Herderite

Herderite is a phosphate mineral belonging to the apatite, phosphate group, with formula CaBe(PO₄)(F,OH). It forms monoclinic crystals, often twinned and variable in colour from colourless through yellow to green. It forms a series with the more common hydroxylherderite, which has more hydroxyl ion than fluoride.^[2]

Herderite
General
Category	Phosphate mineral
Formula	CaBe(PO4)(F,OH)
IMA symbol	Hrd[1]
Strunz classification	8.BA.10
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/a
Unit cell	a = 4.81, b = 7.7, c = 9.82 [Å]; β = 90.1°; Z = 4
Identification
Colour	Colourless, pale yellow, greenish-white
Crystal habit	Occurs as prismatic tabular crystals, pseudo-orthorhombic or pseudo-hexagonal; fibrous botryoidal to spheroidal aggregates
Twinning	On {100} or {001} as fishtail contact
Cleavage	Indistinct on {110}
Fracture	Subconchoidal
Mohs scale hardness	5 - 5.5
Lustre	Vitreous
Diaphaneity	Transparent - translucent
Specific gravity	3.02
Optical properties	Biaxial (-)
Refractive index	nα = 1.556 - 1.592 nβ = 1.578 - 1.610 nγ = 1.589 - 1.620
Birefringence	δ = 0.033
2V angle	Calculated: 70°
Ultraviolet fluorescence	Fluoresces violet under UV; cathodoluminesces and phosphoresces pinkish orange under X-rays
References	[2][3][4]

It is found in many parts of the world, often in pegmatites and associated with other apatite minerals.

It was first described in 1828 for an occurrence in the Sauberg Mine, Ore Mountains, Saxony, Germany. It was named for Saxon mining official Sigmund August Wolfgang von Herder (1776–1838).^[2]