Serra da Mina Mine, Cercal, Santiago do Cacém, Setúbal, Portugal

MnO₂

Crystal System: tetragonal
Colour: iron-black, light steel-gray
Lustre: metallic, dull or earthy
Habitus: Commonly as reniform, concretionary, dendritic, granular and earthy aggregates. Rarely, as prismatic crystals with square cross-section
Hardness: 6-6.5; 2 when in earthy aggregates
Fracture: irregular/uneven
Cleavage: perfect on {110}
Density: 5.189 g/cm³ 
Origin and geological occurrence: pyrolusite is strongly oxidized Mn-bearing ore deposits and rocks. It is a common alteration product of Mn-minerals. It forms in bogs and lakes, under shallow marine, oxidizing, conditions.

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Samples:

4.12 Pyrolusite Microcrystalline vein in chert #oxides