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
INTERPRETATION OF CHEMICAL COMPOSITION AND SUBDIVISION OF MAGNESIUM IRON MICAS, PART A BASIC SUBSTITUTION AND BIOTITE PLANE
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摘要: 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(RⅥ+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(RⅥ+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。Abstract: 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 = (αRe1, αRe2, ...αRen, v) is a vectorial form used to represent substitution, here, aRei(.i=1,2,...n) represents the SUBSTITUTIONCOMPONENT of ion R with valence ei in V, vector. LENGTH L= (ΣαR2ei)1/2, COMPONENT SUM (CS = ΣαRei) and ELECTROVALENT SUM ES = Σei·αRei 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(Rv1+3)2□v1 (Fe-muscovite substitution vector) and Fe-1+2Alv1 Si-1Al1v (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(Rv1+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(Rv1+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. K2Fe3.75+2 Al1.50 Si6Al2O20(OH)4 (ideal siderophyllite) and K2Fe2.40Al2.40Si6Al2O20(OH)4 (ideal Al-siderophyllite) are not the ideal end members, but only the ideal minerals in the phlogopite-annrenom.
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Key words:
- Mg-Fe micas /
- biotite /
- chemical composition /
- subdivision /
- substitution /
- substitution vector
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