Plumbogummite

Plumbogummite
Plumbogummite from the Yangshuo Mine, Guangxi Province, China. Specimen size 4 cm.
General
Category	Phosphate minerals
Formula (repeating unit)	PbAl3(PO4)2(OH)5·H2O
Strunz classification	8.BL.10
Dana classification	42.7.3.5
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol: (3 2/m)
Space group	R3m
Identification
Formula mass	581.14 g/mol
Color	Blue, grey, greenish or yellow
Crystal habit	Crystals, rare, have a hexagonal outline
Cleavage	None
Fracture	Uneven or sub-conchoidal
Tenacity	Brittle
Mohs scale hardness	4 to 5 or 4.5 to 5
Luster	Resinous or dull
Streak	White
Diaphaneity	Translucent
Specific gravity	4.014
Optical properties	Uniaxial (+), segments of crystals may be biaxial
Refractive index	no = 1.653 or 1.653 to 1.688 ne = 1.675 or 1.675 to 1.704 no = 1.722 and ne = 1.742 for Ga-rich plumbogummite
Pleochroism	None
Solubility	Soluble in hot acids
Other characteristics	Non-fluorescent, not radioactive
References

n_o = 1.653 or 1.653 to 1.688
n_e = 1.675 or 1.675 to 1.704

Plumbogummite is a rare secondary lead phosphate mineral, belonging to the alunite supergroup of minerals, crandallite subgroup. Some other members of this subgroup are:

Plumbogummite was discovered in 1819 and named in 1832 from the Latin "plumbum" for lead, and "gummi" for gum, in allusion to its lead content and appearance, which at times resembles coatings of gum.

Plumbogummite crystallizes in space group R3m. The reported lattice parameters (the lengths of the sides of the unit cell) vary in detail according to the source, but all agree that normal plumbogummite has "a" close to 7 Å and "c" close to 17 Å, with Z=3. Various reported values of "a" and "c"are:

Mills et al. investigated a gallium-rich sample of plumbogummite from Tsumeb, Namibia, and found larger cell parameters, with a = 7.0752 Å and c = 16.818 Å.

The basic structural units of plumbogummite are PO₄tetrahedra, with phosphorus atoms (P) at the center and oxygen atoms (O) at the corners, together with AlO₆ octahedra, aluminium atoms (Al) at the center and oxygen atoms at the corners. The tetrahedra and octahedra combine by sharing corners, to form composite layers. Lead atoms (Pb) occupy sites between the layers.

Plumbogummite is found in the oxidized zones of lead-bearing deposits. It commonly occurs as botryoidal, kidney shaped, stalactitic or globular crusts or masses, frequently with a concentric structure; rare crystals have a hexagonal outline. Pyromorphite and baryte are common associated minerals, and plumbogummite may be pseudomorphic after them. Other associated minerals include mimetite, duftite, cerussite, anglesite and wulfenite.