Jaffeite, ideally Ca6Si2O7(OH)6, is the natural analogue of the phase C3SH1.5(3CaO·SiO2·1.5H2O; tricalcium silicate hydrate; TSH) that occurs as a component of cement. It is hexagonal, space group P3, with a = 10.026(5), c = 7.482(Å) Å, and Z = 2. The strongest lines in the X-ray powder-diffraction pattern are [d (Å), I, hkl] 8.66,100,010; 3.279,50,210; 2.996,90,112; 2.887,70,030; 2.833,90,022; 2.466,30,212; 2.083,30,222; 1.872,25,042,004; and 1.757,30,232. Jaffeite is colorless with a vitreous luster; Dmeas = 2.65(3), Dcalc = 2.58(2) g/cm3. Optical properties include uniaxial (+); ω = 1.596(2), ε = 1.604(2). Jaffeite occurs at the Kombat mine, Namibia, in a low-grade metamorphic setting, with major defemite and hausmannite, and minor brucite, hillebrandite, vesuvianite, glaucochroite, apatite, galena, and native copper.