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A double-walled, copper calorimeter has been used to measure the heats of solution of cuspidine, 3Ca0,2Si02,CaF2, and the mineral phase, 3Ca0,3A120,,CaF2, in 40% aqueous hydrofluoric acid at 25". These values have been combined with the heats of solution of calcium silicate-calcium fluoride and calcium aluminate-calcium fluoride mixtures of correct stoicheiometry (and known heats of formation) to yield reliable estimates for the heats of formation (AHf") of the three-component phases a t 25", as follows: 3Ca0,2Si0,,CaF2 -1219.9 (f4.5) and 3Ca0,3A1,0,,CaF2 -1845.3 (A5.5) kcal. molep1. CUSPIDINE, 3Ca0,2Si02,CaF2, is present to approximately 30% by weight in most silicate slags obtained from present phosphorus-furnace 0peration.l The aluminous phase, 3Ca0,3Al2O,,CaF2, has similarly been identified in small quantities in aluminous slags but has only recently been described in detail.2 For convenience, this material will be referred to as '' Calfidine." *