Nepheline syenite from the Pennsylvanian of Maine, USA. (public display, Geology Department, Wittenberg University, Springfield, Ohio, USA)
Medium gray = nepheline
Whitish-gray = feldspar
Black = biotite mica
Yellowish = cancrinite
The unusual rock shown here if from an alkaline igneous intrusion in New England famous for producing yellow cancrinite and blue sodalite mineral specimens. The combination of cancrinite and sodalite in the nepheline syenites near Litchfield, Maine is given the rock name "litchfieldite". This specimen lacks the blue sodalite content, which is less common at the locality.
Nepheline and cancrinite are feldspathoid minerals - the "feldspar-like minerals". Feldspathoids have more aluminum and less silicon than the feldspars. They are less stable than feldspars and usually decompose in strong acids - they don't effervesce, but become gels.
Nepheline is an aluminum-rich version of sodium feldspar - it has the chemical formula NaAlSiO4 (sodium aluminosilicate). The formula has also been cited as (Na,K)AlSiO4 (sodium potassium aluminosilicate).
Nepheline has a nonmetallic luster, is usually whitish to grayish in color, is moderately hard (H = 5.5 to 6 on the Mohs Hardness Scale), forms hexagonal crystals, and has poor cleavage. It is an important rock-forming mineral in many syenites. It is usually mixed with albite feldspar (= sodium feldspar, NaAlSi3O8).
Cancrinite is a complex feldspathoidal mineral with the formula (Na,Ca)8(Al6Si6O24)(CO3,SO4)2ยท2H2O - hydrous sodium calcium carbonate-sulfate-aluminosilicate. It's a good example of a "garbage can mineral". Cancrinite varies in color, but is often yellowish to orangish to reddish.
Geologic unit & age: Litchfield Pluton, Early Pennsylvanian, ~321 Ma
Locality: unrecorded/undisclosed site between the towns of Litchfield & West Gardiner, near Cobbosseecontee Stream (possibly from the Dennis Hill locality), southern Kennebec County, southwestern Maine, USA
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Some info. from:
West et al. (2016) - The Litchfield Pluton in south-central Maine: Carboniferous alkalic magmatism in northern New England, USA. Atlantic Geology 52: 169-187.
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