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Category Phosphate mineral
(repeating unit)
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
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 [1][2][3]

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.[1]

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, Erzgebirge, Saxony, Germany. It was named for Saxon mining official Sigmund August Wolfgang von Herder (1776–1838).[1]