The nature and stability of end-member illite (I), K0.88±0.01/O10(OH)2, a K-deficient mica, has been a subject of much controversy. Evidence for the metastability of “illite” with respect to ideal muscovite (Ms) + pyrophyllite (Py) has been discussed in the literature but conflicting evidence from studies of natural and synthetic systems point to the stability of K-deficient micas with respect to Ms below ~360 °C. Py coexists with K-deficient micas in natural assemblages thought to have formed between 300 and ~360 °C.

Available evidence suggests that end-member illite (I) has an ordered, domain structure. Structural strain due to K-deficiency in micas may be accommodated by the creation of Py domains. The resulting structure could be stable with respect to Ms + Py at low temperatures only if the domains are ordered. Recent FTIR studies have established the presence of local Py domains and molecular water in Al-rich illite. Py stability (~360 °C at 1 kbar) limits the stability of the domain structure; Ms coexists with andalusite + water at 400 °C.

The prograde, stepwise transformation of S (smectite) → I-S → I probably leads to a metastable, compositional end-member (Iw) containing “excess,” interlayer water. Dehydration of Iw results in the formation of a metastable, disordered K-deficient mica (Id) that may recrystallize to form a stable, ordered, Py domain structure (Io). Iw may persist metastably but solid-state alteration of Ms can only yield Io. Fine-scale, Py domains have not been observed in K-deficient micas by TEM as yet, but available evidence suggests that end-member illite (Io) is a distinct, ordered, domain structure, stable below ~360 °C.

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