Palygorskite (formerly called attapulgite) is a lath-like or fibrous clay mineral with a structure consisting of short and alternating inverted 2:1 sheets or ribbons. These ribbons have an average width (along the Y direction) of two linked tetrahedral chains in palygorskite (and three linked chains in sepiolite). The tetrahedral sheet is continuous across the ribbons, but with apices pointing up and down in adjacent ribbons. On the other hand, the octahedral sheet is discontinuous. Because of its sheet-like and ribbon like structural characteristics, it, and its companion mineral, sepiolite, have been variously classified as 2:1 phyllosilicates, inosilicates, and “unclassified” silicates. It is currently classified as an “unclassified silicate”.

The general structural formula for palygorskite can be presented as:
(Mg_5-y-zR³⁺_y)(Si_8-xR³⁺_x)O₂₀(OH)₂(OH₂)₄ · R²⁺_(x-y+2z)/2 (H₂O)₄

where R²⁺_(x-y+2z)/2 and (H₂O) represent the charge balancing cations and water in the rectangular cavities, y is the fraction of Mg substituted by Al in the octahedral ribbon sheet, x is the fraction of Si substituted by Al in the tetrahedral ribbon sheets.

The finite width of the ribbons limits the number of octahedral sites per formula unit to five; thus, a true trioctahedral mineral would have 5 cations in the octahedral sites. In palygorskite it is common for some of the octahedral sites to remain vacant, with occupancy usually varying from about 4 to 5 of 5 sites filled. Intermediate values are common.

Isomorphous substitution in palygorskite is relatively low in the tetrahedral sheets, with Al commonly occupying 0.01 to 0.09 of 8 tetrahedral sites. However, isomorphous substitution is common in the octahedral sheet, with Al filling 28 – 59% of the occupied octahedral sites. Other cations are also present, including Fe²⁺, Fe³⁺, Mn, and a variety of other transition metals in small quantities.

Total layer charge is relatively low, as expressed by very low CEC values, usually in the range of 5 to 30 cmol kg-1. Exchangeable cations such as Ca and Mg and variable amounts of water are contained in the rectangular channels that lie between the opposing 2:1 ribbons and extending parallel to the lath direction.

Distribution

Palygorskite is a common mineral in magnesium rich, arid soil environments. It is known to form in ocean sediments and may form in Mg rich soils, though conclusive data are still lacking.

Brindley, G.W. and G. Brown. 1980. Crystal Structure of Clay Minerals and their X-ray Identification. Mineralogical Society, London, UK.

Singer, A., 1989. Palygorskite and Sepiolite Group Minerals. Ch. 17, p. 829-872. In: J.B. Dixon and S.B. Weed (ed.), Minerals in Soil Environments, 2nd Edition. SSSAJ Special Publication No. 1.