Ice­land spar, also known as opti­cal cal­cite, is a vari­ety of cal­cite with a unique prop­er­ty of dou­ble refrac­tion. In its crys­tals, it is easy to observe how a light beam pass­ing through the min­er­al splits into two sep­a­rate rays. Due to this prop­er­ty, Ice­land spar is of high val­ue to sci­en­tists, min­er­al­o­gists, and jew­el­ers.

Ice­land spar is a vari­ety of cal­ci­um car­bon­ate (CaCO₃) that forms as a result of long geo­log­i­cal process­es. It is found in both pri­ma­ry and sec­ondary deposits and most often orig­i­nates through hydrother­mal activ­i­ty, and less com­mon­ly as a sed­i­men­ta­ry min­er­al. It forms from hydrother­mal bicar­bon­ate-chlo­ride solu­tions that crys­tal­lize in cav­i­ties of basic effu­sive and car­bon­ate rocks. The crys­tals may exhib­it var­i­ous col­ors, usu­al­ly pale shades (blue, pink, yel­low, col­or­less, or oth­ers). The col­or depends on impu­ri­ties of man­ganese, iron, mag­ne­sium, and less com­mon­ly bar­i­um, lead, stron­tium, or bitu­men.

The crys­tals are char­ac­ter­ized by a wide vari­ety of crys­tal­lo­graph­ic forms and can be beau­ti­ful­ly faceted, with some reach­ing sizes of sev­er­al tons. The main prop­er­ties of Ice­land spar are trans­paren­cy, homo­gene­ity, and strong bire­frin­gence. This is what makes it a high­ly valu­able opti­cal mate­r­i­al. Ice­land spar is con­sid­ered one of the most per­fect forms of cal­cite, both struc­tural­ly and chem­i­cal­ly.

Ice­land spar 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 opti­cal and piezo-opti­cal appli­ca­tions.

Discovery of the mineral

Ice­land spar attract­ed sig­nif­i­cant sci­en­tif­ic atten­tion as ear­ly as the 17th cen­tu­ry. One of the first researchers to study it was the Dan­ish sci­en­tist Ras­mus Bartholin, who in 1669 described the unique phe­nom­e­non of dou­ble refrac­tion. This dis­cov­ery became an impor­tant mile­stone in the devel­op­ment of opti­cal sci­ence. The name “Ice­land spar” orig­i­nates from the his­tor­i­cal­ly most famous deposit at Hel­gus­tadir in east­ern Ice­land, which in the 18th–19th cen­turies sup­plied Europe with large, defect-free crys­tals for sci­en­tif­ic and applied pur­pos­es. Although the min­er­al was first exten­sive­ly mined in Ice­land, it is now found in many coun­tries world­wide. His­tor­i­cal­ly, it was already known in the Viking Age and was lat­er wide­ly used in opti­cal research, con­tribut­ing sig­nif­i­cant­ly to the under­stand­ing of light refrac­tion.

The prop­er­ties of Ice­land spar were first described by Eras­mus Bartholin in 1669. The first large crys­tal clus­ters were dis­cov­ered in Ice­land, par­tic­u­lar­ly near the Hel­gus­tadir farm and the Djúpivogur area. Here, crys­tals of enor­mous size were found, includ­ing a spec­i­men weigh­ing about 60 tons. Min­ing began in the 17th cen­tu­ry, and sys­tem­at­ic extrac­tion con­tin­ued until the 1920s.

Distribution of iceland spar

Ice­land spar is rel­a­tive­ly rare in nature, as the for­ma­tion of large, trans­par­ent crys­tals requires very spe­cif­ic con­di­tions. Its main deposits are asso­ci­at­ed with lime­stones and dolomitesthat have under­gone recrys­tal­liza­tion process­es. The most famous deposit is locat­ed in Ice­land (the Reyk­janes region and east­ern Ice­land), which gave the min­er­al its name. In the past, high-qual­i­ty crys­tals up to sev­er­al tens of cen­time­ters in size were extract­ed there.

In addi­tion to Ice­land, deposits are known in Nor­way, Den­mark (Green­land), the Unit­ed States (Mon­tana, Arkansas, South Dako­ta), Cana­da, Mex­i­co, Peru, Chi­na, and Mon­go­lia. Small­er occur­rences are also found in the Czech Repub­lic, Slo­va­kia, Ger­many, and Ukraine (Podil­lia, the Carpathi­ans, and Donbas—within cal­cite veins).

The most valu­able deposits are those with high­ly trans­par­ent crys­tals free of cracks and inclu­sions, as only such mate­r­i­al is suit­able for opti­cal appli­ca­tions. Indus­tri­al extrac­tion today is lim­it­ed, as demand for large crys­tals has part­ly been replaced by syn­thet­ic mate­ri­als.

In the 20th cen­tu­ry, deposits were also dis­cov­ered in the USA (Mon­tana, Cal­i­for­nia), South Africa (Cape Province), Aus­tralia, Spain, Argenti­na, and Rus­sia. Dur­ing World War II, inter­est in Ice­land spar increased due to its opti­cal impor­tance. Today, South Africa remains the main glob­al sup­pli­er, with indus­tri­al min­ing start­ing in the 1920s and con­tin­u­ing to this day as an impor­tant raw mate­r­i­al base.

Extraction

The extrac­tion of Ice­land spar can be chal­leng­ing, as it often requires tar­get­ed explo­ration and min­ing in spe­cif­ic geo­log­i­cal areas. It is usu­al­ly mined by open-pit meth­ods with lim­it­ed use of explo­sives. The crys­tals must be care­ful­ly extract­ed to pre­serve their clar­i­ty and opti­cal prop­er­ties, which are impor­tant for sci­en­tif­ic as well as aes­thet­ic and col­lec­tor pur­pos­es.

Ice­land spar min­ing is gen­er­al­ly small- to medi­um-scale; how­ev­er, the asso­ci­at­ed risks are sim­i­lar to those of oth­er quar­ry­ing oper­a­tions, includ­ing land­scape and soil dis­tur­bance, local changes in drainage pat­terns, dust and noise, and waste rock dumps.

Uses of iceland spar

Due to its strong bire­frin­gence and trans­paren­cy, Ice­land spar is used in opti­cal sys­tems for polar­iza­tion and con­trol of light beams, par­tic­u­lar­ly in Nicol prisms. Only sin­gle crys­tals with­out cracks and with min­i­mal inclu­sions are used for this pur­pose. Two grades of raw mate­r­i­al are dis­tin­guished: Type A—used for a broad spec­tral range, and Type B—for red and infrared ranges. The min­er­al is also used in cam­era polar­iz­ing fil­ters to reduce glare.

There is a hypoth­e­sis that Ice­land spar was the “sun­stone” used by the Vikings to nav­i­gate in cloudy weath­er. In 1967, Dan­ish archae­ol­o­gist Thork­ild Ram­sk­ou pro­posed this idea, and a crys­tal of spar was found among nav­i­ga­tion­al instru­ments on a ship that sank in 1592. In 2011, French physi­cists con­firmed its poten­tial use for nav­i­ga­tion, and fur­ther exper­i­ments sup­port­ed this con­clu­sion. Obser­va­tion through the crys­tal allows the posi­tion of the Sun to be deter­mined even under over­cast skies due to polar­ized light caused by Rayleigh scat­ter­ing.