• journal_title：Mineralogical Magazine
• Contributor：Sun Shihua ; Yu Jie
• Publisher：Mineralogical Society of Great Britain and Ireland
• Date：2000-
• Format：text/html
• Language：en
• journal_abbrev：Mineralogical Magazine
• issn：0026-461X
• volume：64
• issue：4
• firstpage：755
• section：Articles

To date, Fe-Li micas have been defined differently from other micas. The purpose of this paper is to reinterpret the actual Fe-Li mica series with new concepts of ‘essential replacement’ (the evolution direction) and ‘composition track’ (the sequence of mica varieties). Two hundred and fifty-eight analyses from the literature are used for this study in the form of eight data groups. The common compositional and substitutional characters of Fe-Li micas have been reinterpreted in light of principal component analysis and a geometric frame of ideal Fe²⁺-Al-Li micas in space with (Si, Al_IV, Al_VI, Fe²⁺, Li, □_VI, K)-coordinates.

In our new interpretation, the actual Fe-Li micas are essentially neither Al_IV- nor Al_VI-, but □_VI-constant. The actual Fe-Li micas are the weakest fluctuant relative to the Annite-Polylithionite-Trilithionite-Siderophyllite (APTE) plane. About 90% of variations of actual Fe-Li micas range from the trioctahedral trend described as a sequence along the segment K₂Al₂Fe_5/2²⁺ Li□_1/2Si₆Al₂O₂₀(OH,F)₄ +x[AlFe²⁺ _–2Li₃SiAl_–2] (–1/3⩽ x⩽ 5/8). The substitution AlFe_–²⁺₂Li₃SiAl_–2 (i.e. 2Al_IV +4Fe²⁺ → 2Si +Al_VI +3Li) is the main mechanism that keeps actual Fe-Li micas trioctahedral. More than 8% of variations arise from the dioctahedral trend involving created Al_VI- and □_VI-increasing replacement. The actual Fe-Li mica series comprises the composition trend from Fe²⁺-biotite to lepidolite. This series is not on the siderophyllite-polylithionite join, but can be expressed ideally as K₂Al_1+xFe²⁺_4–4xLi_1+3x(Si_6+2xAl_2–2x)O₂₀(OH,F)₄ (–1/3⩽ x⩽ 1).