Bull. Bull. Kan aga w a pr efec t. Mu s . (Nat. Sci .), No. 22 , pp . 25-32 , Jan. 1993 25 Siegenite in a caryopilite ore from the Okutama mine, Tokyo Daiji Daiji HIRATA K anagawa Prefectural Mu seum, Yokoham a 231, Japan and and Akira KATO Departmen t of Geolo gy, National Science Museum 3-23 3-23 1 H yakunin- cho , Shinjuku 169, Japan 東京都奥多摩鉱山産カ リオピラ イ ト質 鉱石中のシ ーゲン鉱 平田大 二 （神奈川県立博物館） 加 藤 昭 （国立科学博物館地学研究部） 東京都奥多摩鉱山のマンガン鉱床中に産するカリオピライ トを主と する鉱石 中からシ ーゲン 鉱を記載した。 S = 4 として 計算された実験式は（ Coi. 12 Ni i.19 Mn o.09 h:J. 00 S4.oo を与え， 副成分 としての マンガンの存在は 硫ス ピネル系鉱物としては慣石中のもの を除 いて 異常な例 と言え る。なおカリオピライト質鉱石中にマンガン層状珪酸塩があり，実験式（ Mn, Mg, FeMSi, Al , Fe) 601 s(OH, H 20) 5 が与えら れたが，分類上の位置は不 明である。 Abstract Sieg enite from a weakly metamorpho s ed bedd ed manganese or 巴 depo sit of the the Okutama mine is found asa v er y minor con sti tuent of a ca ryopilit e ore including rhodochrosite rhodochrosite and a manganese-b e aring la yer ed sili cate s. The aver a ge of five analyses is: Co 3 3.29, Ni 22.96, Mn 1.59, S4 2. 17 , total 100.01 %, and yield s the empirical formula (Co 172 N iug Mn o.o9b.oo S4.oo ,a rare manganif e rous variet y. One of the chemi ca l anal ys es of ryopilite ca ryopilite gives: Si0 2 38.65, Al 20 3 0.72 , FeO 2.14, MgO 2.53, CaO 0.05, MnO 46 .07, total 90.16 %. That of rhodochrosite is: MnO 55 . 95, CaO 4.78 ,FeO 0.47, MgO 0.33 ,total 61.53 % . The chemi cal anal ys es of th 巴 manganese-b earing la y ered silicat 巴 gi ves: Si0 2 45.78, Al 20 3 3.02 , FeO 1.90, MgO 3. 77, MnO 32.60 , CaO 0.33 , K 20 2.13 , total 89.53 % . It yi 巴Id s an empirical empirical formula K o.32 Ca o.04 (Mn 3.2s Mg o.s1 Fe 2+ o.osb.oo (Si s.44 Al o.42 F e3 + o.14hs.oo 0 1s.oo 0( Hh .84 ・3.16H 20 if the diff ere nce to 100 % is regarded as H 20 . The mineralogi cal status is un s pe cified. cified. INTRODUCTION There are many bedded manganese ore deposit s in Mesozoic accretional bodies in Japan . All All of them accompany chert as their most principal wallrocks, in which subordinate amounts of of mudstone , greenstone and lim es tone exist. Ores therefrom undergo various grades of 26 D. HIRATA and A. KATO regional regional or contact metamorphism by which many manganese oxides, silicates, carbonates and rarely rarely borates are formed. The original state of manganiferous beds is considered to be manganese nodules precipi- tated tated on the deep sea floor even currently. The nodules are also a carrier of heavy elements, which are absorbed or involved in manganese dioxide minerals forming nodules. After the diagenesis diagenesis and subsequent metamorphism, thes 巴 elements take different geochemical behav- iours. iours. Chalcophile elements such as nickel and cobalt form sulphides. Siderophile ones such as as iron behave in a partially similar manner to manganese. In the cases of nickel and cobalt, their their behaviours are essentially same and produced minerals of these elements involve them in the the form of mutual substitution as seen in the present case. The present siegenite includes a minor fraction of manganese, which is unusual to the members of thiospinels, except for manganoan daubreelite(FeCr2S4) and its unnamed Mn2 + an analogue analogue (KEIL and BRETT, 1974), both being exclusively found in meteorites. The implication of of the occurrence is briefly considered . In In the examined caryopilite were analysed four phases, siegenite, caryopilite, rhodochro- site, site, and a probably hydrous manganese silicate with the probable chemical formula (Mn, Mg, FeMSi, Al, Fe)6015(0H, H20)5, but the status has not been mineralogically specified. ORE DEPOSIT, ORES AND THE OCCURRENCE OF SIEGENITE The ore deposit of the Okutama mine is located about 500 m east of Okutama station, the endpoint endpoint of Ome line, Eastern Japan National Railway Co. Ltd. (Fig. 1), or about 70 km westward of the center of Tokyo. Although the history of the mine has not been known in detail, detail, it is said that it had been worked during the World War II. Still the portal and dumps are are left, where the examined materials were collected. The heavily ruined condition of the gallery gallery impedes the entrance but the profile of ore bearing bed could be recovered after the observation observation of materials forming the dumps and of outcrops exposed along a valley leading to the the portal in which the dumps are developed. The wallrocks of manganiferous bed are dominantly occupied by very weakly recrystal- lized lized grey to white massive chert accompanying minor hematite-bearing red one. Both of them are are minutely veined by quartz without any apparent regularity. Since the lower grade ores found found in the dumps consist of the mixture of caryopilite and quartz of various ratios, the ores in in direct contact with the wallrock must have included caryopilite as the most important member in quantity. Besides chert, minor blocks of mudstone and greenstone are found in the dump. From the geological situation that the deposit is located in the southern part of Kanto Mountainland, Mountainland, where regional metamorphic rocks belonging to Sanbagawa belt are exten- sively sively exposed, the wallrocks and ores suffered from a very low grade of regional metamor- phism, phism, though apparently unmetamorphosed. The mudstone is also found in the outcrop along along the valley in a small quantity. Although the existence in chert beds could not be realized, it it is very likely that the greenstone exists within chert beds. Also, blocks of sandstone are found found in dumps, too, but the origin could not be specified. Sieg 巴nite in a car yopilite ore from the Okutama mine 27 Fig. Fig. 1. Inde x m ap of the Okutama min e. Map: 1/25 , 000 Okutamako. Th e location is: N 35 °48 ’23", E 139°06 ’16 " Th e recognized ore minerals include caryopilite, rhodo chrosite, rhodonite, ganophyllite, hausmannite , tephroite , neotocite and mangano sit e in the order of apparent decrea s ing quan- tity, tity, and the first one occupies the prin ci pal m ember of the ores. The gangue miner a l is quartz with with very minor hallo ysite. The most principal and nea rly unique or e mineral is caryopilite with whi c h variable amount of of quartz coexists. Most of caryopilite or es hav e bandings composed of vari-coloured caryopilite. caryopilite. The colours includ e light brown , greyish sa lmon pink ,a nd light brownish grey. The thickness of bands is from a few millim ete rs to a centimeter order. All of ores are very fine-grained fine-grained and minutel y inters ecte d b y veinlets of th e following assemblages: 1) rhodochrosite, rhodochrosite, 2) rhodochrosite-rhodonite, 3) rhodonite-ganophyllit e, and 4) neotocite. The first first one co nsi sts of rather coarse-grained white to grey white rhodo chrosit e. The second one is occupied by a pink material composed o f fine-grained rhodochrosite and rhodonite. The third third on e is exclusively found in high er grade ores. Th e rhodonite is so mewhat coar se r in grain size size and the colour is rather rosy. The neotocite veinl ets are found in caryopilite or es only and devo id of any other associated phase s. It is brown in co lour and se mi-tran spar ent in the fr es h state. state. All the veinlets are of millimet er to submillimeter thick, and found in ores of mod erate to to high grades. Rhodochrosite Rhodochrosite is exclusively found as veinlets cutting caryopilite ores. Rhodonite occurs principall y in ve inlet s as stated above. Be sid es this, the aggr eg at e of rhodonite and rhodochrosite rhodochrosite is found betwe en wider band s in caryopilite ores, where the aggregate embodies light light bluish grey mass of tephroite. Hausmannite is found as cho cola te brown thin seams of millimeter millimeter thick intercalating with brown caryopilite forming the highest grade of ore in this 28 D. HIRATA and A. KATO deposit . Some caryopilite bands are interposed by rhodochrosite spots in which tin y grains of manganosite manganosite with or without hausmannite are involved. In the examined caryopilite ore, very small small pearly-lustered white spots composed of fragile flaky mineral are found. The chemical analysis analysis gives an empirical formula (Mn, Mg, FeMSi, Al, Fe) 601 5 (0H, H20)s if the difference is is entirely occupied by H 2 0. Siegenite Siegenite is generally found as extremely tiny grains in a caryopilite ore without banding, which is rare in the dump. It is barely visible under magnifier and in direct contact with caryopilite caryopilite and rhodochrosite. No external form is observed (Fig . 2). It is unique opaque mineral mineral in this ore. Under the microscope, caryopilite forms aggregates composed of random oriented oriented felty aggregates of very fine flakes. It is moderately pleochroic with yellowish brown to to brown colour, and has rather high birefringence. The extinction is parallel with positive elongation. elongation. The aggregate involves variable amounts of rhodochrosite and quartz along with the the examined siegenite-bearing material , the former being of visible dimension (Fig.