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Formula: CaBe(PO4)(OH)
Anhydrous phosphate containing hydroxyl, beryllium-bearing mineral
Crystal System: Monoclinic
Specific gravity: 2.95 measured, 3.00 calculated
Hardness: 5 to 5½
Streak: White
Colour: Colourless, grey, brown, pale yellow, greenish white, light blue, purple; colourless in transmitted light. May be blue-green or
blue in daylight, lavender or light violet in incandescent light
(Mindat).
Solubility: Soluble in acids
Environments:
Hydroxylherderite is found usually as a pegmatite mineral formed during late stage hydrothermal deposition in crystal-lined
cavities
(Mindat, Webmin, HOM).
It may form from the alteration of beryl or
beryllonite
(HOM, AM 63.913).
Associated minerals include elbaite, topaz,
cassiterite, albite,
microcline, muscovite,
lepidolite, fluorapatite,
fluorite and quartz
(HOM, R&M 87.5.449).
The presence of these species can indicate very high fluorine activity at the time of formation. Hydroxylherderite forms late,
when most of
the fluorine may have been consumed by the other fluorine-bearing mineral species, and only in rare cases will sufficient fluorine
be left over to form herderite
(R&M 87.5.449).
Localities
At Minas Gerais, Brazil, hydroxylherderite is associated with lepidolite,
elbaite, topaz and
microlite
(Dana).
At the Sapo mine, Minas Gerais, Brazil, hydroxylherderite is a rare accessory mineral in the pegmatite, normally found inside
tourmaline-bearing cavities
(Min Rec 40.4.290).
In the Erongo Mountains, Namibia, hydroxylherderite is found in crystal-lined cavities associated with
schorl,
muscovite and orthoclase
(MinRec 37.5.419).
At Klein Spitzkoppe, Namibia, hydroxylherderite has been found associated with euclase
(MinRec 36.4.325).
At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, hydroxylherderite is common, and commonly associated with
fluorapatite and earlier pegmatite minerals including
muscovite, quartz and
feldspars. The Emmons pegmatite is an example of a highly evolved
boron-lithium-cesium-tantalum
enriched pegmatite
(R&M 94.6.509).
At the Crooker Gem Pegmatite, Newry, Oxford county, Maine, USA, the mitridatite-coated
fractures are rich
in phosphate species. The mitridatite is the first phosphate to develop in the fracture, followed by
jahnsite, laueite,
phosphosiderite, pseudolaueite,
rockbridgeite, stewartite,
strunzite, whitmoreite, and
xanthoxenite, all of which have overlapping formation.
Fluorapatite, eosphorite,
hydroxylapatite, hydroxylherderite and
zanazziite
are found in the Crooker gem pegmatite but are not found in late fractures
(R&M 87.5.446).
At the Newry mine, Oxford county, Maine, USA, hydroxylherderite is an alteration product of
beryllonite
(Dana, Mindat).
The Fletcher mine, Groton, Grafton County, New Hampshire, USA, is the premier New Hampshire locality for
hydroxylherderite. Crystals found here are not in the phosphate pods derived from the alteration of
triphylite but in pockets that are some distance away from them. The
pockets are 10 to 15 cm in diameter and contain millimeter-sized muscovite
crystals and hydroxylherderite crystals to 5 cm, ranging in colour from dark brown to a light greenish brown to
yellowish brown or cream-coloured
(R&M 97.3.224-226).
At the Palermo No. 1 mine, Groton, Grafton County, New Hampshire, USA, hydroxylherderite occurs as thick tabular
to pseudohexagonal crystals as well as radiating fibrous aggregates. They can range from colourless, pale green,
yellow-green and tan, to reddish-brown
(R&M 97.3.224-226).
At the Keyes Mica Quarries, Orange, Grafton County, New Hampshire, USA, the
pegmatites are beryl-type
rare-element (RE) pegmatites.
The Number 1 mine exposed a pegmatite that shows the most
complex zonation and diverse mineralogy of any of the Keyes
pegmatites. Six zones are distinguished, as follows, proceeding
inward from the margins of the pegmatite:
(1) quartz-muscovite-plagioclase
border zone, 2.5 to 30.5 cm thick
(2) plagioclase-quartz-muscovite
wall zone, 0.3 to 2.4 metres thick
(3) plagioclase-quartz-perthite-biotite
outer intermediate zone, 0.3 to 5.2 metres thick, with lesser muscovite
(4) quartz-plagioclase-muscovite
middle intermediate zone, 15.2 to 61.0 cm thick
(5) perthite-quartz inner intermediate zone, 0.9 to 4.6 meters thick
(6) quartz core, 1.5 to 3.0 metres across
The inner and outer intermediate zones contained perthite crystals up to
1.2 meters in size that were altered to vuggy
albite-muscovite with
fluorapatite crystals. This unit presumably was the source of the
albite, muscovite,
fluorapatite, quartz and other
crystallised minerals found in pieces of vuggy albite
rock on the dumps next to the mine.
The middle intermediate zone produced sheet mica with accessory minerals including
tourmaline, graftonite,
triphylite, vivianite,
pyrite, pyrrhotite, and
beryl crystals to 30.5 cm long and 12.7 cm across.
Hydroxylherderite from the Keyes No. 1 mine forms white to tan crystals to at least 6 mm in
albite vugs. Associated minerals include crystals of
fluorapatite, pyrite,
quartz and vivianite. Tiny
submillimeter glassy beige botryoids of hydroxylherderite have been collected at the Keyes No. 2 mine on just one
specimen, which also contains moraesite and
lazulite-scorzalite.
An unusual variety of hydroxylherderite was found as prismatic crystals, striated longitudinally, that
fluoresce pale yellow under shortwave UV
(R&M 97.4.319-320).
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