Mg-Fe云母化学成分的解释和分类(Ⅰ)——基本置换和黑云母平面

 中国东部花岗岩类141个Mg-Fe云母的化学成分将近90%的变化属于八面体层内的类质同象置换,置换矢量Mg ₁Fe⁺²和Fe_-3⁺²(R_Ⅵ⁺³)_-2□_Ⅵ组成了天然黑云母平面,大约80%的变化应当解释为基本置换8Mg ₁Fe⁺²+Fe_-3⁺²(R_Ⅵ⁺³)₂□_Ⅵ.这些是Mg-Fe云母在广泛的自然条件下表现出来的最主要的晶体化学关系。文中还提出了置换矢量的长度、分量和以及电价和三个参数,用以识别矿物化学成分变化的类质同象置换特征。

INTERPRETATION OF CHEMICAL COMPOSITION AND SUBDIVISION OF MAGNESIUM IRON MICAS, PART A BASIC SUBSTITUTION AND BIOTITE PLANE

In this paper the chemical composition of 141 Mg-Fe micas from granitoids in Eastern China was interpreted in terms of SUBSTITUTION VECTOR The substitution vector V = (α_R^e1, α_R^e2, ...α_R^en, v) is a vectorial form used to represent substitution, here, aR^ei(.i=1,2,...n) represents the SUBSTITUTIONCOMPONENT of ion R with valence ei in V, vector. LENGTH L= (Σα_R²e_i)^1/2, COMPONENT SUM (CS = Σα_Re_i) and ELECTROVALENT SUM ES = Σe_i·α_Reⁱ are three parameters of the substitution vector. The length representsthe intensity of the substitution. The component sum represents the change in ions occupancy, and the electrovaient sum — the change in ions valence in structural sheet or bulk of mica due to the substitution The type of a substitution can be distinguished in terms of its CS and ES values.Mg_-1Fe⁺² (Mg-Fc substitution vector), Fe_-3⁺²(R_v1⁺³)₂□_v1 (Fe-muscovite substitution vector) and Fe_-1⁺²Al_v1 Si_-1Al_1v (Fe-Tschermark’s substitution vector) are the three basis vectors of the compositional space of Mg-Fe micas. It was demonstrated by principal components analysis and graphic method that about 80 percent of the compositional changes in the 141 Mg-Fe micas could be interpreted by the basic substitution 8Mg_-1Fe⁺² + Fe_-3⁺²(R_v1⁺³)₂□_v1, and the Tschermark’s substitution was not related to the basic compositional trend. It was also demoestrated that near 90 per cent of the compositional changes were due to octahedral substitutions respricted within the octahedral sheet, and the natural biotite plane composed of Mg_-1Fe⁺² and Fe_-3⁺²(R_v1⁺³)₂□_v1 Was more suitable for subdivision of Mg-Fe micas. These features are the most important crystallochemical relations of Mg-Fe micas occurred in common conditions in K₂Mg₆Si₆Al₂O₂₀(OH)₄ (phlogopite), K,Fe₆Si₆Al₂O₂₀(OH)₄ (annite) and K₂Al₄Si₉Al₂ O₂U(OH)₄ (muscovite) are the most important ideal end-members, which situated in the preceding natural biotite plane. K₂Fe₄Al₂Si₄Al₄O₂₀(OH)₄ (Fe-eastonite), K₂Mg₄Al₂Si₄ Al₄O₂₀(OH)₄ (castonite) and K₂Al₃Fe⁺²Si₇AlO₂₀(OH)₄ (phengite) are the another three ideal members related to Tschermark’s substitution. K₂Fe_3.75⁺² Al_1.50 Si₆Al₂O₂₀(OH)₄ (ideal siderophyllite) and K₂Fe_2.40Al_2.40Si₆Al₂O₂₀(OH)₄ (ideal Al-siderophyllite) are not the ideal end members, but only the ideal minerals in the phlogopite-annrenom.