Franklinfurnaceite, a Ca-Fe (super 3+) -Mn (super 3+) -Mn (super 2+) zincosilicate isotypic with chlorite, from Franklin, New Jersey

Franklinfurnaceite, ideally , is monoclinic, space group C2, with a = 5.483(7), b = 9.39(3), C = 14.51(1) Å, β = 97.04(8)°, and Z = 2. The strongest lines in the X-ray powder-diffraction pattern are (d,I/I₀,hkl): 2.305(100)(204,222,133), 2.707(80)(130,131), 2.602(70)(201,132), 14.4(50)(001), and 3.35(50)(023). It is dark brown with vitreous luster and perfect {001} cleavage; D_meas = 3.66 and D_calc = 3.737 g/cm³. Optically, it is biaxial (-), 2V = 79°, α = 1.792(4), β = 1.798(4), and γ = 1.802(4); dispersion r < v, moderate; strongly pleochroic with X = very dark brown, Y = brown, Z = deep brown; orientation is Z = b, with X and Y in the (010) plane, Y Λ c = 29°. Absorption is intense, X ≫ Z > Y. Franklinfurnaceite is associated with hetaerolite, clinohedrite, hodgkinsonite, and willemite from Franklin, Sussex County, New Jersey. Crystal-structure analysis shows it to be a zincosilicate isotypic with minerals of the chlorite group.