Herderite
Herderite, CaBe(PO4)F: a phosphate species split from hydroxylherderite at 5.86% fluorine. Hardness 5 to 5.5, fluoresces violet; the Brazil gem; localities.
Herderite is CaBe(PO4)F, the fluorine end of a series with hydroxylherderite, CaBe(PO4)(OH); both are on the IMA list. Hardness 5 to 5.5, density 3.02, biaxial (−). Its sheet sets the line at 5.86% fluorine, which checks exactly, and accepts one proven herderite, a gemstone from “Brazil” at 7.0% F; the other analyses on the two sheets are hydroxylherderite.
In short
- Two names, one series. The September 2026 IMA-CNMNC Master List lists herderite, CaBe(PO4)F, status G, 1828, Germany, and hydroxylherderite, CaBe(PO4)(OH), status Rn, 2007 s.p., USA. Both Handbook of Mineralogy sheets say the two form a series; the name depends on whether fluorine or hydroxyl is in the majority.
- The 5.86% line. The herderite sheet writes “F > 5.86%, the series midpoint”. Worked on standard atomic weights, CaBe(PO4)(OH0.5F0.5) weighs 162.06 and is 5.86% F; the hydroxylherderite sheet’s ideal 1:1 column (P2O5 43.80, BeO 15.43, CaO 34.60, F 5.86, H2O 2.78) reproduces to within 0.01. Pure herderite would be 11.65% F.
- The analyses, placed. The Brazilian gemstone’s F 7.0% puts 59.8% of the (F,OH) site as fluorine: herderite. The Virgem da Lapa, Brazil, analysis at F 5.31% is 45.3% fluorine: hydroxylherderite, close to the line. The Paris, Maine, analysis has fluorine only as a trace: hydroxylherderite.
- Documented failure: telling them apart by hand. Both sheets print hardness 5 to 5.5, both are biaxial (−), and both say their X-ray patterns cannot be distinguished. Density differs by 0.07 (3.02 against 2.95), 0.16 g in water on a 20.00 g piece; the index ranges meet at α 1.59, β 1.61 and γ 1.62. A fluorine analysis is what settles a name near the line.
- Beryllium. Worked, herderite is 5.53% beryllium by mass and hydroxylherderite 5.60%, close to beryl’s 5.03% and a third of phenakite’s 16.37%. Beryllium hazard questions are routed to toxic minerals in a collection; nothing here is advice.
| Material | F wt% | F share of the (F,OH) site | Name it takes | Source on the sheet |
|---|---|---|---|---|
| CaBe(PO4)F, end member | 11.65 (worked) | 100% | Herderite | IMA formula |
| Gemstone from “Brazil” | 7.0 | 59.8% | Herderite | Herderite sheet, electron microprobe |
| Series midpoint, OH:F = 1:1 | 5.86 | 50% | The line | Both sheets; worked 5.86 |
| Virgem da Lapa, Brazil | 5.31 | 45.3% | Hydroxylherderite | Hydroxylherderite sheet, column (2) |
| Paris, Maine, USA | Trace | Near 0% | Hydroxylherderite | Hydroxylherderite sheet, column (1) |
| CaBe(PO4)(OH), end member | 0 | 0% | Hydroxylherderite | IMA formula |
| Density, measured | Herderite 3.02 | Hydroxylherderite 2.95 | 20.00 g reads 13.38 against 13.22 g in water | Both sheets |
| Beryllium by mass (worked) | Herderite 5.53% | Hydroxylherderite 5.60% | Phenakite 16.37%, beryl 5.03% | IMA formulas |
What is herderite? A calcium beryllium phosphate
Herderite is calcium beryllium phosphate with fluorine, and its sheet is thin because proven material is scarce. The Handbook of Mineralogy herderite sheet gives the crystal system in brackets, [monoclinic, pseudo-orthorhombic], by analogy to hydroxylherderite, point group 2/m, and the habit simply as crystalline. Hardness 5 to 5.5; measured density 3.02, calculated not determined. Transparent, light green, colourless in transmitted light, vitreous. Biaxial (−), α 1.556 to 1.59, β 1.578 to 1.61, γ 1.589 to 1.62, 2V calculated 70°. The sheet adds that its X-ray pattern cannot be distinguished from hydroxylherderite’s and is similar to datolite’s.
Luminescence, as printed. “Fluoresces violet under UV; cathodoluminesces and phosphoresces pinkish orange under X-rays.” The hydroxylherderite sheet says that it “may fluoresce weak yellow under SW UV, perhaps with bright yellow-orange phosphorescence.” Those are the sheets’ words; this page adds no activator. Background is on fluorescent minerals explained, and lamp exposure limits are in the ICNIRP 2004 guidelines on limits of exposure to ultraviolet radiation; this is not safety advice.
Name. The sheet names it to honour Siegmund August Wolfgang von Herder (1776–1838), mining official, Freiberg, Saxony, Germany; the IMA list gives 1828 and Germany. Hydroxylherderite is named for the predominance of hydroxyl over fluorine and its relation to herderite; the sheet’s references spell it hydroxyl-herderite, and the IMA list records its status as Rn, 2007 s.p.
Herderite or hydroxylherderite? The 5.86% line
The rule: more than 5.86% fluorine by mass is herderite; less is hydroxylherderite. The herderite sheet prints the figure as “the series midpoint”. Worked on standard atomic weights: CaBe(PO4) with half a fluorine and half a hydroxyl weighs 162.06, and 0.5 × 18.998 is 9.50 of it, 5.86%. The hydroxylherderite sheet’s column (3), CaBe(PO4)(OH,F) with OH:F = 1:1, prints P2O5 43.80, BeO 15.43, CaO 34.60, F 5.86, H2O 2.78 and −O = F2 2.47; worked, the same formula gives 43.79, 15.43, 34.60, 5.86, 2.78 and 2.47.
Placing the analyses. Solving for the fluorine share of the site: the Brazilian gemstone at 7.0% F is 59.8% fluorine, a herderite. The Virgem da Lapa analysis, by electron microprobe with BeO calculated, reads F 5.31%: 45.3% by the same method, and 0.45 F per phosphorus from the oxides directly, a hydroxylherderite 4.7 points below the line. The Paris, Maine, analysis prints fluorine as a trace, with Ca 0.98 and Be 1.04 per phosphorus: a hydroxylherderite near its end member.
Documented failure: the hand specimen. Both sheets give hardness 5 to 5.5 and biaxial (−) optics; both say the X-ray patterns cannot be told apart. The index ranges meet rather than separate, herderite reaching α 1.59, β 1.61 and γ 1.62 where hydroxylherderite starts. Density, 3.02 against 2.95, means a 20.00 g piece reads 13.38 against 13.22 g in water, 0.16 g apart, but the herderite density is a single printed value; the method is on how to measure specific gravity at home. The sheet’s own position is that a gemstone from “Brazil” is the only known fluorine-dominant herderite proven by modern analytical methods, so on the sheet’s own account a specimen labelled herderite without a fluorine analysis is best read as a member of the series, and more likely hydroxylherderite.
Herderite crystals: what the hydroxylherderite sheet records
The crystals collectors see in the series are described on the hydroxylherderite sheet. The Handbook of Mineralogy hydroxylherderite sheet gives monoclinic, pseudo-orthorhombic or pseudohexagonal, space group P21/a; stout prismatic crystals elongated along [001] or [100], may be thick tabular {001}, typically with complex but rounded form development, to 17 cm; also botryoidal to spherical and radial fibrous. Twinning on {100} or {001} or both, as “fishtail” contact twins, is common. Cleavage on {110}, irregular; fracture subconchoidal.
Colour. Colourless, grey, brown, pale yellow, greenish white, light blue and purple; it may be blue-green or blue in daylight and lavender or light violet in incandescent light. Lustre vitreous to subvitreous, resinous. The occurrence is late-stage, in miarolitic cavities in complex granite pegmatites, and the sheet says it may form from the alteration of beryl or beryllonite. Its associates are elbaite, topaz, cassiterite, albite, microcline, muscovite, lepidolite and quartz; the bertrandite sheet, in turn, names herderite among bertrandite’s associates.
Beryllium, worked. Herderite is 5.53% beryllium by mass, 15.34% BeO; hydroxylherderite 5.60%, 15.53% BeO. On the same arithmetic phenakite is 16.37% and beryl 5.03%, as on those pages and on is phenakite a beryl. Beryllium hazard questions are routed to toxic minerals in a collection; nothing here is advice.
Where is herderite found? The two Distribution fields
Herderite’s own field is two sentences long. “Originally found in Germany, at Ehrenfriedersdorf, Saxony, but the analysis showing F > OH is suspect and any material from this locality seems unavailable. A gemstone from “Brazil” is the only known fluorine-dominant herderite proven by modern analytical methods.” The sheet marks no type locality, and this page does not supply one; see what is a type locality.
Hydroxylherderite’s field says many localities, but uncommon at most. For analysed material it lists, in Maine, Paris, Hebron, Greenwood, Stoneham and Newry, Oxford Co., Topsham, Sagadahoc Co., and Poland and Auburn, Androscoggin Co.; in New Hampshire, the Fletcher and Palermo #1 mines near North Groton and the Keyes mine, Orange, Grafton Co.; the Blue Chihuahua pegmatite near Oak Grove, San Diego Co., California; the Epprechtstein and Waldheim quarries, Fichtelgebirge, Bavaria; the Viitaniemi pegmatite near Eräjärvi, Finland; Mursinsk, Ural Mountains, Russia; Ungursuay, Kazakhstan; and in Minas Gerais, Brazil, large crystals from the Xandá mine, Virgem da Lapa, and the Golconda mine near Governador Valadares.
On a label. Write the name an analysis supports; without one, the series name with the locality is the honest form. More phosphates and beryllium species are in the species index. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.