You are looking at an extraordinary and exceptionally rare mineral specimen — a polished cross-section through a scapolite-andradite garnet calc-silicate skarn breccia recovered from the southern ring fault zone of the Taum Sauk Caldera, Iron County, Missouri. This is Half A of a matched pair cut from a single large specimen. Half B is available separately

Geological Significance

This specimen originates from one of the most geologically remarkable localities in the American Midwest — the Ketcherside Gap area along the southern ring fault of the Taum Sauk Caldera, a 1.48 billion year old trap-door collapse caldera in the St. Francois Mountains of southeastern Missouri. This caldera system is part of the Mesoproterozoic St. Francois Mountains igneous terrane, a region formally recognized by the USGS as containing iron oxide-copper-gold (IOCG) deposits and one of the highest potentials for undiscovered rare earth element deposits in the contiguous United States.

Peer-reviewed geological literature (Lowell & Noll, Mineralogical Magazine, 2001) specifically documents Fe-Cu-Au-bearing scapolite skarn at Ketcherside Gap — the exact locality from which this specimen was recovered. The skarn formed when magma-derived hypersaline brines at 400-450°C infiltrated stromatolitic carbonate moat-fill sediments inside the caldera, producing a complex calc-silicate mineral assemblage that has been compared in the literature to the world-class Olympic Dam IOCG deposit in South Australia.

What You Are Seeing

This polished slab reveals multiple generations of mineralization frozen in a single cross-section, representing a complete record of the hydrothermal system:

The white to cream bladed mineral clasts suspended throughout the gray matrix are scapolite — a sodium-calcium aluminum silicate that is the signature mineral of this skarn system. Under shortwave UV light these fluoresce a vivid diagnostic orange, one of the most reliable fluorescent responses in mineralogy and the definitive identification test for scapolite. Some clasts show partial to complete silicification — the center replaced by chalcedony while the outer rim retains orange fluorescence — preserving a textbook metasomatic replacement front visible at hand specimen scale.

The gray matrix material surrounding the scapolite clasts displays a stunning plumose or feather-pattern internal texture under magnification — chalcedony that pseudomorphed after the original skarn silicate minerals, preserving their crystal geometry perfectly in silica. This pseudomorph texture fluoresces pale blue-white under UV. The zig-zag chevron patterning visible in the polished face represents the original scapolite crystal habit preserved in the replacement chalcedony.

The dark brown to black crystals visible on the natural unpolished face of the specimen are andradite garnet — the iron-calcium garnet characteristic of iron-rich calc-silicate skarns. These show the classic dodecahedral crystal form with resinous to subadamantine luster and range from dark reddish-brown to near-black depending on iron content.

A dense reddish-brown iron oxide horizon separates the garnet zone from the scapolite-chalcedony zone, representing a chemical boundary layer where iron-rich hydrothermal fluids precipitated a massive hematite band before garnet crystallized above it. This layered sequence records the actual progression of the hydrothermal event in cross-section.

The white banded material forming the bold visual patterns throughout the specimen consists of multiple generations of scapolite veining and replacement, with thin dark iron oxide selvages marking each fluid infiltration event. The overall breccia texture records a shatter and recementation event in which the original skarn was fragmented by fluid overpressure or tectonic movement along the ring fault and then recemented by later silica and carbonate-rich fluids.

Under shortwave UV this specimen is spectacular — scapolite clasts glow vivid orange, chalcedony zones glow pale blue-white, fluorite matrix phases glow cyan-turquoise, and scattered calcite phases flash bright blue-white. Multiple fluorescing mineral phases responding simultaneously makes for one of the most visually complex UV responses possible in a single specimen.

Matched Pair Note

This is  Half B (4 lbs 6.9 oz, 5.75"x4.75"x4.75") half A is listed separately and bears all the same minerals and characteristics. Purchasing both halves gives you the complete original specimen as it existed in the field. A link to Half B is included in this listing. A discount is available if purchasing both — message before buying.

Provenance

Collected from within the ring fault zone of the Taum Sauk Caldera. GPS-documented locality. Surface collected float from a documented IOCG skarn exposure.

This specimen is appropriate for advanced mineral collectors, fluorescent mineral collections, university teaching collections, and anyone interested in Proterozoic IOCG geology. It is a legitimate scientific specimen from a documented deposit with peer-reviewed literature support.

Kw

scapolite, fluorescent scapolite, orange fluorescent mineral, UV fluorescent mineral, shortwave UV, SW UV fluorescence, andradite garnet, garnet skarn, calc-silicate skarn, IOCG deposit, iron oxide copper gold, hydrothermal breccia, skarn breccia, polished mineral slab, mineral specimen Missouri, St. Francois Mountains, Taum Sauk Caldera, Ketcherside Gap, Iron County Missouri, Arcadia Missouri, Mesoproterozoic, Proterozoic mineral, 1.48 billion years, caldera ring fault, chalcedony pseudomorph, plumose chalcedony, feather chalcedony, scapolite pseudomorph, metasomatic replacement, hydrothermal skarn, calc-silicate mineral, wollastonite skarn, fluorescent rock, multimineral specimen, rare mineral specimen, mineral collector, advanced collector, museum quality mineral, teaching specimen, university mineral, fluorescent collection, matched pair mineral, polished slab, lapidary specimen, cut and polished, natural mineral display, cabinet specimen, Missouri mineral, Ozark mineral, rare Missouri mineral, iron oxide mineral, hematite skarn, magnetite skarn, scapolite marialite, scapolite meionite, garnet dodecahedron, andradite dodecahedron, hydrothermal vein mineral, ring fault mineral, caldera mineral, volcanic mineral Missouri, Precambrian mineral, billion year old mineral, geological specimen, documented locality, provenance mineral, scientific specimen, fluorescent garnet, multimineral fluorescent, chalcedony breccia, silicified skarn, replacement mineral, pseudomorph mineral, iron formation, banded iron mineral, hematite horizon, mineral investment, rare earth adjacent, IOCG terrane, Olympic Dam analog, St. Francois terrane, Missouri geology, Ozark geology, Precambrian Missouri, deep time mineral, ancient mineral specimen