Anhydrous phosphate containing halogen, forms a series with hydroxylherderite.
There is some doubt whether the true end-member herderite exists in nature, and many reported occurrences may be hydroxylherderite rather than herderite (Mindat).
Specific gravity: 3.02
Hardness: 5 to 5½
Colour: Colourless, pale yellow, greenish-white; colourless in transmitted light.
Common impurities: H,OH
Most herderite-series minerals are the product of late-stage processes in
and significant fluorine enrichment in the fluids appears to result in the domination of herderite
over hydroxylherderite (AM 93.1545-1549). The most commonly
associated minerals are quartz, albite,
muscovite and clay.
Microcline, if present, is etched (AM 63.913-917).
At the type locality, the Sauberg Mine, Ehrenfriedersdorf, Erzgebirge District, Germany, herderite is found as a pegmatite mineral formed during late-stage hydrothermal deposition (Mindat).
At the Skayangi pegmatite, Mogok, Myanmar, herderite is associated with a complex rare-element beryl-type granite pegmatite (AM 93.1545-1549, Mindat).
Chywoon quarry, Cornwall, England, UK, is a muscovite-biotite granite quarry. Small crystals of herderite-hydroxylherderite have been reported here. The crystals occur mainly on the prism faces of quartz crystals, although a few have been found associated with fluorite and fluorapatite on mica. Herderite-hydroxylherderite series minerals typically occur within granite pegmatites and associated miarolitic cavities and greisen. If granite is enriched in fluorine the likelihood of herderite occurring, as opposed to hydroxylherderite, is increased. This appears to be the case at Chywoon quarry where an enrichment in fluorine has resulted in late-stage fluorine-bearing minerals, fluorite and fluorapatite, crystallising in cavities in the pegmatites (JRS 19.24-27).
At Meldon, near Okehampton, Devon, England, UK, herderite has been identified as an intergrowth with beryllonite in aplite, close to a pegmatite veinlet (JRS 19.24-27).
At Stoneham, Maine, USA, herderite has been found on quartz crystals in pockets in the pegmatite (USGS bulletin 445).
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