Mereheadite

Mereheadite is a rare oxychloride that can be found with Mendipite at Merehead quarry, Cranmore, Somerset, in the United Kingdom. Most specimens are associated with calcite, mendipite or hydrous cerussite in the Manganese pods on vein two at torr works quarry (merehead quarry). This mineral is associated with symesite which is also light yellow to orange. Symesite is found in small blotches on the calcites or mendipites; Mereheadite does not, as it is most often found in veins.

Mereheadite
General
Category	Halide
Formula	Pb47Cl25(OH)13O24(CO3)(BO3)2
IMA symbol	Mrh[1]
Strunz classification	3.DC.45
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Unit cell	a = 17.372(1) Å, b = 27.9419(19) Å, c = 10.6661(6) Å; β = 93.152(5)°; Z = 2
Identification
Color	Pale yellow
Mohs scale hardness	3+1⁄2
Luster	Vitreous, resinous
Streak	White
Optical properties	Biaxial
Refractive index	nα = 2.190 nγ = 2.280
Birefringence	δ = 0.090
References	[2][3][4][5]