Mg-Fe云母化学成分的解释和分类(Ⅱ)——Mg-Fe云母的自然分类

INTERPRETATION OF CHEMICAL COMPOSITION AND SUBDIVISION OF Mg-Fe MICAS, PART B: THE NATURAL SUBDIVISION OF Mg-Fe MICAS

给出云母化学成分比值FM=6(Fe^(+2)+Mn)/(Fe^(2+)+Mn+Mg),用以表示Mg-Fe云母成分变化的基本趋势。根据291个云母的化学成分相对于FM值的异常变化,提出了天然Mg-Fe云母自然分类的新方案:把FM≌1.20(1.05—1.35)、4.80作为区分金云母、黑云母和铁叶云母的界线;FM>0.60的金云母称为铁金云母;以FM=2.70、3.60把黑云母再细分为富镁黑云母、镁铁黑云母和富铁黑云母。Mg-Fe云母自然分类的本质是各成员云母之间白云母置换方式和限度的潜在差异在不同的自然结晶条件下的反映。

It has been demonstrated that about 90% composi tional variability of Mg-Fe micas from granitoid, can be explained on the Foster's Mg-(Fe+2 + Mn) - (R+3 + Ti)vi triangle. In this paper, now, a natural subdivision of Mg-Fe micas is proposed modified from Foster's based on 291 mica analyses in terms of anomalous Changes in composition, and the essence of the natural subdivision is also discussed.The ratio FM = 6(Fe+2 + Mn)/(Fe+3+ MN+ Mg) is available to indicate the basic compositional trend by which about 80% variability of Mg-Fe mica composition can be explained. Most octahedral cations anomalously change their occupancies at around FM = 1.20(1.05-1.35) and 4.80, which are considered as the boundaries between phlogopite and biotite and between biotite and siderophyllite, respectively. By the same reason of anomalous changes in occupancies, phlogopite with FM>0.60 is named as ferrophlogopite, and biotite is further subdivided into three subgroups; magnobiotite (-1.20 FM 2.70), magnoferrobiotite (2.70 FM 3.60) and ferrobiotite (3.60 FM 4.80). Every cation occupany extends continuously to a certain limit, but the change in compositions as a whole from a variety to the neighbour one is discontinuous. In the light of substitution vectors it is found out that the discontinuous changes are due to the different extents of magnesium-or iron-muscovite substitution between two neighbour varieties. Ts-chermak's substitution of Al into lattice is very common in true micas within the limits of maintenance of the comfortable junction plaines between the octahedral and tetrahedral sheets, but it shows much fewer influence on composition than muscovite substitution and it is of no importance to natural subdivision of Mg-Fe micas.High-valent cations play a different role from Mg-rich to Fe+2-rich varieties in crystallo-chemistry. On the other hand, a variety of mica has its distinctive occurrence in nature. Thus, the natural subdivision of Mg-Fe micas essentially reflects the potential difference in the manner and extent of muscovite substitution among various micas during crystalization under distinctive natural conditions.