In addition to spurrite, Ca5(SiO4)2(CO3), and tilleyite, Ca5(Si2O7)(CO3)2, galuskinite, C is the third mineral in the CaO—SiO2—CO2 ternary system. Galuskinite, monoclinic, space group P21/c (a = 18.79, b = 6.72, c = 10.47 Å, β = 90.79°, V = 1322 Å3, 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—CO2 system. The crystal structure of galuskinite, refined from single crystal X-ray data to Ri = 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.