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Towada-Hachimantai NP, Akita
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“Hokutolite” honors Beitou, Taiwan—not a Japanese place. Yet the mineral was recorded at Tamagawa’s old Shibukuro Onsen in 1898, before the Taiwanese name was coined; the two were linked in 1911.
Just 2 natural localities are known: Beitou, Taiwan, and Tamagawa. Beitou’s spring source was cut off by development in the 1950s, making new Hokutolite hard to form there; Tamagawa’s hot water still deposits it.
Scientists measured radium-isotope ratios in 3 Tamagawa samples to estimate Hokutolite’s growth at 0.06–0.15 mm per year—just 0.6–1.5 mm per decade. Even this naturally radioactive stone builds itself at a patient pace.
A cross-section looks like a dark-brown-and-white barcode. The stripes aren’t decoration: their colors vary with the proportions of barium sulfate and lead sulfate, as well as the iron-oxide content of each layer.
As the spring river cools, it leaves more than Hokutolite: sulfur, silica, and barite also precipitate. The fine mixed sediment called yunohana is gathered for bath salts—a local product born from the same mineral-rich flow.
The deposit became a Natural Monument in 1922 and a Special Natural Monument in 1952. Its slow-growing riverbed crust is protected by law, so collecting or removing Hokutolite is illegal.
The acid-rich water that forms Hokutolite also earned a grim local nickname: Tamagawa-doku (“Tamagawa poison”), for harming rivers and rice fields. Acidification of nearby Lake Tazawa also made most of its fish disappear.
