Thermoanalytical Techniques and Mineral Studies: Interpreting Dolomite’s Behaviour

A thermoanalytical technique is one that measures the physical properties of a substance based on its response to a thermal load [1-2]. In a typical experimental design, a thermal program is run on a known amount of sample, and the material’s response to the applied thermal load is observed on a plot created by specific software. There are at least a dozen thermoanalytical techniques, but the most widely used are those that provide chemical (rather than physical) information. Specifically, we are talking about the following techniques: thermogravimetric analysis, differential thermal analysis, differential scanning calorimetry, and microthermal analysis. Among the physical information that thermoanalytical techniques can provide are: crystalline transition, second-order transition, fusion, vaporization, sublimation, absorption, adsorption, and desorption.

Regarding chemical information, we can mention: chemisorption, desolvation (especially dehydration), decomposition, oxidative degradation, solid-state reactions, and solid-gas reactions (e.g., oxidation or reduction) [3]. Among the applications of thermoanalytical techniques in the mining industry, decomposition products and treatment temperatures (in industrial furnaces) are of vital importance. It is known experimentally that thermogravimetry, together with differential thermal analysis, can determine whether the decomposition of a mineral occurs in one or multiple stages, allowing us to infer the appearance of successive and distinct products, which enables the destination and use of each of these products [4-28].

In 2023, I undertook a research project on the thermal behaviour of the dolomitic mineral [29]. To carry out this titanic task (conducting research in Argentina presents significant challenges), I had to conduct an extensive bibliographic review on all the technological aspects of this mineral [30] and a specific review on its thermal behaviour [31]. In this research, I came across various articles that explained that dolomite is a mixture of two carbonate minerals: magnesite and calcite [30]. In relation to this assertion, I have given a discussion with the appropriate experimental justifications [29] and why this statement is not correct. And one of the articles that caught my attention the most is that of Wiedemann & Bayer [32] an older article whose postulate is still valid in the field of mineral chemists, in which they explain that a TGA test carried out in an air atmosphere would show a two-stage process but that the thermogram presents only one signal due to strong overlapping of these signals. This is where I want to focus. This article focuses on analyzing empirical evidence. It questions to what extent researchers should rely on experimental facts versus speculation. Thermoanalytical techniques are complex, primarily because they have proven over the years to be essentially “extensive” techniques, as Rodriguez-Navarro et al. [33] clarify: the quantities of mass placed in the crucible, the heating rates, and the working atmosphere all influence the plot architecture of the thermogram. That is, there are external physicochemical phenomena (work variables) and internal ones (internal energy, Gibbs free energy, enthalpy, entropy, etc.) that determine the instrument’s response, and it is this response that must be interpreted. Like any analytical technique, thermoanalytical techniques can exhibit peak overlap. This overlap should be verified by observing split signals, such as doublets or triplets. From this situation, a peak deconvolution can be decided upon to clarify the thermal phenomenon present. However, interpreting peak overlap based on independent and isolated phenomena assumes that physicochemical phenomena combine in a linear and summative manner, which is not always true.

In summary, when interpreting the signals from the thermograms provided by the different thermoanalytical techniques, it must be taken into account that the kinetic, physicochemical, chemical, and physical factors in the study of solid materials are complex, and their study and understanding are not yet fully clear [34]. Given this framework, the greatest interpretative and scientific rigor must be applied and speculations that only try to force a convenient result for the researcher who simply wants to publish an article must be abandoned.

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