各种斜长石成分的分布规律和低结构态斜长石分类

斜长石是自然界最常见的造岩矿物,一般认为它是完全类质同象系列的典型代表。然而近代研究发现,斜长石在地质体中出现的概率是不均等的,呈不连续的众数分布。在斜长石的X射线衍射、电子衍射、光学性质以及红外吸收光谱所记录的相关曲线上,出现反映结构变化的不连续点,这些不连续点与上述稳定斜长石(呈众数者)的成分点一致。另外,斜长石的非布拉格“卫星”衍射等都说明斜长石超结构对斜长石的稳定性起着重要的制约作用。本文以大量的实际资料,把斜长石的内部结构和斜长石各种物理化学性质联系起来,讨论各种斜长石成分的分布规律,并提出低结构态斜长石分类。

ON THE DISTRIBUTION REGULARITY OF COMPOSITION OF DIFFERENT PLAGIOCLASES AND THE CLASSIFICATION OF LOW-PLAGIOCLASE

The measurements made on a universal stage of the composition of 3310 plagioclase grains from a certain diorite body in East China indicate that the composition frequency curves of these plagioclases always consist of one or more normal distribution with peak-values of An32-33, An38, An45, and An53-54, which agree very well with the composition frequency curves of plagioclases obtained elsewhere in the world. In our frequency curve some composition points of plagioclase are present either as a mode or as a sub-mode, and others form background value, depending on the structure, especially on the superstructure of this mineral.According to our model formula of superstructure (N/18),there are three groups of stable plagioclase as follows:1. When N = 2n (n = 1, 2, 3,…), we obtain more stable plagioclase, named even point plagioclase, such as An11.1, An22.2… An33.3, An44.4, An55.5, An66.6, An77.7, An88.8, and pure auorthite (An100), among which the four-fold points of plagioclase (such as An22.2, An44.4, An66.66 and An88.8) are the most, stable. The composition points of the stable plagioclase often coincide with the discontinuous points on the curves of 2V, refractive index, some X-ray diffraction and infrared absorption as well. This is, in our opinion, related with the stable structure of the plagioclase.2. When N = 2n + 1 (n = 1, 2, 3,…), we obtain the less stable plagioclase, named odd point plagioclase, such as An5.5, An16.6, An27.7, An38.8, An50, An61.1, An72.2, An83.3 and An94.4.When N is not an integer, these plagioclases are shown as background-distribution on the statistical curves.According to the theory of plagioclase superstructure, the stability of this mineral is mainly associated with the distribution pattern of the subcell, the dimension of "domain", the nature of domain boundary, the degree of replacing boundary and polarizing deformation, the amount of connected surface in all subcells and so on.’ The authors interprete the plagioclase with the so-called exsolution texture (as peristerite exsolution, Boggild loop and Huttenlocher exsolution) ; in fact, as an inter-growth of a kind of stable plagioclases, i.e. even point plagioclase or odd point plagio-elase in a single crystal. Basing on the data obtained by means of X-ray diffractions and others the authors would offer seven types of low-plagioclase as follows:1. Type of C 1 structure (An0-An2-3) gives only "a" diffraction.2. Type of "peristerite structure" (An2-3-An22.2) gives sometimes the weak "e" diffraction in "peristerite" due to the reflection of An22.2 lamela in it.3. Type of main-Cl structure (An22.2-An44.4) seems to be C1 > I1 and appears clearly as an "e" diffraction and becomes gradually more intensive with the increase of An component. Then the "f" diffraction begins to appear from An32 but in a diffusive state.4. Type of intermediate structure (An44.4-An50) gives a standard parameters of superstructure, δa + δc = 160 and δa + δb = 40, and the lattice is CI≌ I1.5. Type of main-I1 structure (An50-An77.7) gives a sharp "e" diffraction and the intensity of "f" diffraction is obviously significant with the increase of An component, while the "b" diffraction also starts to appear and becomes sharper with the increase of An component. All the characteristics mentioned above show that the lattice of the plagioclase is dominated by I1 with the P1 in a less amount, for there exists a weak "c" diffraction.6. Type of I1-P1 structure (An77.7-An94.4) gives the "e" and "f" diffractions in diffusive form, while the "c" diffraction becomes distinctly more intensified. As a result in the lattice of plagioclase the I1 gives way to P1.7. Type of P1 structure (An94.4-An100) gives the various diffractions of original lattice in addition to the "e" and "f" diffractions.