Obira Mine is one of Japan’s classic axinite localities and, for many collectors, the name most strongly associated with sharp Japanese axinite in old collections. The best pieces have a look that is immediately recognizable: lustrous brown to clove-brown wedges and blades, sometimes with mauve or purplish internal tones, clustered on skarn or forming dense, architectural groups. They are not usually the glassy, gemmy single crystals associated with some Alpine or Russian occurrences; Obira’s strength is its sculptural, skarn-grown character—crystals with knife-like edges, steep wedge forms, and a burnished luster that rewards strong side lighting.
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Mineralogically, Obira belongs to the boron-rich skarn and tin-polymetallic system of the Sobo–Katamuki mountain region on Kyushu. The mine worked a complex assemblage of tin, zinc, lead, copper, arsenic and related sulphide ores, but the collector fame rests on the borosilicate suite: axinite, danburite, datolite, tourmaline, fluorite, quartz and calcite, with pyroxene, garnet, amphibole and other skarn minerals. In this setting, boron-bearing fluids interacted with calcareous and mafic wall rocks; tourmaline dominates in more siliceous or granitic hosts, while axinite is most characteristic where the fluids met iron-magnesian to calcareous rocks.
The locality is historically important because Obira specimens entered both Japanese institutional collections and the international mineral trade. Japanese sources have long described the mine as a celebrated axinite occurrence, and modern locality databases record both axinite-(Fe) and axinite-(Mn) from the mine. Older labels may simply say “Axinite, Obira Mine, Oita, Japan,” and that older naming should not be automatically dismissed: many classic pieces were labeled before the modern species suffixes became routine.
Collectors look for several things in Obira axinite. The first is crystal definition—sharp, wedge-like faces rather than massive brown crusts. The second is luster, because the finest Obira pieces can be highly reflective even when only translucent. The third is composition of the specimen: clusters with many upright crystals, an attractive matrix, or associations with quartz, fluorite, datolite, danburite pseudomorphs, sulphides or skarn minerals are more desirable than loose, bruised fragments. The best cabinet specimens also carry the intangible premium of a closed Japanese classic: locality, age, and label history matter nearly as much as size.
Search for specimens: View all axinite specimens from Obira Mine, Japan
Obira Mine, written 尾平鉱山 in Japanese, lies in the Ogata district of present-day Bungo-Ono City, Oita Prefecture, Kyushu. The locality is in steep mountain country east of Mount Sobo, and older mining literature places the mine roughly 30 km south of Ogata Station on the Hohi Line. Modern locality coordinates place it near 32°50′30″N, 131°34′40″E.
The deposit is best understood as a Miocene tin-polymetallic skarn and vein system related to acid intrusions. The regional rocks include Paleozoic sedimentary and calcareous rocks, older granitic rocks, serpentinite or ultramafic components, volcanic rocks of the Sobo region, and younger acid intrusive rocks. Miocene granite and granite-porphyry intrusions supplied heat and mineralizing fluids; where those fluids encountered limestone, diabase tuff, calcareous rocks, hornfels and mafic wall rocks, they produced skarn and borosilicate assemblages. The important point for axinite collectors is that Obira is not a simple “vein crystal pocket” locality. It is a multi-stage contact-metasomatic, pneumatolytic and hydrothermal system, and the axinite reflects that complexity.
The mine’s ore system includes both contact metasomatic deposits and veins. Published descriptions divide the deposits into zinc- and lead-bearing garnet–hedenbergite–axinite–datolite–ilvaite skarn masses, tourmaline–fluorite–cassiterite–quartz veins, and later sulphide veins. In calcareous and mafic wall rocks, boron was fixed chiefly as axinite and, more rarely, danburite; in more siliceous or granitic rocks it more commonly formed tourmaline. That distinction explains why Obira axinite is so strongly tied to specific skarn bodies and mixed wall-rock contacts rather than to every part of the mine.
Mining history is long. Local histories record silver mining as early as 1547 and the start of tin mining in 1617. The mine later became an important tin operation, with zinc, lead, copper, arsenic and sulphide products also present in the ore system. In the twentieth century, mining passed through a modern industrial phase: Mitsubishi Obira Mine was formed around the mid-1930s by consolidating earlier holdings, and a concentrator was built in 1940. One production figure recorded for 1941 gives 53,222 tons of ore and 391 tons of tin metal. The principal mine closed in 1954, and some local histories note a brief later Kurauchi Obira phase before final closure in 1959.
The old workings were not a single simple mine. Geological descriptions distinguish Mitsubishi Obira and Kurauchi Obira workings, with different adits, veins and ore bodies. Names encountered in older Japanese descriptions include the Sugi-Iwadani tin vein, Sugi-Iwadani copper-arsenic vein, Koumori vein or adit, Hajikami valley tin vein, Hinata vein, Sensuji vein, Showa vein, Okura deposit and Ginshiki or Ginpo deposit. For collectors, those names matter because axinite quality was not evenly distributed. Some bodies were mainly tourmaline–quartz–cassiterite veins; others were axinite-bearing skarn masses or mixed skarn-vein bodies.
The Okura deposit is especially relevant. It was described as a massive contact deposit in Paleozoic sandstone rich in basic igneous material, containing skarn minerals, axinite, fluorite, quartz, fine short-prismatic adularia, minor tourmaline, sulphides and oxidized ore. Axinite there was reported to form attractive individual crystals. The same description notes rare fist-sized fluorite crystals and hedenbergite crystals reaching 20–30 cm in diameter, a reminder that Obira’s skarn was capable of producing unusually coarse mineral growth.
Another key axinite locality within the mine was the Ginshiki deposit northeast of the Hajikami valley tin vein. It was interpreted as a contact deposit formed by replacement of a limestone lens enclosed in hornfels. It contained calcite, quartz, skarn minerals, sulphides and especially abundant axinite; the axinite crystals occurred in numbers sufficient to make fine specimen groups. Sphalerite and galena were among the major sulphides in that deposit, and germanium-bearing ilvaite was also recorded.
The mine is closed. Collecting access should be treated accordingly: this is a historical mining locality, not an open public collecting ground. Old underground workings, unstable dumps, private or managed land, environmental-control areas and steep mountain terrain all make casual collecting inappropriate. Modern specimens on the market are best regarded as old stock, recycled collection pieces, dealer-held material, or small quantities from older dumps and estate collections rather than active mine production.
Obira axinite is most often seen as brown, clove-brown, smoky brown, dark purple-brown or brown with mauve overtones. Some pieces show translucent edges or windows when backlit, but the locality’s collector appeal is less about gem transparency than about form, luster and old-classic character. Strong examples can appear almost metallic-bronze on the faces under direct light, then shift to brown-violet or smoky clove tones at the edges.
The classic habit is wedge-shaped to bladed, consistent with the Japanese name “ono-ishi,” or axe stone. Good crystals show sharp flattened forms, oblique terminations, beveled edges and striated or stepped faces. Some Obira specimens consist of many upright crystals covering a matrix surface; others are radiating, fan-like, curved or intergrown clusters. A less common but attractive style consists of relatively isolated crystals perched on skarn or quartz. Dealer and museum descriptions record individual crystals from millimetric size to around 1 cm on many cabinet specimens, with some trade examples reaching roughly 2 cm crystals and larger aggregate groups.
The species identity can be complicated. Mindat records axinite-(Fe), axinite-(Mn) and axinite group from Obira. Japanese museum records also include “manganaxinite” from Obira, using older or translated terminology for Mn-dominant axinite. In practical collecting, older labels may not distinguish the species suffix; analytical confirmation is needed before confidently calling a specimen axinite-(Fe) or axinite-(Mn). In the absence of analysis, “axinite group” or simply “axinite” is the safest descriptive label for older specimens.
Associations are one of the pleasures of the locality. Axinite occurs in a mineralogical neighborhood that may include datolite, danburite, quartz, calcite, fluorite, tourmaline, hedenbergite, garnet, ilvaite, epidote, grunerite or other amphiboles, pyrite, pyrrhotite, arsenopyrite, sphalerite, galena and chalcopyrite. Some commercial specimens are described with quartz pseudomorphs after danburite, a pairing that strongly evokes Obira’s borosilicate-rich skarn environment. Fluorite from Obira is also well known, and colorless to pale green fluorite, quartz and tourmaline associations help place axinite in the broader mine suite.
In thin-section and ore-deposit descriptions, axinite at Obira is not merely a pocket mineral. It may fill spaces between earlier skarn minerals such as hedenbergite–diopside and garnet, include small pyroxene grains, grow along cracks or zoning boundaries in garnet, and show euhedral relationships against later quartz, calcite and many sulphides. This paragenesis is important for collectors because it explains the common matrix textures: axinite may look embedded in dense skarn rather than freely grown in open cavities, and even fine crystals can be partly crowded by later minerals.
Quality is judged by sharpness, luster, placement and lack of bruising. A fine Obira axinite should have crisp wedge or blade outlines, reflective faces and a visually coherent cluster. Color should be rich enough to show brown, purple-brown or mauve tones rather than a dead black mass. Matrix can add value when it frames the crystals or displays associated minerals; it can detract when the axinite is swallowed by massive skarn or iron-stained crusts. Because many pieces are old, a historic label, especially one predating the modern trade, is a meaningful quality factor.
The most common authenticity issue is not treatment but locality confidence. Obira is a famous Japanese name, and old Japanese axinites from other Oita and Miyazaki skarns, including Toroku and related Sobo-region localities, can look superficially similar. A reliable old label, dealer provenance, museum-style catalog information or a well-documented collection history is valuable. Specimens labeled only “Japan” or “Oita” should not automatically be upgraded to Obira without supporting evidence.
No established cutting, dyeing, irradiation or coating tradition is associated with Obira axinite specimens. The species is generally collected as natural crystals, and obvious surface enhancement would be unusual. Repairs are possible on any bladed mineral, especially on protruding axinite groups, but repairs should be judged specimen by specimen. Use magnification to check for glue at contact points, mismatched luster on broken edges, and suspiciously isolated crystals on unrelated matrix.
Condition is a serious matter. Axinite has good hardness, but the sharp wedge edges and thin terminations chip readily. Many Obira pieces have small edge nicks, bruised high points or cleaved-looking scars from extraction in dense skarn. Dark lustrous crystals can hide damage until viewed under low-angle light. On clustered specimens, look especially at the tallest or most exposed crystals; these tend to take the first impact. Iron staining, sulphide oxidation, and old dump weathering can also reduce visual quality.
Rarity is relative. Obira axinite is not a one-specimen rarity, and examples do appear in the trade, but fine, undamaged, well-crystallized cabinet pieces are legitimately scarce. Small clusters, fragments, and modest old stock pieces are more available than display-quality specimens with crystals over a centimeter and good aesthetics. Pieces with strong historic labels, named Japanese collections, or associations such as fluorite, datolite, danburite-related quartz, or attractive skarn matrix are more competitive.
Current market availability is episodic. Japanese dealers occasionally offer small to mid-sized Obira axinite specimens, and international dealers have handled classic pieces ranging from thumbnail and miniature groups to small cabinet specimens. The best examples tend to surface through old collections rather than new production. A buyer should be prepared for variable spelling—Obira, Obira Mine, Obira Kozan, Ono-gun, Ogata-machi, Bungo-Ono or Bungo-Ohno—and for older labels that use “Oita,” “Kyushu,” or “Bungo” rather than the current municipal name.
The most vivid way to understand Obira is to picture it not as a single mine hole but as a mountain system of named veins, adits and skarn lenses. The Geological Survey of Japan’s postwar description preserves a remarkably concrete underground geography: veins bending along granite-porphyry dikes, skarn bodies replacing limestone lenses, old adits cutting toward unseen ore, and the same mineralizing event changing character depending on the rock it entered.
At the Sugi-Iwadani tin vein, the main Mitsubishi ore channel was traced for more than 1,200 m along strike. The vein generally ran northeast–southwest, steeply dipping, with a usual width of about 1.5 m underground. Where it met the Koumori vein, however, it swelled dramatically to about 6 m. The vertical difference from the highest outcrop, near 1,250 m elevation, down to the lowest explored level below the New O切 adit was about 480 m. That is the sort of scale that made Obira an industrial mine rather than a mere specimen occurrence.
The Hajikami valley tin vein reads like a mineral collector’s dream and a miner’s problem at the same time. It had two main veins, one roughly 230 m long from outcrop to the lowest level, and two richer ore shoots. One rich shoot was about 40 m long and 5–6 m wide; another, developed around the junction of two veins, extended about 50 m and widened to 7–8 m near the meeting point. The cavities in the southern rich shoot yielded handsome crystals of quartz, tourmaline, fluorite, arsenopyrite, sphalerite and siderite. The old description lingers over details: tourmaline needles gathered radially like chestnut burrs, their tips grading from black through green to colorless silky ends; fluorite appeared as pale grass-green octahedra or cubes; arsenopyrite formed pencil-thick or larger prismatic crystals, sometimes standing like props on cavity walls or gathered in parallel bundles.
The siderite in those cavities was described in even more memorable terms. It coated comb-like quartz as globular droplets or thin crusts, and in some places formed flat disks 2–3 cm across and only 2–3 mm thick. These “senbei”-like disks—named after the round Japanese rice cracker—stood upright in loose stacks of more than ten on walls of quartz and tourmaline crystals. Nearby marcasite formed radial nodules and botryoidal crusts, their black surfaces often wrinkled like a brain. Such details matter because they reveal the same pocket environment that could frame axinite elsewhere in the mine: open space, repeated pulses of fluid, and mineral growth crowded into complex three-dimensional cavities.
The Okura deposit gives the axinite story its skarn texture. At the surface it was a massive contact body in Paleozoic sandstone rich in basic igneous material. It contained skarn minerals, axinite, fluorite, quartz, fine short-prismatic adularia, minor tourmaline, sulphides and oxidized ore. The axinite was noted for forming attractive individual crystals. Fluorite was said to reach fist-size crystals on rare occasions, and hedenbergite reached extraordinary crystals 20–30 cm in diameter. About 50 m below the outcrop, an adit from the Koumori level passed through the projected continuation of the body, where the geology changed: a narrow concealed ultramafic body carried a vein about 50 cm wide and about 60 m long. The same ore body could therefore present itself as skarn above and vein below simply because the host rock changed.
Another underground scene comes from the contact deposit inside the Koumori workings. About 80 m beyond the lower vein section beneath Okura, miners encountered a skarn body more than 10 m wide in actinolite hornfels. No matching outcrop existed at the surface. The interpretation was that a small hidden limestone lens had been entirely replaced. The body contained skarn minerals, calcite, tourmaline, axinite and limonite. There were sparse aggregates of single quartz crystals larger than a thumb, clouded by inclusions and bubbles, and old accounts said danburite once occurred with that quartz.
Then there was the Ginshiki deposit, one of the most important names for axinite collectors. It lay northeast of the Hajikami valley tin vein, just above the mine road. It was interpreted as a contact body formed by complete replacement of a limestone lens enclosed in hornfels. In addition to calcite, quartz, skarn minerals and sulphides, it carried especially abundant axinite. The old description says the axinite crystals gathered in numbers to make “splendid” specimens. Sphalerite and galena were abundant among the sulphides, and germanium-bearing ilvaite was recorded there.
The human history has its own sharp edges. Local accounts describe Obira as officially opening in 1617 for tin, after an older record of silver mining in 1547. In the twentieth century the mine became a modern mountain community under large-scale operators, including Mitsubishi. Local retellings call it a “fuyajo,” a castle that did not sleep, with entertainment facilities and drinking streets during its boom. One account gives a peak population of about 3,500 and says film premieres in Oita Prefecture reached Obira early, an astonishing image for a remote mountain mining settlement now known to most collectors only through small brown crystals in old boxes.
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