**3. Niggli and Rittmann norm**

The Niggli norm (or Niggli's molecular catanorm) was developed by P. Niggli in 1933 [2, 3, 15]. It was later renamed the equivalent norm. It is in contrast to the CIPW standard, flexible in the choice of minerals to be considered. On the basis of the equivalent numbers standardized to 100%, the chemical rock analysis (**Table 1**) are the so-called base molecule groups developed. These are the simplest chemical compounds of the most common elements in the earth's crust. A standard mineral inventory can now be calculated from these basic molecule groups. The standard minerals calculated in this way correspond to the minerals that are present at high temperatures and high-pressure conditions arise. This is why, this standard was given the name Katanorm. Niggli's work was taken up by Barth (1952) [3, 16] and the norm was converted into the atomic equivalents. Barth developed the other methods Mesonorm and Epinorm for lower metamorphic rocks [17]. A computer program has also been developed for this purpose [18].

Even so, the Niggli norm never really caught on. The greater flexibility of the Katanorm compared to the CIPW norm brings one greater computational effort. In addition, the Katanorm minerals do not come from the modal mineral composition of rock closer than the standard minerals to the CIPW standard [3].

Rittmann (1973) [3, 19] developed a calculation method based on the Katanorm Niggli. It is closer to nature, but also much more complicated. The calculation method is particularly important for volcanites, in which the modal mineral inventory can only be partially recognized. The minerals are often submicroscopic, only partially or not at all crystallized. The Rittmann norm uses information that remain unused in the CIPW norm and the Niggli norm, such as the degree of oxidation, the H2O and CO2 content, or the geological situation, and the structure of the rock. The method, therefore, includes several different standard calculation methods that take this information into account. A computer program was developed at an early stage [20].
