Montebrasite

montebrasite

lepidolite

petalite

amblygonite

Images

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:

Pegmatites

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).

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).
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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, almandinespessartine, 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, childreniteeosphorite, whitmoreite, beraunite, bermanite, laueite, stewartite, messelitefairfieldite, 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).

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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