Povondraite

Povondraite is a rare silicate mineral from the tourmaline group with formula: NaFe³⁺₃(Fe³⁺₄,Mg₂)(BO₃)₃Si₆O₁₈(OH)₃O.[2] It is a dark brown to black nearly opaque mineral with a resinous to splendent luster. It crystallizes in the trigonal crystal system as equant, distorted prisms with trigonal pyramid terminations.[3][2]

Povondraite
The crystals here are black, lustrous, and 1-2mm in size. They richly cover one side of the matrix, and the povondraite winds its way through the fissures in the rock.
General
Category	Cyclosilicates
Formula (repeating unit)	NaFe3+3(Mg2Fe3+4)(Si6O18)(BO3)3(OH)3O
IMA symbol	Pov[1]
Strunz classification	8/E.19-75
Dana classification	61.3.1.6
Crystal system	Hexagonal-Rhomboidal
Crystal class	Hemimorphic trigonal
Space group	R3m (No. 160)
Unit cell	a=16.18Å, c=7.44Å
Identification
Formula mass	1156.50 gm
Colour	Dark brown to brown-black, black
Crystal habit	Prismatic, striated
Cleavage	None observed
Fracture	Irregular, conchoidal
Tenacity	Brittle
Mohs scale hardness	7
Luster	Resinous
Streak	Brown
Diaphaneity	Translucent
Refractive index	1.74 - 1.82
Birefringence	0.069
Common impurities	K,Mg,Al,Cu,Pb,Sn,Ti,V,H2O

It occurs as rare fracture and cavity encrustations within schists derived from sedimentary rocks. Associated minerals include quartz, potassium feldspar, muscovite, schorl, riebeckite and magnesite.[3]

Discovered at the San Francisco mine, near Villa Tunari (in Alto Chapare), Bolivia, in 1976, originally it was called ferridravite, for the composition and the assumed relationship to dravite, i.e., "ferric dravite".[4] However later investigations yielded a new empirical formula which had no relation to the dravite. This called for renaming, and the new name, after Pavel Povondra (1924–2013),[5] a mineralogist at the Charles University in Prague, was approved by the International Mineralogical Association in the 1990s.[6]