals of alman­dine, which is par­tic­u­lar­ly suit­able for use as an abra­sive mate­r­i­al.

The high den­si­ty and con­sid­er­able mechan­i­cal strength con­tribute to the accu­mu­la­tion of gar­nets (main­ly pyrope and alman­dine) in allu­vial, lake and coastal-sea plac­ers. Plac­er deposits of gar­net are usu­al­ly small in terms of reserves and are char­ac­terised by the pres­ence of small, heav­i­ly rolled crys­tals and frag­ments bro­ken by numer­ous deep cracks.

One of the new types of abra­sive raw mate­ri­als for the coun­try is gar­net. Its high phys­i­cal and mechan­i­cal prop­er­ties (hard­ness, strength and wear, shape of the abra­sive grain, micro­c­rack­ing, abil­i­ty to pro­duce frag­ments with sharp-cut­ting edges when crushed, high degree of self-sharp­en­ing, abil­i­ty of gar­net grains to firm­ly attach to paper or fab­ric back­ing) make it a suc­cess­ful abra­sive raw mate­r­i­al.

Deposits in Ukraine

There are two types of gar­net deposits and occur­rences in Ukraine: igneous and meta­mor­phic.

The Lozne deposit is locat­ed in the Khmel­nyt­skyi dis­trict of Vin­nyt­sia region. In plan, it is rep­re­sent­ed by a shaft-shaped body of biotite-gar­net gran­ites stretch­ing in a north­east­er­ly direc­tion for 700 m with a width of 550 m. These are grey medi­um-coarse-grained dense rocks with point and rec­tan­gu­lar gar­net inclu­sions, the size of which does not exceed 5–7 mm, more often 2–4 mm. The gar­net con­tent in the rock ranges from 9.2 to 17.4, with an aver­age of 12.1%, and the raw mate­r­i­al resources are esti­mat­ed at 7.6 mil­lion tonnes.

The Slo­bidske deposit is locat­ed on the right bank of the Piv­den­nyi Buh Riv­er in Vin­nyt­sia Oblast. It is rep­re­sent­ed by out­crops of biotite-gar­net gran­ites stretch­ing for 1 km and 600–700 m wide, sur­round­ed by migmatites. The migmati­tised rocks have a band­ed struc­ture, with alter­nat­ing dark bands (1–2 cm) enriched in biotite and gar­net and light bands dom­i­nat­ed by quartz and feldspar. The gar­net con­tent is 14–17%, and the raw mate­r­i­al resources are esti­mat­ed at 10 mil­lion tonnes down to a depth of 50 m.

In the Kryvyi Rih dis­trict of the Dnipro region, gar­net is a rock-form­ing min­er­al in the Sak­sagan shale of the Kryvyi Rih series. Gar­net-bear­ing shales are a promis­ing raw mate­r­i­al for the pro­duc­tion of abra­sive gar­net.

The Zavalivske graphite deposit is locat­ed on the bor­der of Kirovohrad and Ode­sa regions. It belongs to the Zavalivs­ka fold­ed struc­ture. The mid­dle part of the struc­ture is com­posed of biotite-graphite and biotite-gar­net gneiss­es and quartzites, which are the source of gar­net and rep­re­sent the pro­duc­tive stra­tum.

The gar­net-biotite gneiss­es are devel­oped in the ore zone in the form of bands and lens­es up to 1 km long and 1–2 to 100 m thick. The gar­net con­tent in the pro­duc­tive thick­ness varies from 6 to 31% and aver­ages 15%.

Lab­o­ra­to­ry and tech­no­log­i­cal tests have estab­lished the pos­si­bil­i­ty of pro­duc­ing gar­net con­cen­trate that meets the require­ments for con­cen­trates for the pro­duc­tion of abra­sive leather and pow­ders for glass grind­ing. The gar­nets of the Zavalivske deposit are not infe­ri­or in phys­i­cal and mechan­i­cal prop­er­ties to those of the Slo­bo­da deposit. Reserves of gar­net-biotite gneiss­es amount to 20323.4 thou­sand tonnes, with a gar­net con­tent of 3426 thou­sand tonnes.

Gar­nets are an impor­tant group of min­er­als that show con­sid­er­able diver­si­ty in terms of their phys­i­cal and chem­i­cal prop­er­ties and genet­ic types of deposits. Deposits in Ukraine have sig­nif­i­cant poten­tial for the extrac­tion of gar­nets that meet the needs of the abra­sive indus­try. The unique phys­i­cal and mechan­i­cal prop­er­ties of these min­er­als and their reserves cre­ate the pre­con­di­tions for fur­ther devel­op­ment of the min­ing indus­try in the region.

Thus, gar­nets are not only aes­thet­i­cal­ly pleas­ing but also strate­gi­cal­ly impor­tant min­er­als with a wide range of indus­tri­al appli­ca­tions. Their extrac­tion and use open up prospects for the devel­op­ment of new tech­nolo­gies and indus­tries.