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Formula: LiAl(PO4)(OH)
Anhydrous phosphate containing hydroxyl, amblygonite group, and forms a
series with it, lithium-bearing mineral
Crystal System: Triclinic
Specific gravity: 2.98 to 3.04 measured, 3.03 calculated
Hardness: 5½ to 6
Streak: White
Colour: White, colourless, gray white, very pale brown, pale pink, pale yellow
Solubility: Soluble with difficulty in acids. Colours a flame red (Mindat)
Common impurities: Na
Environments:
Montebrasite is a late primary and secondary phosphate in
lithium-bearing
granite
pegmatites, often forming very large crystals up to one metre
in length.
Associated minerals include hydroxylapatite,
fluorapatite, cassiterite,
spodumene, lepidolite,
petalite, pollucite,
topaz, tourmaline,
triplite, lithiophilite,
lacroixite, crandallite,
muscovite and
albite (HOM, Dana, Mindat).
Localities
At the type locality, the Montebras Mines, Creuse, Nouvelle-Aquitaine, France, montebrasite occurs in a
granite
pegmatite
(Mindat).
Montebrasite from Montebras - Image
At the Karibib district, Namibia, montebrasite occurs in lepidolite
pegmatite with
petalite (Dana).
At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, montebrasite occurs as pods to 25 cm across in
the core margin.
In many cases the pods are associated with
triphylite-lithiophilite or
with
pollucite and beryl. One mass has
been found which
is an intergrowth of montebrasite with barium-bearing goyazite. The
Emmons
pegmatite is an example of a highly evolved
boron-lithium-cesium-tantalum
enriched pegmatite
(R&M 94.6.511-512).
Montebrasite from the Emmons Pegmatite -
Image
At the Plumbago North pegmatite, Newry, Oxford County, Maine, USA, the currently exposed (in 2019) portion of the
pegmatite is principally composed of
quartz, albite,
muscovite and spodumene;
microcline is sparse and occurs in the
spodumene zone and in rare miarolitic cavities. Montebrasite,
beryl, cassiterite,
almandine–spessartine,
fluorapatite and
columbite group species are also present;
schorl, lepidolite and
pollucite are extremely rare.
The columbite group and similar minerals include
columbite-(Fe),
columbite-(Mn) and
tapiolite-(Fe).
Triphylite pods up to 50 cm across occur throughout the
spodumene-rich zone. Numerous
secondary phosphates related to metasomatic alteration of
triphylite and montebrasite have been identified including
ferrisicklerite,
heterosite, vivianite,
strunzite,
childrenite–eosphorite,
whitmoreite, beraunite,
bermanite, laueite,
stewartite,
messelite–fairfieldite,
rockbridgeite,
phosphosiderite,
jahnsite group minerals,
paravauxite, mitridatite,
diadochite,
hydroxylherderite and
moraesite.
Siderite-rhodochrosite
occurs replacing massive triphylite.
Most primary Fe-Mn phosphates are Fe-dominant as are the
secondary phosphates. Only a few (such as
bermanite) have Mn > Fe.
Montebrasite occurs in very large single crystal masses, some exceeding 1.5 meters across
(CM 57.795-797).
Montebrasite from Newry - Image
Alteration
Montebrasite may alter to mixtures of clay minerals or
kaolinite and mica, often as rims
surrounding
rounded nodules of unaltered material. It is also observed to alter to
turquoise,
wavellite, wardite or
morinite (Mindat).
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