Montebrasite and associated secondary phosphates from the Palermo #2 mine, 
North Groton, New Hampshire

Jim Nizamoff¹, Robert W. Whitmore², Alexander U. Falster¹ William B. Simmons^1 
1Dept. of Geology and Geophysics, University of New Orleans, New Orleans, LA 70148
²924 South Stark Highway, Weare, NH 03281

Recent mining activity at the Palermo #2 pegmatite in North Groton, New Hampshire, has revealed the presence of primary montebrasite and a corresponding suite of secondary aluminum-bearing phosphates.   The Palermo #2 pegmatite can be classified as a moderately evolved LCT beryl-phosphate granitic pegmatite.   It is approximately 400 m in length, 7 to 50 m in width and is hosted by the Devonian age Littleton formation (quartz-mica schist).  The internal zonation is characterized by a weakly defined border, wall, intermediate, core marin and core zones.

Primary montebrasite occurs in the core margin as masses that are generally less than 1 cm in maximum dimension and is typically found in contact with, or proximal to, pods of triphylite and hydroxylapatite.  Scorzalite-lazulite is present in blue masses to 1 cm as a high temperature metasomatic alteration product of montebrasite.

 In addition, numerous low-temperature hydrothermal secondary phosphates derived from the alteration of montebrasite and associated minerals have been identified.
 

  Gormanite-souzalite is found as greenish-blue acicular crystals that form nodular masses to 10 cm associated with montebrasite, scorzalite-lazulite and chlorapatite.  Small masses (<100 µm) of augelite occur sporadically in gormanite-souzalite.  Goyazite and crandallite are found as platy laths forming rims on montebrasite or as pseudohexagonal plates assoiciated with fluorapatite in cavities in siderite. Childrenite-eosphorite forms slender prisms to 2 cm associated with siderite, hydroxylapatite, ludlamite, messelite-fairfieldite and scorzalite-lazulite.  Whiteite-(CaMnMg) occurs as radiating sprays of nearly colorless to light brown prisms to 2 mm associated with messelite-fairfieldite, paravauxite and scorzalite-lazulite.  Whiteite-(MnFeMg) occurs as aggregates of bladed browtn to orange crystals up to 1 mm associated with laueite, paravauxite, and jahnsite-(CaMnFe).

Jahnsite-(MnFeMg) is present as light brown striated bladed crystals to 1 mm associated with messelite-fairfieldite, paravauxite-gordonite and scorzalite-lazulite.  Jahnsite-(CaMnMn) also forms patchy to zoned overgrowths on whiteite-(MnFeMg).  Jahnsite-(CaMnFe) occurs as brown to orange prisms to 1 mm associated with laueite, paravauxite and whiteite-(MnFeMg).  In addition, jahnsite-(CaMnFe) appears as zoned overgrowth on whiteite-(CaMnMg)

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Paravauxite is found as sharp prismatic colorless to orange crystals to 2 mm associated with scorzalite-lazulite, gormanite-souzalite, whiteite-(MnFeMg) and whiteite-(CaMnMg).  Gordonite occurs as prismatic colorless crystals to 1 mm associated with siderite, gormanite-souzalite, hydroxylapatite and messelite-fairfieldite-collinsite.  Gordonite may also occur as zoned overgrowths on paravauxite crystals

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The low-temperature hydrothermal alteration of montebrasite cannot solely account for the substantial number of secondary phosphates at Palermo #2.  As montebrasite typically occurs proximally or interstitially with triphylite or hydroxylapatite, the alteration of these primary phases affects the nature of the hydrothermal fluids by contributing additional ions not available from the decomposition of montebrasite alone.

 

 To complicate matters even further, silicates, carbonates, sulfides, arsenides, and oxides occurring in the mineral assemblage of the core margin also contribute ions to the hydrothermal fluids responsible for the formation of secondary phosphates.  Alteration of the mineral assemblage in a given area may create a characteristic suite of secondary products whereas a completely different suite of secondary products can appear only a few centimeters away.