Encyclopedia of Japanese Minerals (Go to Intro Page)
by Alfredo Petrov
Minerals Starting with "O"
OHMILITE (ohmi-ishi; ohmi-seki)
Niigata: The type locality for this rare Sr-Ti silicate is at Kanayamadani in Ohmi town (from which name is derived), where it occurs as light pink to pale tan or whitish needles to 1.5mm long and 0.01mm diameter, as spherulitic aggregates or divergent sprays with vitreous to silky luster, associated with albite, strontio-orthojoaquinite, magnesioriebeckite and rare benitoite in vugs in riebeckite-syenite or an albitite rock body in serpentinite.
OKAYAMALITE (= IMA # 97-002) (okayama-seki)
Okayama: The type locality for this anhydrous calcium borosilicate (the boron analogue of gehlenite) is Fuka, where it occurs as creamy white, anhedral grains to 0.03mm, with no cleavage, forming compact equigranular masses with earthy luster to several mm across, in a pentahydroborite-bearing body in pyrometasomatosed marble. Associated with calcite, johnbaumite, vesuvianite and wollastonite.
OKHOTSKITE (ohohtsuku-seki)
Hokkaido: The type locality for this Ca-Mn-Mn member of the pumpellyite group is the Kokuriki mine. Okhotskite forms orange vitreous cleavages, and small granular veinlets to 2mm wide, in Mn-bearing hematite ore, associated with quartz, Mn-bearing pumpellyite, piemontite, bementite and fluorapatite.
OLENITE (ohren-denki-seki)
Iwate: In the Sakihama pegmatite (Qv Elbaite).
OLIVENITE (oriibu-dohkoh)
Hiroshima: Fine, white to greenish grey olivenite needles, with the crystal points always yellowish, occur as rosettes or spherical sprays, fairly abundantly in fissures in a limonite-rich pneumatolytic copper-tungsten vein crossing what is now a granite quarry, previously the Setoda mine, on Ikuchi island. Nearby, in quartz-rich greisen, olivenite forms green-black dipyramidal crystals.
Miyazaki: Translucent green crystals around 1.5mm long sprinkled over fissures in whitish rock at the Kano mine.
Yamaguchi: Olive green, platy crystals to 2mm, with large (100) faces and minor (110) and (011), in vuggy limonite in the oxide zone of the Kitabira iron sulphide mine. Also as acicular xls to 3mm. Analysis shows minor Mg (0.17 wt% MgO) and Zn (0.07 wt% ZnO) replacing Cu, but no Mn'' or Ca. May be associated with conichalcite.
OLSHANSKYITE (orushansukii-seki)
Okayama: Forms pure white masses of microcrystals at Fuka, the second world locality for olshanskyite, in a high-temperature massive Ca-borate skarn band, 3m long, a few cm to 2m thick, in calcite marble near contact with gehlenite-spurrite skarn. The borate body, discovered around 1992, is composed of takedaite, nifontovite, olshanskyite, sibirskite, parasibirskite, frolovite, henmilite and pentahydroborite. Vuggy parts may hold colorless to whitish olshanskyite crystals ranging from insignificant fibrous to acicular and even 5cm(!) bladed twins, but more usually as 1mm long, colorless thin spears. Formed by late-hydrothermal alteration of sibirskite and parasibirskite, with which it is most closely associated. (Kusachi & Henmi (1994) Min. Mag. 58, 279-284)
OMINELITE (= IMA # 99-025) (ohmine-seki)
("OHMINELITE" would have been a better transliteration.)
Nara: The type locality for this vitreous blue, very rare Fe-Al borate-orthosilicate (the Fe''-analogue of grandidierite) is on the Misen river in Tenkawa village in the Ohmine Mountains (whence the name). It occurs as transparent grains to 0.5mm diameter, elongated euhedral to equant anhedral, intergrown with sekaninaite as a component of Miocene granite porphyry and granodiorite, with disseminated mm-size grains of dumortierite, andalusite and topaz. Sometimes altered to schorl tourmaline. Ominelite has one perfect cleavage direction, hardness 7, density 3.17, pale blue streak, and is pleochroic from colorless to pale blue-green in polarized light. Other associated minerals are K-feldspar, plagioclase, quartz, garnet, biotite, muscovite, zircon, apatite, monazite, ilmenite, pyrite, cordierite, sericite and chlorite. (Minor Mg,Mn,Zn replace Fe'', and about 1% of silicate is replaced by phosphate.)
OMPHACITE (onfasu-kiseki)
Ehime: With pyrope in eclogite on the Doi-cho side of Higashiakaishiyama. Crystals in metasomatic hornblende-bearing omphacitite rock at Gongenyama in neighboring Besshi give an empirical formula (Ca 0.794, Na 0.202, K 0.002) (Mg 0.654, Al 0.197, Fe'' 0.103, Fe''' 0.082, Ti 0.006, Mn 0.001) (Si 1.869, Al 0.131) (with 0.02 wt% P2O5). This rock is sometimes considered to be an eclogite, but probably formed at a slightly lower grade of metamorphism. (An "extremely low sodium" diopsidic clinopyroxene from a "normal eclogite" here is listed in the omphacite section of DHZ. -qv Diopside.)
Niigata: A considerable portion of the classic green "jadeite" from Hashidate, used as gem material since ancient times, turns out to be mostly omphacite, although the associated white and violet jade is true jadeite! Titanian omphacite occurs in the "blue jade". Omphacite and jadeite co-exist in some Ohmi-area deposits, showing a miscibility gap (Jd 65<86) between these two pyroxenes. Similarly in pebbles on Oyashirazu beach and other Itoigawa city beaches.
Okayama: Omphacite-diopside vein in omphacitite block from Ohsayama serpentinite melange.
OPAL (tanpaku-seki; opaaru; opahru; hyalite = gyokuteki-seki)
Aichi: Opal in deep sky blue to pale blue, yellow, white and several other translucent color tones forms stringers and fractured irregular to rounded masses inside chalky white to brown, mammillary nodules from a few cm to several decimeters across at Ebi, associated with altered obsidians and tuffs, and jasper and chert. The blue ones are highly sought after and are now difficult to find in cuttable sizes.
Akita: White opal spherulites to 3mm ("siliceous oolites") are precipitated as aggregates in a hot spring at Yunotai. Spiky irregular balls of "siliceous sinter" to almost 2cm form at Goshogake hot springs around variously shaped rock fragments, and are found in abundance on the surface of a "mud volcano". Siliceous sinter also at the Akinomiya hot springs.
Aomori: "Siliceous sinter" is not uncommon as a finely laminated hot spring deposit on acid-leached dacitic lavas in the Osorezan hydrothermal field. The internal structure is like a porous honeycomb intercalated with highly wavy layering and micro-fibrous thiobacillus casts. Dark red layers get their color from colloidal inclusions of amorphous Tl-Sb-As sulfide and stibnite.
Fukuoka: "Amorphous silica" formed a speleothem in a granite cave in Kitakyushu city (Aizawa, J. and Fujii, A. (1993) Jour. Speleol. Soc. Japan, 18, 11-16). Red and brown opal from Tsuno. Opaque brown banded opal from Abumibata.
Fukushima: Precious opal, with a principally green play of colors in a colorless background, is found as masses to 3cm, associated with concretions to 5cm of white to grey common opal, at Ohsaka. Also at Hohsaka (same place???). Sometimes cut as a gemstone. Opaque opal, usually brown and sometimes banded, at Nihonmatsu. Patchy brown and red, almost opaque, from Minowa.
Gifu: In miarolytic cavities in biotite granite at Tawara, abundant botryoidal "hyalite" opal ranging from colorless to white or yellowish, coats crystals of microcline, albite, smoky quartz and micas. Fluorescent bright green, or sometimes white (sometimes both colors in two generations of hyalite on the same specimen).
Hokkaido: In Shikaoi-chou as banded translucent orange to yellowish cream compact masses. At Kamuiwakka waterfall, unicellular microbes (Cyanidium caldarium) living in a sulfuric, strongly acidic (ph < 2) hot spring, are precipitating uniform granular microspherules of amorphous silica.
Ishikawa: Glassy masses of opal with conchoidal fracture, light yellow to yellowish brown and deep sky-blue or bluish white, rarely with greenish schiller, fill amygdules in rhyolite at Akase; sometimes cut as cabochons. At Bodai, pale bluish to milky white, occasionally weakly iridescent, with stalactitic protruberances, in liparite and liparitic tuff. Bluish grey opal is found in rhyolite amygdules at the Yuusenji copper-lead-zinc mine. In Suzu city on the Noto peninsula, white opaque opal concretions to 8cm form distorted shapes evocative of fantastic animals.
Kagoshima: Crusts of red hyalite, probably colored by hematite derived from fumarolic ferric chloride vapors, are widespread on the acid-leached rhyolite opaaru-keiseki rock on Ioudake (Iwojima). Botryoidal hyalite occurs in white fibrous sulfate masses around a fumarole on Akuseki island in the Tokara chain. At Sandaidou-Shimojugashira, quite uniform opal spherules from 1 to 2.5mm, colorless to brownish white, were formed as layers and loose aggregates by a hot spring. These "fish eye stones" show opalescence and play of colors when hydrated.
Nagano: Fibrous opal is found at Ishiyasuba in the Suwa iron mine. "Silica gel" at Nakabusa hot spring.
Okinawa: Amorphous silica occurs coating barite and sulphides in the "Jade" seafloor hydrothermal field in the Izena Cauldron. "Silica gel" reported from Kitadaitou-jima.
Ouita: Emerald-green to greenish yellow masses of opal, containing sprays of millerite (qv) embedded in the opal and lining dehydration fissures, occur in a vein in serpentinite at the Wakayama mine. The opal is apparently colored by secondary Ni minerals. Hyalite at Onogawa.
Saga: Colorless to transparent brown "hyalite" accompanies the rare-earth carbonates in vugs in alkali basalts at Niikoba. Fluorescent brilliant green.
Toyama: Highly unusual opal oolites are precipitated from Shinyu hot spring at Tateyama. The oolites are often loose, or can be very lightly cemented together, sometimes as compacted well-cemented globular hyalite. Individual spherules are transparent, colorless, milky or pale yellowish, almost perfectly uniform "fish eyes", and can reach 2cm diameter but are usually 1.5 to 3mm. Usually dull white on the surface when dry. A tiny sand grain forms the nucleus of each oolite. Such "fish eye stones" are also found at other localities in Japan, but those at Shinyu are the best and most abundant. Formed by opal precipitation around a quartz particle during constant rotation induced by agitation in the boiling spring. Most spherules show banded internal structure, with amorphous bands alternating with fibrous bands giving broad x-ray diffraction peaks between 5.2 and 3.4 angstroms; some spherules have no banding and only the microfibrous structure. At Kurodake, rounded grains of hyalite are frequently included in transparent quartz crystals or attached to their faces.
ORIENTITE (oriento-seki)
Hokkaido: In the Wakasa manganese mine, with macfallite.
Shiga: Reported as fracture-fillings and veinlets in hausmannite-manganosite-carbonate ore with accessory barite xls at the Ioi mine.
ORPIMENT (seki-oh; yuuoh)
Aomori: Orpiment occurs as light grey to yellow-grey, loose spiky radial spheres to 2cm, sometimes aggregated as crudely botryoidal multi-cm concretions, in argillized caldera sediment at Bakuchizawa, Osore-zan, an 874m high volcano. Formed where As-H2S-rich hydrothermal fluid met cold, very acid lake water. Associated with kaolinite, "low" cristobalite, montmorillonite, pyrite and barite. Also as small spheroidal inclusions in barite; and as yellow to reddish yellow, roughly botryoidal to stalactitic earthy incrustations in layered shells associated with realgar xls and sulfur in multi-cm cavities.
Gumma: Reported from the Nishinomaki mine, but this might be pararealgar or wakabayashilite?
Hokkaido: Excellent cabinet-size groups of yellow to orange-yellow aggregates of lamellar to flattened pyramidal crystals to 13mm, also as thin cleavages or lamellar masses, sometimes associated with realgar, have been found in a quartz vein in altered andesite at Jouzankei. Also from Kinaushi and from the Tohya mine.
Okinawa: Orpiment is presently being deposited around undersea vents in the "Jade" hydrothermal field, Izena Cauldron, as tabular xls to 0.02mm, sometimes as inclusions coloring orange barite. In polished section, can form finely banded agate-like structures in siliceous matrix, or radial fibrous aggregates.
ORTHOCLASE (sei-chohseki; seh-chohseki)
Aichi: Barium-rich K-feldspar occurs in contact with shirozulite in metamorphosed manganese ore at the Taguchi mine.
Iwate: "Barium adularia" was described from the Isagozawa hematite mine; analysis corresponds to (K 3.336, Ba 0.248, Na 0.150, Ca 0.032) (Al 4.573, Fe''' 0.041, Si 11.527) apfu (Yoshimura & Shirozu, 1953; in DHZ). (Qv Hematite.)
Kagoshima: White, dull rough Carlsbad twins to 15cm(!) size occur abundantly, loose in scree at around 2,000m elevation, weathered out of porphyritic biotite granite at Sanbonsugi, and at several other spots on Miyanoura mountain, on Yakushima island. (National Park)
Nagano: Rhomb-like 1cm xls of "adularia" grow on quartz crystals with hastingsite from the Kobushi mine. Also at the Tenryuu river.
Okinawa: White "adularia" crystals occur with clinochlore in alpine-type veinlets cutting schists and slates at Nerome and other localities in the Yanbaru region.
Osaka: White orthoclase phenocrysts to over 2cm, many as Carlsbad twins, weather out at Nagatani.
Tokyo: Barium-bearing orthoclase occurs with celsian and tamaite at the Shiromaru mine, in slightly metamorphosed manganese ore.
OSARIZAWAITE (osarizawa-ishi; osarizawa-seki)
Akita: The type locality for this Pb-Cu member of the alunite group (the Al analogue of beaverite) is the Osarizawa mine. In the Showa-hi vein and elsewhere it forms light greenish yellow ("veronese green") earthy, porous, multi-cm size scoriaceous masses and friable aggregates of minute scaly crystals with hexagonal outlines and perfect basal cleavage, rarely with rhombohedral faces. The most beautiful specimens form very botryoidal greenish aggregates. It occurs in a large limonitic oxide zone of Pb-Zn-Cu ore veins, associated with anglesite, kaolinite, goethite, and minor linarite, azurite, brochantite, malachite, djurleite, covellite, sulfur, quartz, chalcedony and hydrous Mn oxides. Analysis (oxide wt%) Pb 32.72, Cu 11.27, Zn 0.22, Fe''' 4.43, Al 12.35, S 22.92, Si 2.18, C 0.45, and water 8.50, no Ca, Mg or As. (D 3.89-4.02; mean RI 1.735-1.757; yellowish green in thin section.) Also at the Kisamori mine.
Niigata: Greenish brown crystalline crusts at the Mikawa Cu-Pb-Zn mine.
OSMIUM (shizen-osumiumu; iridosumin)
(In most cases no analyses exist for reported osmium from Japan, and the so-called "osmium" or "iridosmine" might in fact be other PGM species.)
Ehime: Small quantities were found during gold placering in the Dohzan river (Dozangawa). (Perhaps derived from the dunites around the Akaishi chrome mine?).
Hokkaido: "Iridosmine" (often ruthenium-rich) is much more abundant in HokkaidoÕs "platinum" sands than platinum itself (30 to 50 times as abundant in one study). As flattened grains, sometimes with hexagonal outline. It is easily distinguished from platinum/isoferroplatinum by its greater hardness, sharper grain edges and brilliant white metallic luster (but is not so easily distinguished from ruthenium). Only known from alluvium, always with other PGM minerals and magnetite, often also gold and cinnabar. An "iridosmine" from the Pehchan river in Esashi district had a specific gravity of 22.275. The most prolific locality was a placer in the Tomarinai river, where steel-grey "iridosmine" grains up to 0.5mm formed 65% of the final concentrate, together with 35% gold grains to 0.5mm. Also in the Yuubari river as flattened "iridosmine" grains from 0.1mm to 3mm (although the majority measure around 0.5mm, and only 0.2% of them exceed 2mm) in placers with platinum and gold. As irregular or flattened grains to 0.8mm with gold grains and cinnabar pebbles in the Uryuu river. Also from Teshio. A nugget from Usotannai in the Esashi district weighed 1.883g. An osmium nugget measuring 1.5cm and weighing 9.4g has been reported from Hokkaido. Alluvial "iridosmine" is locally known by the kanji ideograms for "sand white gold" and is reported also from Niseu and Toikambetsu and Horokanai.
Ibaragi: Reported in small amounts from auriferous veins at Daigo and Tage ("Keda" sic). (Perhaps merely as solid solution in native gold?)
Miyagi: Minor quantities with gold in a beach placer on a river mouth at Tsuya.
Niigata: Reported in the Sasagawa.
Tokushima: Small quantities were found with gold during placering operations at Yamashirodani.
OSUMILITE (OSUMILITE-(Fe)) (ohsumi-ishi; ohsumi-seki)
Kagoshima: The type locality for this potassium-iron-aluminum silicate is Sakkabira hill on the upper Osumi peninsula (whence the name), where it occurs as dark blue to black (translucent light blue in thin section) vitreous, short prismatic to thick tabular (on 0001 face) hexagonal crystals to 5mm, with no cleavage, in vugs in hypersthene plagio-liparite (hypersthene rhyodacite) lava, associated with andesine, thin plates of tridymite, quartz, hypersthene, biotite, magnetite and black granular fayalite. Five years before the description of osumilite, these crystals had been misidentified as cordierite (Morimoto, 1948), an understandable mistake, considering the crystal habit and the extreme blue to colorless pleochroism. Other faces developed are m(1010), a(1120), j(2130), p(1011), q(1012) and e(1122). (D 2.64) Analysis corresponds to (Na 0.342, K 0.320, Ca 0.115) (Fe'' 1.481, Mg 0.459, Mn'' 0.059, Ti 0.001) (Al 2.225, Mg 1.309, Fe''' 0.260) (Si 9.021, Al 2.979) O 30 (H2O) 1.027 (0.40 wt% MnO) ((Note that Na slightly dominates over K, whereas the published formula is K-dominant, and that Mg dominates over Fe'' - The Mg must be distributed over the B and C sites in order for Fe'' to dominate the B site, otherwise there is no difference between "osumilite-(Fe)" and osumilite-(Mg).)) Some references state that the Sakkabira type locality is on Sakurajima, or that the host lava is associated with Sakurajima volcano, but in reality the Sakkabira lavas are older, although they are only a few hundred meters away from Sakurajima and its more recent lava flows. Also at the bottom of Sakkabira hill, at Hayasaki. Black tabular crystals to over 2mm (but usually less than 1mm) in vugs at Shimizu, the 7th locality for osumilite in Japan.
OSUMILITE-(Mg) (kudo-ohsumi-seki)
Gifu: Considered by Japanese collectors to be the "find of the year" in 2005, a new osumilite locality was discovered in Tertiary rhyolite, not associated with any current volcano. Crystals are unusually large for osumilite, reaching 8mm! Some collectors have been labelling these "osumilite-(Mg)", while others claim they are roughly on the midpoint between Mg- and Fe-endmembers.
Kagoshima: Deep sapphire-blue, transparent, tabular hexagonal xls to 1.5mm, standing on edge in 5mm vugs, associated with colorless tridymite xls and phlogopite xls, in dacite at Yamanoguchi in Iriki-cho.
Ouita: Black to transparent deep blue crystals to 1.6mm in vugs in lava at Minamihata. Similarly as black microcrystals in vugs at Mannen-yama.
Tokyo: In rhyolite at Akasaki(?) on Kouzu-shima (Satoru Matsubara, Tokiko Chiba, Akira Kato (1995) 57).
"OYAMALITE"
= REE-phosphate-rich ZIRCON (qv) from Oyama, Ehime
OYELITE (oue-seki)
Mie: Oyelite was found in rodingite rock, derived from a gabbroic pegmatite, in serpentine at Suisho-dani in Ise city.
Okayama: The type locality for this hydrous calcium borosilicate is the Jiro Oye roadcut outcrop (whence the name) at Fuka. It occurs there as white aggregates of vitreous acicular crystals from 1 to 3mm long, with scawtite as a veinlet in spurrite skarn, also associated with bultfonteinite, xonotlite and calcite. (Note: this is the same species which had previously been erroneously reported from the Crestmore quarry in California as "10A-tobermorite".)
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