Nordstrandite

nordstrandite

dawsonite

nahcolite

diaspore

Images
Formula: Al(OH)₃
Hydroxide, triclinic paramorph of monoclinic bayerite, triclinic doyleite and monoclinic gibbsite
Crystal System: Triclinic
Specific gravity: 2.43 to 2.436 measured, 2.45 calculated
Hardness: 3
Streak: White
Colour: Colourless, coral pink and reddish brown
Environments

Plutonic igneous environments
Pegmatites
Sedimentary environments
Hydrothermal environments

Nordstrandite occurs in four different types of occurrence:
1. As a weathering product of bauxitic soils derived from limestone
2. As a vein or fissure-filling mineral in dolomitic oil shale
3. As an alteration product of certain aluminium carbonate minerals
4. As a late-stage mineral in pegmatite pockets in nepheline syenite
(Dana).

Localities

There are two co-type localities, Gunong Kapor, Bau, Kuching Division, Sarawak, Malaysia, and Mt Alifan, Lamlan Ridge, Guam, USA.

At the Berry formation, Sydney Basin, New South Wales, Australia, nordstrandite occurs as an alteration product of dawsonite and alumohydrocalcite (Dana).
Nordstrandite rims around the margins of dawsonite nodules in New South Wales suggest that it is a secondary product formed through loss of soda from the dawsonite, according to the reaction:

NaAlCO₃(OH)₂ + H₂O ⇌ Al(OH)₃ + NaHCO₃
dawsonite + H₂O ⇌ nordstrandite + nahcolite
(AM 60.285-291)

At the Poudrette quarry, Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Quebec, Canada, nordstrandite occurs in pegmatites, miarolitic cavities and biotite-rich xenoliths in the nepheline syenite. It is commonly associated with natrolite, analcime, sodalite, albite and microcline. At least one of the carbonate minerals calcite, siderite, dolomite, ancylite and dawsonite is always present in small amounts, except in xenoliths where apatite takes the place of carbonates. Other minor minerals that may be present include aegirine, pyrite, zircon, rutile, fluorite and a member of the serpentine group. Nordstrandite commonly forms tabular rhombic microcrystals in random aggregates in cavities and interstices of natrolite, analcime or albite. It also forms globular aggregates and radiating clusters commonly with a core of analcime or siderite. Less commonly it forms relatively larger(1-2 mm) crystals of a distinct blocky habit in natrolite vugs. These crystals are translucent to opaque with a transparent outer zone. The opaque parts of the crystals contain numerous minute needle-shaped inclusions of dawsonite. Indications are that the order of crystallisation is microcline, albite, natrolite or analcime or sodalite, carbonates and nordstrandite, then pyrite and serpentine (CM 20.77-85).

At the Narssârssuk pegmatite, Narsaarsuk Plateau, Igaliku, Kujalleq, Greenland, nordstrandite occurs in the nepheline syenite pegmatite (Dana).

At Gunong Kapor, Bau, Kuching Division, Sarawak, Malaysia (one of the co-type localities), nordstrandite occurs in soil from the edge of a sinkhole in a limestone cliff (Dana). Associated minerals include quartz, kaolinite, diaspore and goethite (HOM).

At the Green River Formation, Colorado, USA, nordstrandite occurs as very thin layers of white platy or fibrous aggregates in fractures in dolomitic siltstone and oil shale.
The following is a proposed explanation of the excess of acid-extractable alumina in Colorado dawsonite rocks.

Nordstrandite rims around the margins of dawsonite nodules in New South Wales suggest that it is a secondary product formed through loss of soda from the dawsonite, according to the reaction:

NaAlCO₃(OH)₂ + H₂O ⇌ Al(OH)₃ + NaHCO₃
dawsonite + H₂O ⇌ nordstrandite + nahcolite

This same equilibrium equation is may explain the excess of alumina in the Colorado rocks. It is suggested that nordstrandite was precipitated from strongly basic lake waters charged with Na⁺, (CO3)^2- and aluminate ions (AlO₂)^-. In the water trapped in the sediment, the increasing concentration of CO₂, arising from organic matter caused the reaction

Na₂CO₃ + H₂O + CO₂ → 2NaHCO₃
natrite + H₂O + CO₂ → nahcolite

with an accompanying shift in pH from near 11 (strongly alkaline) to about 9 (somewhat less alkaline).
Under these conditions, the aluminate ion precipitates as nordstrandite, and the nahcolite combines with the nordstrandite to produce dawsonite according to the following equation:

NaHCO₃ + Al(OH)₃ ⇌ NaAl(CO₃)(OH)₂ + H₂O

It is suggested that since nordstrandite is the phase of Al(OH)₃ that is thus formed, this mineral, rather than the commoner gibbsite as was previously assumed, is responsible for the excess alumina in the Green River rocks, but there are other suggestions (AM 60.285-291).

At Mt Alifan, Lamlan Ridge, Guam, USA, one of the type localities, nordstrandite occurs in secondary solution cavities in limestone near the contact with deeply weathered basalt flows (Dana). It forms from its paramorph bayerite during aging at intermediate to high pH values (moderately to strongly alkaline). Near absence of bayerite in nature may indicate that it is metastable and will invert to nordstrandite in alkaline solutions.
Nordstrandite from Guam occurs as clusters of tiny crystals radiating out into microscopic solution cavities in limestone. Calcite and red clay occur with the nordstrandite as cavity fillings. Abundant nordstrandite occurs only in a part of the limestone near the contact with residual soils developed on basalt flows and tuff. It is considered probable that the nordstrandite precipitated from aluminous solutions of pH 8.5 to 9 (alkaline) flowing along the contact between the limestone and underlying basaltic soil. The enclosing limestone environment can be used to infer a pH above 7 (alkaline) during nordstrandite crystal growth, as was found in laboratory experiments (AM 55.43-77).

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