PHOSPHATES ARE PHUN: STEWARTITE

 

I am always on the lookout for phosphate minerals and have documented several in this Blog.  Famous mines producing secondary phosphates, often microcrystals, are the Tip Top in the Black Hills of South Dakota, the Foote Mine in North Carolina, the Branchville in Connecticut, and the Palermo #1 in New Hampshire.  I recently acquired a micromount of stewartite, a manganese iron phosphate [Mn⁺⁺Fe⁺⁺⁺(PO₄)₂(OH)₂-8H₂O], collected from Palermo #1 and mounted by R. Stevenson in 1972.

The micromount with a cluster of stewartite crystals in the lower 1/3 of the specimen and scattered crystals throughout. Maximum width of specimen ~3.5 mm.

The Grafton Pegmatite Field, with the Palermo Pegmatites #1, #2, #3, is in the Acadian Orogenic Belt, a tectonic area that represents the Devonian (~420Ma --~360 Ma) uplift of mountains in the northern section of the Appalachian Orogen, around southern Virginia to Newfoundland.  What we also know is that the active tectonic zone supplied magmatic plutons, volcanos, hydrothermal fluids, and lots of heat for metamorphic transformation of preexisting rocks. As I noted in a previous posting (29 Oct. 2020), the pegmatites within the area of Palermo mines are quite complex but have as their primary phosphate (with PO₄) minerals triphylite (Li, Fe) AND/OR lithiophylite (Li, Mn), and montebrasite (Li, Al) AND/OR ambygonite (Li, Al, F) (Nizamoff, 2006).  These primary minerals then interacted with post-magmatic aqueous fluids to produce several tens of secondary phosphate minerals including stewartite. Nizamoff (2006) also determined that primary montebrasite [LiAl(PO₄)(OH,F)] and/or triphylite (lithium iron phosphate) PLUS muscovite (to add aluminum) were subjected to high temperatures, ~500^oC—300^oC, by infusions of hydrothermal fluids.  These fluids also leached out lithium and fluorine and left behind iron, magnesium, manganese, aluminum, and hydroxyl ions and the result was the formation of the iron manganese phosphate, stewartite (among many others). So, the primary phosphates were heated by hot fluids which also leached out some of the elements and left behinds others producing the often-colorful secondary phosphates.

Photomicrograph of submillimeter-length stewartite crystals with arrow pointing to inclined termination of bladed crystal.

Photomicrograph of submillimeter-length stewartite crystals. 

Stewartite usually is found as yellow, golden yellow, orange, lemon yellow or brownish yellow, very tiny crystals where one needs magnification to observe them.  Crystals are translucent to transparent, almost vitreous, acicular to thin bladed with terminations that are inclined, soft (~3.0 Mohs), and leave a white streak (if one wants to ruin a crystal).  They often appear in tuffs and sprays or bundles.  Crystals of stewartite are quite attractive under magnification and often are difficult to visually distinguish from laueite (another phosphate).

REFERENCES CITED

Nizamoff, J., 2006, The Mineralogy, Geochemistry and Phosphate Paragenesis of the Palermo #2 Pegmatite, North Groton, New Hampshire: University of New Orleans Theses and Dissertations. 398.