The structure of ferro-pedrizite—a new lithium mineral of the amphibole supergroup—has been studied by single-crystal X-ray diffraction. Fe 2+ ions dominate over Mg in the chemical composition of this mineral. The parameters of the monoclinic unit cell are a = 9.3716(4) Å, b = 17.649(1) Å, c = 5.2800(6) Å, and β = 102.22(1)°. The experimental set of intensities contains a large number of significant reflections (~10%), which violate the C lattice. Consideration of pseudomerohedral twinning (matrix [1 0 0/0 $$\bar 1$$ 0/–3/4 0 $$\bar 1$$ ]) allowed us to solve and refine the structure within the sp. gr. C 2/ m to the final value R = 3.9% in the anisotropic approximation of atomic displacements using 4843 I > 2σ( I ). The twin components are found to be 0.681(3)/0.319(3). Twinning has been revealed for the first time in amphiboles of the pedrizite family. The idealized formula of ferro-pedrizite ( Z = 2) is determined as NaLi 2 ( $$Fe_2^{2 + }$$ Al 2 Li)[Si 8 O 22 ](OH) 2 .