The Breitenau sparry magnesite deposit in the Paleozoic of Graz (Eastern Alps/Austria) is situated in Upper Silurian parts of the Middle Hackensteiner Fm. of the Laufnitzdorf Group. The individual dolomite and magnesite rocks of the deposit compose the Breitenau member which is described as a new lithostratigraphic unit. Dolomicrosparite is the magnesite precursor. Two fragments of conodonts deriving from long living (Late Silurian – Early Devonian) conodont multielements were isolated from dolomicrosparite. There is only one generation of magnesite. “Rosszahn”-dolomite, coarse pigment-rich dolomite crystals and very fine grained dolomite were formed after the magnesite formation; magnesite redolomitisation occurs along cleavage planes and crystal boundaries. Late generations of dolomite were formed along faults, joints and in cavities. The footwall of the Breitenau member consists of dark grey, green and red schists, which contain hypidiomorphic breunnerite and mesitine crystals in dolomite-quartz veins. The hangingwall is composed of metatuffite with intercalations of purple limestone, diabase, siltic dolomite and black, partly tuffaceous shale with intercalations of lydite and metatuff. Contacts of magnesite and dolomite, petrografic results, as well as geochemical and isotope data indicate a metasomatic magnesite formation. Pigment that consists mainly of Corg and phyllosilicates in zoned and black magnesite crystals images the former structure of dolomite crystals. Occasionally magnesite crystals contain primary dolomite inclusions. Increased CaO-contents in the magnesite are due to this dolomite inclusions and redolomitisation. Fe and Mn substitute for Mg in the magnesite lattice, FeO is also contained in accessory minerals. Chlorite and quartz in the pigment cause elevated SiO2-contents. The quality of the magnesite in the lower and central parts of the deposit is good, CaO-, SiO2- and Fe2O3- contents are usually lower than in the hanging part. Some trace element contents (Sr, Ba, Cr, Ni, Co) are comparable to magnesite formed by Mg2+ metasomatism and in marine-evaporitic environments. Similar patterns of REE in dolomites and magnesites indicate a formation of the magnesite through hydrothermal alteration respectively Mg2+ metasomatism. Magnesite that lacks pigment shows a depletion of light REE. δ13C- und δ18O-data are similar to that of sparry magnesite of the Austroalpine Greywacke Zone. Anthracite/Metaanthracite in magnesite displays the same rank of coalification as neighbouring rocks. The depositional environment could be a passive continental margin, where basins formed due to intracontinental rifting in the presence of volcanic islands or seamounts. The time of the magnesite metasomatism, however, is still a “hot” topic of discussion.