论斜长石超结构——关于e衍射参数及其矢量模的一些探讨

矿物的单位晶胞的晶体结构研究,曾使矿物学从外观形态描述进入微观研究。现代对矿物的非布拉格衍射--“卫星”衍射的研究,揭示了矿物具有超结构,从而使矿物学进入了新的发展阶段。矿物的超结构研究,表明矿物不是单位晶胞的简单重复,每一种矿物从宏观到微观,在单位晶胞,超晶胞等不同数量级范围内,都具有各自的组合、分布规律。无疑,超结构研究更能解释矿物的各种物理和化学性质,因而它具有深远的矿物学意义。

ON THE SUPERSTRUCTURE OF PLAGIOCLASES——A DISCUS-SION ON PARAMETERS AND VECTORS OF E DIFFRACTION

Basing upon the Megaw’s model of superstructure of plagioclase (a0, 9b0, 2c0) we studied some e diffractions of this mineral and proposed for e-plagioclases (An33.3 to An72 a correction formula of e diffraction, δa+δc=160°+N°, where N is the number of increased anorthite-like subcells more calcic than An50. Therefore, δa + δc will be increased by one degree for each increased anorthite-like subcell after An50.If the proposed formula holds true, we may establish following group of equations :δa + δb = 40°δa + δc = 160° + N°where a*, b*, c*, α*,β*, γ* are reciprocal parameters of An80 with typical IT lattice.The calculated values of (δa+δc) of twenty e-plagilclases are shown in Table 1. The calculated ualues coincide well with the observed ones, and the error of mean square root is only±2°. So, it is necessary to rewrite δa+δc = 160° as δa + δc=160°+N°. An obvious deviation of e-diffraction parameters, however is noted for the plagioclases more sodic than An33.3 or more calcic than An72, which is caused by the predominance of CT or PT umits over IT units in their lattices.According to Megaw’s superstructure model, we get An%= N/18. 100, where N is the number of anorthite-like subcells in the supercells. Therefore the formula of width of anorthitelike subcell comain in the direction of b axis is given as follows:where n is the number of anorthite-like subcells in the direction of b axis, and b is the length of the cell edge in the direction of b axis of An80 with typical JT lattice =12.87A). Hence, we obtainIt can be seen from Table 1 that all the calculated values of dtheor]. b coincide fairly well with the observed ones. Therefore, it is appropriate to take Smith’s T (periodicity of supercells) as the width of an anorthite-like domain in the supercells on b axis. Moreover, the calculated values also agree well with the observed ones in the* Smith (1974) wrote this formula as T =1/2|△s| diagrams of T (d) versus An% (see Figs., 5 and 6), with the exception of An<33.3 or An>72. We hold that the e-diffraction of plagioclase is related to the anorthite-like domains.As is well known,Hence, s + S0 + △s. When △s = 0 (or δh, δk and δ1 equal to zero), the above equation corresponds the condition of normal Bragg ’s diffraction; When s≠O, it corresponds the non-Bragg’s diffraction.Because we always get δa + δb=-40° and δa + δc = 160°+N°, as a result, we haveConsequently, the periodicity of all e-plagioclases is 9, whereas only in the case of N = 0(An44.4 - An50) that of 2δh-δl will be 9, because the latter changes with the composition of plagioclases.