Nelenite, (Mn,Fe)16Si12O30(OH)14[As33+O6(OH)3], is a polymorph of schallerite and a member of the friedelite group. X-ray diffraction patterns can be indexed on a supercell with a = 13.418(5) and c = 85.48(8)Å, space group R3¯m, but by analogy with TEM results on mcGillite and friedelite, the structure is based on a one-layer monoclinic cell with a = 23.240, b = 13.418, c = 7.382 Å, β = 105.21°, and space group C2/m. Chemical analysis yields SiO₂ 31.12, FeO 17.12, MgO 0.12, ZnO 3.63, MnO 29.22, As₂O₃ 12.46, H₂O 6.42, sum = 100.09%. Analysis of a number of samples indicates that Fe substitutes for Mn up to 5.8 of the 16 octahedrally coordinated cations, but that the Si: As ratio is constant. The strongest lines in the X-ray powder diffraction pattern (d, I/Io) are: 2.552,100; 2.878,70; 1.677,60; 3.55,60; 1.723,50.

Nelenite is brown in colour with a vitreous luster and perfect {0001} cleavage, which easily distinguishes it from schallerite. The Mohs' hardness is approximately 5. The density is 3.45 g/cm³ (calc.) and 3.46 g/cm³ (obs.). Nelenite is uniaxial negative with ɛ = 1.700 and ω = 1.718 (both ± 0.004). Nelenite was formerly known as ferroschallerite, which is a misnomer. It was found in the Franklin Mine, Franklin, Sussex County, New Jersey, in the 1920s. It occurs in several parageneses, associated with actinolite, tirodite, albite, garnet, feldspars, and several members of the stilpnomelane group in coarse-grained assemblages with pegmatitic texture and a breccia likely derived from this rock. Nelenite is named in honour of Joseph A. Nelen, chemist at the Smithsonian Institution.