BH250-134
Title
Subject
Description
BH250-134 and BH250-134b are feldspar samples collected from the Ingersoll Mine near Keystone, South Dakota. This mine is renowned for producing some of the largest and most well-formed crystals of microcline. These specimens are part of the broader Harney Peak Granite pegmatite district, known for hosting some of the most spectacular pegmatites in North America.
Microcline crystals from the Ingersoll Mine and nearby pegmatites can exceed 3 meters (10 feet) in length, making them among the largest feldspar crystals ever recorded. (See field photo.)
The crystals are typically massive and blocky, with prominent perthitic intergrowths—intergrown albite and microcline. Many also show vivid amazonite coloring, a striking blue-green hue caused by trace amounts of lead or copper.
Under the microscope, this microcline exhibits distinctive tartan (cross-hatched) twinning, a diagnostic feature that distinguishes it from orthoclase.
These samples commonly display perthitic textures, formed through the exsolution of albite from potassium feldspar during slow cooling. The coarse perthitic intergrowths in this region are textbook examples of exsolution in feldspars and are ideal for teaching purposes.
Some feldspar specimens from Keystone even show perthitic lamellae visible to the naked eye, thanks to the extremely coarse grain size of both host and exsolved phases.
These feldspars are hosted in granitic pegmatites, part of the Harney Peak Granite complex, which was emplaced during the late Paleoproterozoic (~1.7 billion years ago).
The pegmatites are enriched in rare elements, and in addition to feldspar, they also yield quartz, spodumene, beryl, tourmaline, and muscovite.
Coverage
GPS Coordinates: 43°52'50.19"N, 103°25'1.96"W
Creator
Source
Sample BH250-134. Housed at Carleton College in Minnesota.