Holtedahlite

specimen number:
2133396

location:
Tingelstadtjern Quarry, Modum, Viken, Norway

description:
Glassy gray throughout matrix. From the type locality, nice for this rare species.

The Tingelstadtjern quarry, situated in the historic Modum parish of Viken county (formerly Buskerud), Norway, is a small, historic mining excavation renowned for its specialized magnesium-rich mineral assemblages rather than bulk industrial stone production. Geologically, the quarry cuts into a highly metamorphosed Proterozoic terrain characterized by complex magnesite and serpentine bodies embedded within regional quartz-mica schists and amphibolites. Hydrothermal alteration and localized metasomatism within these ultra-mafic sequences precipitated a unique suite of rare phosphate, borate, and carbonate minerals. Formal operations and exploratory extraction at the site occurred sporadically from the mid-nineteenth century through the early twentieth century, with the most active periods aligning with regional prospecting for industrial magnesite and quartz before the small deposit was abandoned and left as a specialized field locality for academic research.

Holtedahlite is a magnesium phosphate mineral that crystallizes in the hexagonal system and was originally discovered and described from the Tingelstadtjern quarry. Geologically, it forms as a rare secondary mineral within the highly altered, serpentinized magnesite deposits, typically occurring as a product of low-grade hydrothermal activity affecting primary phosphate phases. The mineral typically appears as fine-grained, massive aggregates or small, colorless to pale yellow, elongated crystals that can be difficult to distinguish visually from associated minerals like apatite or althausite without analytical testing.

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