A Geoscience Digital Image Library

TitleThin section of nepheline syenite (malignite)
DescriptionThis is a photo of a cross polarized thin section of nepheline syenite (malignite). Nearly half of the rock is composed of euhedral to subhedral, elongated, pale brownish-yellow acmite prisms. Simple and lamellar twinning and zoning is present in some crystals, and grains exhibit a somewhat preferred orientation. Low birefringence nepheline and sanidine make up the other essential minerals. Grains are mostly subhedral and commonly intergrown. Nepheline shows a hexagonal outline in some crystals and lacks the cleavage and tabular form of the sanidine. A few percent of pleochroic amber biotite plates are present, in some cases euhedral and zoned. A percent or two of tiny euhedral magnetite makes up the opaques. Occasional large accessory forsterite anhedra are rimmed with bitotite. An occasional zeolite segregation may be present.
LocationUSA ▹ Montana ▹ Park
PhotographerDarla Sondrol. 2002-04-03.
CollectionWard’s Collection of Classic North American Rocks #18.
Key wordsnepheline, syenite, malignite, Montana
Tech details959 KB. Thin section. Nikon FDX-35.
GeoDIL number2563