Swinefordite is the Mg- and Li-rich trioctahedral member of the smectite group, with the original workers (Tien et al., 1975) determining that the location of Li of the natural sample was split with ~33% in the interlayer and the remainder in the octahedral sheet, and with vacancies in the octahedral site, thereby making this a dioctahedral-trioctahedral intermediate. Köster (1982) redetermined the chemistry based on the cation exchange capacity and determined the composition to be (M⁺_0.4 ^. nH₂O)(Li_1.06Al_0.99 Mg_0.7Fe³⁺_0.10)_Σ=2.85 (Si_3.87Al_0.13)O₁₀(OH)₂ where M is the exchangeable cation and n is variable. The M (interlayer) cation is assumed here as univalent, but it may have other valence states also. The vacancy content in the octahedral site (0.15) is consistent with other trioctahedral smectites (Güven, 1988). Swinefordite occurs as a pseudomorph after spodumene.
Cf., hectorite