Here's a sample of botryoidal psilomelane.  This term no longer applies to a mineral.  Psilomelane turns out to be masses of different manganese oxide minerals.  However, the dominant mineral in such specimens is romanechite, a barium manganese hydroxy-oxide (BaMn+2Mn+48O16(OH)4).


Romanechite has a nonmetallic to “submetallic” luster (I don't see the latter in available materials), a dark gray to black color & streak, is moderately hard (H=5 to 6), and has no cleavage.  It forms fine-grained masses without visible crystals.  Romanechite often occurs as botryoidal masses (seen in cross section on sample below).



Romanechite (field of view 3.9 cm across) makes up most of this botryoidal mass called psilomelane.  Other minerals known to be present in psilomelane rocks are cryptomelane (potassium manganese oxide - KMn8O16), manjiroite ((Na,K)Mn8O16·nH2O), and todorokite ((Mn,Ca,Mg)Mn3O7·H2O).



Photo gallery of romanechite



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