Why is MnO2 called Manganese IV oxide

Manganese (IV) oxide MnO2 

Black, metallic
shiny powder
Mineral pyrolusite
molar mass 86.937 g / mol

AGW 0.2 mg / m3 E (TRGS 900)
density 5.08 g / cm3   
decomposition + 535 ° C
Water solubility   

GHS 07
GHS 08
Hazard classes + category  
Acute toxicity oral 4
Acute toxicity inhalation 4
Specific target organ toxicity inhalation repeat. 2
(Lungs, CNS)
HP rates (see also note)  
H 302, 332, 373
P 260, 280.2-3 + 7, 264, 314

G 4
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CAS 1313-13-9Manganese (IV) oxideManganese (IV) oxide
It is produced by grinding the pyrolusite, which belongs to the brown stones, or by heating manganese (II) nitrate in the air to over 500 ° C:
Mn (NO3)2• 6 H.2O MnO2 + 2 NO2 + 6 H.2O
Today, however, the most important technical process is accessible through the electrolysis of a manganese (II) sulfate solution. The divalent manganese ions oxidize to trivalent Mn at the anode3+that immediately goes back to Mn2+ and Mn4+ disintegrates and disproportionate. Brownstone is deposited on the anode.
Manganese (IV) oxide is a versatile oxidizing agent in organic syntheses, for example in the oxidation of aniline to hydroquinone. During glass production, it is used to discolour carbon and sulphide impurities (brownstone means something like "glassmaker's soap"). In flashlight batteries, the graphite cathode is embedded in manganese (IV) oxide: The manganese (IV) oxide continuously oxidizes the hydrogen occurring at the positive pole to form water. Manganese (IV) oxide is also used for gas cleaning in gas masks and as an oxidizing agent in fireworks. The black pigment manganese black was already used by cave painters.