In the year 1869, 150 years ago, Dmitrij Ivanovič Mendeleev published the classification of the known chemical elements in the form of a periodic table. This scientific goal was achieved thanks to the genius both of Mendeleev, that had recognized the periodicity in the properties of the elements, and of those who had identified all the elements already known at Mendeleev’s time. This discovery process frequently occurred at the edge between chemistry and mineralogy, as a result both of the scientist’s curiosity and of the need to identify the minerals useful to the metals smelting. A brief description of the path that has lead to the discovery of all the elements of the periodic table is not possible; for that reason this work is going to deepen the analysis on the elements whose discovery has involved a mineral and was particularly peculiar. This discovery path had begun already in the ancient time. It is possible to say that the mankind started to isolate and handle the elements during the neolithic age, becoming, over time, more skillfull in recognizing new elements. The path has begun by using the metals already present in nature as native ores, as copper, silver and gold, all already known during the chalcolithic age. From this first step to the invention of the first extraction techniques and smelting, able to yield the metal starting from its minerals, it was a short step. In the ancient time at least nine elements were already known and used. We are talking about “elements”, giving to this word the meaning used in the modern chemistry. This last consideration could lead to another scenario that, however, is out of this speech: the evolution of the concept of “element”. The new elements discovery path, still before the modern definition of “element”, received a huge help by the alchemy: the isolation of four elements was achieved in that period. During the XVIII century the discovery of new elements has seen an acceleration, thanks to the historical context of the Age of Enlightenment. In that period two very similar stories involved the discovery of cobalt and nickel. Both these elements are named from creatures belonging to the miners’ mithology: the miners used to find frequently minerals that, based on their experience, should have contained metals. Those minerals, however, did not yield any known metal and, for this, the miners blamed fantasy creatures: the Kobolds, sprites stemming from Germanic mythology, and Nickel, a mischievous sprite also belonging to German miners mythology. These puzzles were solved by two scientist: George Brandt, that discovered the cobalt, and Axel Frederik Cronstedt, that discovered the nickel. A very peculiar case is represented by fluorine: between the demonstration, occurred in the 1771, that the fluorite contains a new element, and the isolation of elemental fluorine, successfully performed only in the 1886, more than one hundred years have passed. Sometimes the identification of new elements was the product of lucky coincidences. The beginning of the epopee of the rare earth elements discovery was one of these cases; it was determined by two main factors: the presence in Sweden of some important chemists and the discovery, at that time, of a strange mineral, called gadolinite, in a quarry close to the village of Ytterby. Sometimes the identification of a new element was determined by a very clever reasoning about incongruous data and measures. The fact that the radioactivity of the pitchblende (or uraninite) was too high considering only its content in uranium, has lead Marie Sklodowska Curie to the discovery of polonium. The discovery of new elements, in the last century, moved from the edge between chemistry and mineralogy to the edge between chemistry and physics.
Classification of topological invariants related to corner states
