Serandite from St. Hilaire, Quebec, has the following chemical composition: SiO₂ 47.90, Fe₂O₃ 0.08, FeO 0.86, Al₂O₃ 1.20, MnO 33.8, CaO 5.56, MgO 0.03, Na₂O 6.40, K₂O 0.00, H₂O^-1050C .20, H₂O^+1050C 2.42, Y₂O₃ .27. Total 98.73%.

X-ray diffraction data show that this mineral is a pectolite-type mineral. The name serandite was originally given by Lacroix to the manganiferous end member of the series Ca₂NaH(SiO₃)₃ - Mn₂NaH(SiO₃)₃. Serandite from Las Archipelago (Lacroix, 1931) contained 28.99% MnO and 10.42% CaO, whereas serandite from St. Hilaire, Quebec, is closer to the end member with 33.8% MnO and 5.56% CaO.

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