Curable white silicone formulation, a reflective material for optical semiconductor module, and optical semiconductor device

A white curable silicone composition provides excellent elongation characteristics while providing sufficiently great hardness, which as a result can provide a cured product having excellent toughness.The composition comprises: (A) alkenyl group-containing resinous organopolysiloxane which is represented by an average composition formula: (R13SiO1/2)a(R12SiO2/2)b(R1SiO3/2)c(SiO4/2)d(XO1/2)e, where, in the formula, R1 are each independently a monovalent hydrocarbon group, while at least two R1 are alkenyl groups, X is a hydrogen atom or an alkyl group, 0≤a≤0.7, 0≤b≤0.2, 0≤c≤0.3, 0≤d≤0.8, and 0≤e≤0.2, a+b+c+d=1.0, and c+d>0 are satisfied; (B) linear alkenyl group-containing organopolysiloxane which is represented by R2R32SiO(R32SiO)nSiOR2R32, where, in the formula, R2 is an alkenyl group, R3 are each independently a monovalent hydrocarbon group other than an alkenyl group, and n is an integer of 50 or greater; (C) organohydrogenpolysiloxane having at least two hydrogen atom-bonded silicon atoms in one molecule; (D) a catalyst for curing reaction; and (E) a white pigment, wherein the component (A) is included in an amount of 20 mass % or more based on the total mass of the organopolysiloxane components, and the alkenyl group content in the components (A) to (C) is 1.7 mass % or less.