United States Geological
United States Geological
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美国地质
作者：
D. Lindley
各种形式的碳酸钙，以及关于每种形式倾向于发生的条件的许多新工作的说明。结果表明，在一般条件下，无水盐只有方解石、文石和钙石三种确定的改性形式和一种水合形式——六水盐。其他的各种形态，如水晶石、珍珠岩等，被证明是方解石或文石的变种。方解石在所有常温下都是稳定的形式，但在970°时似乎会发生可逆转变，变成一种非常相似的改性«-CaCO3。文石和/t-Ca CO3在一般条件下都是亚稳的，并倾向于过渡到方解石。文石向方解石不可逆转变的速率随温度升高而增加，但在常温下非常缓慢。没有实验证据支持或反对文石在零度以下或高压下是改性稳定的观点。不稳定形式沉淀的条件实际上是未知的，但有利于它们形成的因素包括高度过饱和，同形盐核的存在，溶液中硫酸盐的存在，以及某些有机机构的作用。本文还包括对区分方解石和文石的各种方法的批判性检查。单独使用硝酸钴测试是不可靠的，因为许多其他矿物会产生与文石相同的反应，而少量杂质的影响是如此之大。关于硫酸亚铁试验，以及图格特和尼德施塔特所提倡的试验，也得出了类似的结论，其中影响结果的主要因素似乎是材料的分裂状态。作者得出结论，化学测试通常是不可靠的，应该用光学测试代替，只有当材料是亚微观的时候，光学测试才会失败。答:S。
various forms of calcium carbonate as well as an account of much new work regarding the conditions under which each form tends to occur. It is shown that under ordinary conditions there are only three definite modifications of the anhydrous salt, calcite, aragonite, and jd-CaCOg, and one hydrated form, the hexahydrate. The various other forms, such as vaterite, conchite, and so forth, are proved to be varieties of either calcite or aragonite. Calcite is the stable form at all ordinary temperatures, but appears to undergo a reversible transformation at 970° to a closely similar modification «-CaCO3. Both aragonite and /t-Ca CO3 are metastable under ordinary conditions and tend to pass over to calcite. The rate of the irreversible transformation of aragonite to calcite increases with the temperature, but is very slow at ordinary temperatures. There is no experimental evidence for or against the view that aragonite is the modification stable below zero or under high pressures. The conditions under which the unstable forms precipitate are practically unknown, but the factors favouring their formation include a high degree of supersaturation, the presence of nuclei of isomorphous salts, the presence of sulphate in the solution, and the action of certain organic agencies. The paper also includes a critical examination of the various methods of distinguishing calcite and aragonite. The cobalt nitrate test, when used alone, is unreliable, as so many other minerals give the same reaction as aragonite and the effect of small quantities of impurities is so great. A similar conclusion is reached with regard to the ferrous sulphate test as well as in those advocated by Thugutt & Niederstadt, where the chief factor which influences the result seems to be the state of division of the material. Ths authors conclude that the chemical tests are, in general, unreliable and should be substituted by optical ones, which only fail when the material is sub-microscopic. A. S.