AbstractIn addition to spurrite, Ca₅(SiO₄)₂(CO₃), and tilleyite, Ca₅(Si₂O₇)(CO₃)₂, galuskinite, C is the third mineral in the CaO—SiO₂—CO₂ternary system. Galuskinite, monoclinic, space groupP2₁/c (a =18.79,b =6.72,c= 10.47 Å, β = 90.79°,V =1322 ų,Z =4), occurs in thin veins which cut calcio-olivine, γ-Ca₂SiO₄, skarn with larnite, β-Ca₂SiO₄, relics. Pavlovskyite, Ca₈(SiO₄)₂(Si₃O₁₀), and dellaite, Ca₆(Si₂O₇)(SiO₄)(OH)₂, form a margin between the veins and the calcio-olivine skarn. The sanidinite facies high-temperature skarn formed ∼500 Ma ago when gabbroid rocks of the Birkhin complex (Baikal area, Eastern Siberia, Russia) intruded and contact-metamorphosed limestone xenoliths. Galuskinite is a retrograde product of skarn alteration and has neither been described from cement clinker production processes nor from studies of the CaO—SiO₂—CO₂system. The crystal structure of galuskinite, refined from single crystal X-ray data toRi =3.1%, has a modular character. One may define a polysomatic series with spurrite and larnite as endmembers and galuskinite as a 1:1 polysome built from regular alternating spurrite and larnite modules. Differences between the X-ray powder patterns of galuskinite and spurrite are most obvious in the low 8 region. Galuskinite is named after the Russian mineralogists Irina O. Galuskina and Evgeny V. Galuskin, Faculty of Earth Sciences. University of Silesia, Poland, for their outstanding contributions to skarn mineralogy.