It
was a beautiful bright autumn day, with air like cider and a sky so blue you
could drown in it. Diana Gabaldon
Some
of my Blog readers know that I am partial to blue minerals, and to phosphate
minerals. And if the phosphate minerals
are blue, that is even better. Most of
my minimal understanding of phosphates come from minerals found in Precambrian
rocks of the Black Hills, and by conversations with Tom Loomis at Dakota
Matrix.
Like
many of the interesting phosphate minerals at localities around the world, the numerous
pegmatites in the Black Hills produce most of the phosphate minerals. Perhaps
the pegmatite most studied in the Hills is one exposed at the Tip Top Mine in
the Custer Mining District. This former beryllium mine is the Type Locality for
something like 12 colorful phosphate minerals and tens of others are mostly
hidden away in vugs and fractures; most are microscopic, but all seem to have beautiful
crystals. I have written about several of
these phosphates in this Blog. A couple
of years ago I had a personal tour of the Tip Top with noted mineralogist and
mine owner, Tom Loomis. I was looking
for some of the tiny Tip Top phosphates but no luck—lots of triphylite and
rockbridgeite but no nifty sprays of tiptopite!
Due to the Covid-19 pandemic I missed my fall trip to the Black Hills; however, I keep reading what geological information I can acquire on both the Hills, and on phosphate minerals from other localities in the U. S. One place that tweaked my interest was New Hampshire, home of several pegmatites along the west side of the state. I have had an opportunity to roam around New Hampshire enjoying the camping, mountains, and scenery, but that was in my soft rocks days and I was not really interested in mines and minerals. So, I did not take the opportunity to explore the mines scattered around in the Grafton Pegmatite Field. Like the Black Hills, the Grafton pegmatites are a hotbed for finding phosphate minerals, including the Palermo #1 pegmatite where something like 159 minerals, many of them phosphates, have been identified (MinDat.org).
Rather than feeling sorry for myself about the exile in the basement: I nabbed some coffee for my mug, pulled up my big boy pants, put a sparkle in my eye and a smile on my unshaven face, and decided that yep, I'm ready for the day. Bring on the phosphates.
So,
I started rummaging around my collection during the constant self-quarantine
time looking for phosphates to sort. All phosphates have the PO4 anion
with an oxidation state of 3- (phosphorus with a 5+ and 4 oxygens each with a 2- state leaves a total
anion state of 3-). Among rocks and
minerals (to differentiate from chemically synthesized forms) primary
phosphates crystalize from fluids in late stage magmatic crystallization, for
example the mineral triphylite (a lithium iron phosphate). Secondary phosphates
form many colorful specimens from the primary phosphates that are altered by aqueous
solutions into minerals like strengite (hydrated iron phosphate). Sedimentary phosphates
form at low temperatures from phosphorus-bearing organic material in ocean
basins, epeiric seas, continental shelves and even a few fresh waters (see September 4, 2020 Post). Many of these organic
phosphorites include large amounts of Apatite Group minerals.
What
I came up with in my fumbling around was a specimen containing the blue,
secondary phosphate, scorzalite, collected from the Palermo #1 Mine. The Grafton Pegmatite Field, with the Palermo
Pegmatites, is located 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. This uplift was
the result of plate movement as a microcontinent named Avalonia (parts of
Europe) was banging against Laurentia (proto North America), and being accreted
(the terrane was sticking to Laurentia) while the proto Atlantic Ocean was
being subducted under the Laurentian continental plate. Of course, the orogenic
event was much more complex that this explanation! 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. I always had much respect for the New England
geologists tromping through the vegetation and trying to decipher the
stratigraphy and tectonic sequences, especially those early field workers who
did not have knowledge of plate tectonics.
Cartoon x-section of events in the Acadian Orogeny, New England. Sketch courtesy of Fichter and Baedke at http://csmgeo.csm.jmu.edu.
In the above sketch, the Taconic rocks are roots of Ordovician orogenic events where microcontinent(s) (generally referred to as Taconic Terrane) banged into proto North America creating mountains and accretion. The Acadian terrane is the Avalonia Microcontinent being shoved up and accreted to proto North America. The Catskill rocks are a large clastic wedge of sediments shed off the rising Acadian Mountains. The Acadian rocks (Avalon terrane) were instrumental in the early days of defining "Continental Drift." By the Permian addition rocks and terranes had banged into, and stuck to, proto North America (the Alleghenian Orogeny) helping to create the Pangaea Supercontinent. By the Triassic Pangaea was breaking apart and in proto North America the split sent part of the Avalon rocks with proto Europe and part stayed with North America. Early geologists noted that some North American rocks were identical to European rocks and suggested that at one time they were connected and later "drifted" apart (Continental Drift). At that time geologists did not understand the mechanism for "drifting." Today we know much about plate tectonics and sea floor spreading.
The
pegmatites within the area of Palermo mines #1, #2, and #3 are quite complex
but have as their primary phosphates 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 scorzalite. Nizamoff (2006) also determined that
primary montebrasite [LiAl(PO4)(OH,F)] and/or triphylite (lithium iron phosphate) PLUS muscovite (to add alumnium) were subjected to high
temperatures, ~500oC—300oC, by infusions of hydrothermal fluids. These fluids also leached out lithium and fluorine and left behind iron, magnesium, aluminum, and
hydroxyl ions and the result was the formation of the iron aluminum phosphate,
scorzalite [FeAl2(PO4)2(OH)2], and/or
its magnesium analogue, lazulite [MgAl2(PO4)2(OH)2].
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.
Scorzalite
has a blue color of some sort—blue-green, light blue, dark blue—but mostly a
light sky blue with a resinous or subvitreous or even a dull luster. It may appear something less than transparent
but darker shades are opaque. The hardness is ~6.0 (Mohs) and specimens will
produce a white streak. Crystals may be observed in some specimens; however,
they are quite small. Most specimens are
granular or massive blue blotches.
As I noted above scorzalite [FeAl2(PO4)2(OH)2] has a magnesium analogue named lazulite [MgAl2(PO4)2(OH)2] and they are in a solid solution relationship. As best that I can tell lazulite is usually a darker blue color, at times a midnight blue, and has larger and better formed crystals. I am unaware of intermediate minerals.
Two photomicrographs picking up the dark blue color of lazulite crystals, each has a maximum width of ~ 3 mm. Specimen from Rapid Creek, Yukon Territory, Canada.
According to webmineral.com, the magnesium and iron elements are shown below:
Molecular Weight = lazulite 302.23 gm| Molecular Weight = scorzalite 325.88 gm | |
| Magnesium 1.86 % Mg | |
| Aluminum 16.56 % Al | |
| Iron 12.85 % Fe | |
| Phosphorus 19.01 % P | |
| Hydrogen 0.62 % H | |
| Oxygen 49.10 % O |
The specimen from Palermo #1 also held a bonus mineral that took me some time to identify--the iron manganese phosphate, phosphoferrite [(Fe,Mn)3(PO4)2-3H2O]. And boy was it a surprise--read on. As noted, I had this nifty crystal pegged as phosphoferrite due to physical properties including those displayed in photographs. Many specimens are some sort of a brown to reddish brown (the oxidized forms) to green colors and are transparent to translucent. They belong to the Orthorhombic Crystal System and often occur as tabular to pseudo octahedral crystals. They are soft at ~3.0+ (Mohs) with a vitreous to resinous luster. The twist is that I continued to study the fantastic tome by Nizamoff, (2006) and could not locate his description of phosphoferrite in the Palermo #2 Mine (next door to #1). Instead, he noted that Moore (1971) and Moore and others (1980) suggested that the mineral kryzhanovskite [(Fe,Mn)3(PO4)2(OH,H2O)3] may form from the oxidation of phosphoferrite. With this information Nizamoff (2006) noted "it appears that at Palermo #2 phosphoferrite crystallized in the non-oxidizing assemblage with vivianite, ludlamite and fairfieldite group minerals and was subsequently oxidized to kryzhanovskite. Consequently in the observed assemblage at Palermo #2, kryzhanovskite has a pseudomorphic relationship with phosphoferrite."
REFERENCES CITED
Moore, P.B., 1971, The Fe2+3(H2O)n(PO4)2 homologous series: crystal-chemical relationships and oxidized equivalents: American Mineralogist, vol. 56.
Moore, P.B., T. Araki, and A.R. Kampf, A.R., 1980, Nomenclature of the phosphoferrite structure type: refinements of landesite and kryzhanovskite: Mineralogical Magazine, vol. 43.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.
New
Hampshire is one of the birthplaces of American freedom and independence - a
place with a love and a passion for liberty. Marsha Blackburn