Leucophosphite with Jahnsite-(CaMnMg)

specimen number:
2282058

location:
Tip Top Mine, Custer Co., South Dakota, USA

description:
Choice crystals to 0.2 cm in size on matrix. Quite good for the species.

The Tip Top mine, located near Custer in Custer County, South Dakota, is a historic pegmatite quarry situated within the Black Hills uplift. Geologically, the deposit is a highly fractionated, zoned granitic pegmatite of Proterozoic age that intruded surrounding Precambrian mica schists. The mine is celebrated for its complex internal zoning, which includes a massive quartz-microcline core and extensive hydrothermal replacement zones rich in lithium, beryllium, and phosphate minerals. Industrial mining operations at the site began around World War II, primarily targeting beryl, mica, lithium minerals, and feldspar. Surface quarrying and underground exploration continued intermittently under various operators through the mid-twentieth century, with active commercial extraction tapering off by the late 1980s as focus shifted from industrial production to specialized gemstone and mineral specimen recovery.

Leucophosphite is a relatively rare secondary potassium-iron phosphate mineral that typically forms as an alteration product within the heavily weathered or oxidized zones of complex granitic pegmatites. Geologically, it develops through the late-stage hydrothermal alteration of primary phosphate minerals, such as triphylite or vivianite, often accelerated by the action of meteoric waters or organic solutions in highly specific geochemical environments. The mineral commonly occurs as tiny, distinct crystals that display a monoclinic habit, frequently forming delicate, radiating clusters, spherical aggregates, or fine-grained crusts. Its coloration ranges from a pale, translucent greenish-yellow and brownish-yellow to light pink or completely colorless. Because it requires a precise combination of potassium, iron, and phosphate ions under low-temperature conditions to precipitate, its presence serves as an important indicator of advanced chemical weathering pathways within specialized pegmatite mineral assemblages.

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