“Guarinite” is a typical accessory mineral of the Monte Somma syenite; it belongs to the cuspidine group and displays a domain structure. “Guarinite” contains up to three distinct domains, corresponding to three different ways to connect disilicate groups and walls of octahedra; these may all be simultaneously present in the same crystal. The domains present in the crystals of “guarinite” have cell type I, II and IV, according to the classification scheme proposed for cuspidine-group minerals. Domain IV is the most common, and domain I is the rarest; domain II may occur as the only domain, whereas domain I occurs only in association with domain IV, which invariably predominates. So far, the actual structure of the various domains had not been defined. EPMA and single-crystal structural studies indicate that domain I of “guarinite” displays space group P1, with a 10.973(2), b 10.306(1), c 7.367(3) Å, α 90.03(3), β 109.63(3), γ 90.11(2)°, with a crystal-chemical formula Ca3(Ca0.72 Zr0.28)∑1.00(Zr0.86M0.14)∑1.00(Ca0.59Mn0.25Fe0.16)∑1.00(Na1.20Ca0.76)∑1.96(Si1.98O7)2(F2.88O1.12)∑4.00, where M represents Nb, Ti, Al, Sr, and REE. Domain I is isostructural with hiortdahlite II, and its crystal structure was refined to a final R of 0.072. Domain II of “guarinite” displays space group P1211, with a 10.836(1), b 10.270(1), c 7.296(1) Å, β 109.13(3)°, with a crystal-chemical formula Ca3Zr(Nb0.56Fe0.15Mn0.10Ti0.10Zr0.09)∑1.00(Ca0.72Mn0.18M0.10)∑1.00(Na0.77Ca0.23)∑1.00(Na0.80Ca0.22)∑1.02(Si2O7)2(O2.17 F1.83)∑4.00, where M represents Al, Mg, Sr and Y. Domain II is isostructural with wöhlerite, and its crystal structure was refined to a final R of 0.045. Domain IV of “guarinite” adopts space group P1, with a 10.970(2), b 10.943(2), c 7.365(1) Å, α 109.63(2), β 109.65(2), γ 83.39(1)°, with a crystal-chemical formula Ca4Zr(Ca0.31Mn0.25Fe0.16Zr0.14M0.14)∑1.00(Na1.20 Ca0.76)∑1.96(Si1.98O7)2(F2.88O1.12)∑4.00, where M represents Nb, Ti, Al, Sr and REE. Domain IV is isostructural with hiortdahlite I, and its crystal structure was refined to a final R of 0.067. One should note that the refinements of domain I and domain IV (both twinned) have been carried out on the same crystal. EMPA and SEM studies show the presence of chemically homogeneous crystals as well as crystals with distinct chemical zoning due to a wide variation of the major elements Nb, Ca, Na, F, and pointing to the possible coupled substitution Nb5+ + 2Na+ + O2− ⇄ 3Ca2+ + F− as one of the main mechanisms of chemical variation.