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Formula: CaBe(PO4)F
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).
Crystal System: Monoclinic
Specific gravity: 3.02 measured
Hardness: 5 to 5½
Streak: White
Colour: Colourless, pale yellow, greenish-white; colourless in transmitted light.
Common impurities: H,OH
Environments:
Most herderite-series minerals are the product of late-stage processes in
granite pegmatites,
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).
Localities
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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