J. Japan.Assoc. Min. Petr. Econ. Geol. 79, 267-275,1984

An oyelite-bearing vein at Fuka , the Town of Bitchu, Okayama Prefecture

ISAO KUSACHI*,CHIYOKO HENMI** and KITINOSUKEHENMI**

* Departmentof Earth Sciences,School of Education, OkayamaUniversity, Okayama 700, Japan ** Departmentof Earth Sciences,Faculty of Science, OkayamaUniversity, Okayama 700, Japan

An oyelite-bearing vein was found in the gehlenite- skarn at Fuka, Okayama Prefec ture, Japan. Oyelite, previously called 10 A , was revealed to be a new of approximate composition 1.0CaO • 0.1 B203• 0.8Si02 • 1.25H2O. In addition to oyelite, the vein contains , , and . Among them, oyelite and scawtite are primarily precipitated from fluid and form alternative layers filling the vein fissure. Bultfontein ite and xonotlite are secondarily formed by alteration of scawtite. The thickness of the vein is about 2cm and the thickness of each layer ranges from 1 to 5mm.

Oyelite occurs as a major component of a Introduction vein with width of 1-3 cm penetrating a spurrite Fuka is known as a typical locality of zone of the skarns at an outcrop called Doro- gehlenite-spurrite skarns in Japan. From giwa-roto in Fuka. In addition to oyelite the there, we have reported several rare vein contains bultfonteinite, scawtite and such as tobermorite, perovskite, bicchulite, xonotlite. Scawtite in this vein is well crystal schorlomite, rankinite, kilchoanite, fukalite, 7T lized and so we can use its aggregate as a pure -wollastonite and oyelite . Among them bic specimen of the mineral. Bultfonteinite is also chulite, fukalite and oyelite were reported as well crystallized and is available for some new minerals. Oyelite which had been previ experiments. The present paper deals with the ously described as 10A tobermorite (Heller and mineralogical properties of these minerals and Taylor, 1956; Kusachi et al., 1980) was the mode of occurrence of the vein in which revealed to be a new species different from they occur. tobermorite for its having a high Ca/Si ratio and containing boron as an essential compo- Minerals nent. We, therefore, with A. B. Carpenter, J. Oyelite Ito and H.F.W. Taylor who had studied the The mineral has properties similar to those same minerals from Crestmore, California, of tobermorite, and may be belonged to the named it oyelite in honor of Dr. Jiro Oye, a late same mineral group. The chemical composi professor of mineralogy of Okayama Univer tion of oyelite is similar to that of tobermorite, sity. The name, oyelite, is approved by the but these two are different from each other in Commission on New Minerals and Mineral boron contents and the Ca/Si ratios. Their Names, International Mineralogical Associa crystallographic properties also have mutual tion on February, 1982. similarities and dissimilarities. In contrast to

(Manuscript received February 10, 1984) 268 Isao Kusachi, Chiyoko Henmi and Kitinosuke Henmi

Table 1. Chemical analyses of oyelite, tobermorite, bultfonteinite and scawtite from Fuka

* Kusachi et al ., 1980. ** Excluded an amount of CaO in an app ropriate proportion to form calcite together with the amount of CO2. An oyelite-bearing vein 269

Table 2. X-ray powder data for oyelite

* Kusachi et al. (1980). a=11.25, b=7.25 and c=20.46 A. ** Heller and Taylor (1956).

the 14 or 11 A basal spacing of tobermorite, the cell dimensions calculated from the X-ray

spacing of oyelite is about 10•ð. On the other diffraction data in Table 2 are a=11.25, b = hand, the dehydrated products of the two min 7.25, c = 20.46 A. erals have the nearly same spacings (about The detailed description on this mineral

9 •ð), and their diffraction patterns are very was already reported in Japanese by us (Kusa similar to each other. chi et al., 1980).

Oyelite from Fuka occurs as nearly paral Bultfonteinite lel aggregates of acicular crystals with lengths Bultfonteinite, a flourine-containing cal

of 1-3 mm. The mineral is elongated parallel cium silicate hydrate, is a rather rare mineral

to the crystallographic axis and crystallized in reported from some localities such as the Bult

its axis perpendicular to the vein wall. The fontein mine, South Africa (Parry et al., 1932),

mineral is white in color with vitreous luster on Crestmore, California (Murdoch, 1955 b) and the handspecimen and colorless in thin section. Hatrurim, Israel (Gross, 1977). Miyake (1965)

Its refractive indices are ƒ¿ =1.602,ƒÀ = 1.606 and reported an occurrence of this mineral with X-

ƒÁ =1 .613. Its Mohs' hardness is 5 and its den ray data from the mine of Mihara, Okayama

sity measured with a pycnometer is 2.62 gr/cm3. Prefecture, Japan. The cell dimensions were

The chemical composition of oyelite is reported by Megaw and Kelsey (1955) and the

shown in Table 1. The B203 content of this was determined by McIver mineral is 4.8 wt.%, and the ratio of Ca/Si is (1963). 1.2. The empirical chemical formula obtained Bultfonteinite from Fuka forms parallel or

from the analytical data shown in the Table 1 is radial aggregates of acicular crystals elongat

0.99 CaO•E0.10 B2O3•E0.80 Si02.1.25 H2O. The ing parallel to the b-axis with lengths up to 270 Isao Kusachi, Chiyoko Henmi and Kitinosuke Henmi

Table 3. X-ray powder data for bultfonteinite

* a=10.955, b=8.188, c=5.671 A, ƒ¿=94.29, ƒÀ=91.16 and ƒÁ=89.89•‹. ** Murdoch (1955b). An oyelite-bearing vein 271

Table 4. X-ray powder data for scawtite

* a=10 .127 (3), b=15.188 (4), c=6.631 (2) A and ƒÀ=100.61 (3)*. ** Murdoch (1955a) . 272 Isao Kusachi, Chiyoko Henmi and Kitinosuke Henmi

1mm. The hardness is 4.5 in Mohs' scale and specimens, scawtite from Kushiro has not been the density measured with a pycnometer is 2.73 studied in detail. But scawtite from the pres gr/cm3. It is milky white with vitreous luster ent locality can be obtained as pure specimens, on the hand specimen and colorless in thin and therefore we determined the chemical section. The average refractive index is about composition as shown in Table I by a wet

1.59 and sign of elongation is positive. chemical method. The empirical formula,

The chemical composition was determined Cal.02Si5.99(C03)0.97018.03.2.09 H20 on the bases of by conventional wet chemical analyses. The 0 + CO3 = 19, agrees well with the ideal one content of fluorine was measured by a colori , Ca7 Si6 (C03 )O 18.2 H 2 0 . Ca. metric method described by Numano et al. The scawtite parts of the vein consist of

(1976). As the analyzed specimen contains sutural aggregates of granular crystals with some amounts of calcite, we assumed that all dimensions of 0.02 to 0.7mm. The Mohs' hard- the analyzed amount of CO, is from calcite, ness is 5. It is light gray to white and colorless and, in calculating the mineral formula, exclud in section. The refractive indices and optical ed an amount of CaO in an appropriate axial angle are ƒ¿ 1.595, ƒÀ 1.605, ƒÁ 1.622 and 2VZ proportion to form calcite together with 75.5•‹, respectively. The X-ray powder pattern the amount of CO2. The results are given shown in Table 4 gives the cell parameters a= in Table 1. The mineral formula calculated 10.127(3), b=15.188(4), c=6.631(2) A and ,6= on the basis of O + OH + F = 12 is 100.61(3)•‹. The values are very close to those

(Ca4.08 Na0.01)4.09 (Si 1.92A10.13)2.0504.26F2.00 (OH)5.75, of Pluth and Smith (1973).

which is in a good agreement with the ideal

formula Ca4Si2O4F2OH5. Occurrence

The X-ray powder data are shown in The skarns at Fuka consist mainly of

Table 3. The cell dimensions obtained from somewhat retrogressively altered gehlenite and the data are a = 10.995(5), b = 8.188(4), c = spurrite. The characteristic features of the

5.671(2) A, ƒ¿ = 94.29(6), ƒÀ = 91.16(5) and ƒÁ = skarns are their zonal arrangements from the

89.89(4)•‹. The calculated density is 2.704gr/cm3. central quartz monzonite dike or/and

The Fuka material has the cell dimensions contaminated rock through the gehlenite zone similar to those of the original specimen from and the spurrite zone and finally to the crystal-

Bultfontein, South Africa (Megaw and Kelsey, line limestone. Widths of the spurrite zones

1955). are sometimes up to over 10 meters. Most of

Scawtite the gehlenite crystals in the Fuka skarns were

Scawtite is a calcium carbonate silicate altered to retrograde minerals such as hydrate mineral. After the first report from vesuvianite, garnets and bicchulite. On the

Scawt Hill (Tilley, 1930), scawtite was found other hand, retrograde changes of spurrite are subsequently at Crestmore, California (Mur not extensive and most of the spurrite zones doch, 1955 a), Ballycraigy, N. Ireland (McCon consist of unaltered spurrite. During post nell, 1955) and others. In Japan, the mineral metasomatic stages, spurrite zones were cut was reported from the mine of Mihara, through by veins consisting of calcite, hillebran-

Okayama Prefecture (Miyake, 1965) and from dite, scawtite, foshagite and others. The

Kushiro, Hiroshima Prefecture (Kusachi et al., oyelite-containing vein is one of such veins. It

1971). Owing to its intimate mixing with cal was found at the outcrop called, in Japanese, cite and hence difficulty in separating it as pure Dorogiwa-roto on which a geological and geo- An oyelite-bearing vein 273

Table 5. Vein-forming layers and their constituent minerals

Fig. 1. Somewhat idealized sketch of the oyelite-containisg vein. (1), the outer scawtite layer; (2), the outer oyelite layer; (3), the intermediate scawtite layer; (4), the inner oyelite layer; (5), the inner scawtite layer; (6), the calcite layer. chemical report was already published by us layer ranges from 1 to 5mm. In the spurrite (Kusachi et al., 1983). zone in the vicinity of the vein, spurrite was The oyelite-containing vein penetrates a altered to mixtures of foshagite and calcite spurrite zone of the skarns. The vein, whose accompanied with small amounts of xonotlite strike and dip are N1O W and 80W respective- and scawtite. ly, consists of several layers parallel or some- We can recognize five groups of the vein- what oblique to each other. The thickness of forming layers on the basis of their major the vein varies from 1 to 3cm and that of each constituent minerals as given in Table 5. 274 Isao Kusachi, Chiyoko Henmi and Kitinosuke Henmi

Fig. 2. Photomicrographs of vein-forming minerals. Crossed nicols. (1), oyelite (OY) and scawtite (SC); (2), bultfonteinite; (3), xonotlite; (4), scawtite.

Among them, oyelite and scawtite layers are The structure shown in Fig. 1 suggests the primarily precipitated from fluid, filling the vein sequence of formation of layers. The scawtite fissures and the two layers are nearly parallel and oyelite layers precipitated alternatively as to each other and to the vein walls. Bult- fissure-filling deposits. The growth of oyelite fonteinite and xonotlite layers seem to be sec observed in the layers were onesided and direct- ondarily formed by alteration of scawtite layer. ed toward the inner part of the vein. The Xonotlite layers are found only at rim of the bultfonteinite layers were formed in the vein. Calcite layers are a product at a later scawtite layers or along the boundaries stage of the mineralization and they frequently between the scawtite and oyelite layers. The cut other layers and occasionally even the vein observed growth directions of bultfonteinite boundary. suggest that crystallization started along thin Fig. 1 is a somewhat ideallized sketch of fissures and progressed toward the scawtite the vein. The vein is composed of (1) the outer layers. Xonotlite was formed as randomly scawtite layer, (2) the outer oyelite layer, (3) the orientated acicular crystals in the scawtite intermediate scawtite layer, (4) the inner oyelite layers. Xonotlitization of scawtite was exten layer, (5) the inner scawtite layer and (6) the sive in the outermost parts of the vein and calcite layer. The outermost part of the resulted in xonotlite layers. The photomicro- scawtite layer (1) is rich in xonotlite and forms graphs of vein-forming minerals are shown in a xonotlite layer. Bultfonteinite occurs along Fig. 2. thin fissures in the outer and intermediate The length of the exposure of the vein is scawtite layers, and forms veinlike or about two meters along its dip direction. The lenticular bultfonteinite layers. layered structure varies distinctly from place to An oyelite-bearing vein 275 place. The variation is depend on widths of scawtite. J. Miner. Soc. Japan, 10, 296-304 (in component layers, numbers of alternative depo Japanese). McConnell, J. D.C. (1955), A chemical, optical and X- sitions, and penetrations of calcite layers. The ray study of scawtite from Ballycraigy, Larne, upper parts of the vein are generally simple and N. Ireland. Amer. Mineral., 40, 510-514. poor in oyelite. McIver, E. J. (1963), The structure of bultfonteinite, The conditions of formation of the vein Ca4Si2O10F2H6. Acta Crest., 16, 551-558. Megaw, H. D. and Kelsey, C. H. (1955), An accurate will be discussed elsewhere after studies on the determination of the cell dimensions of bult- fukalite-containing and other veins at the same fonteinite, Ca4Si2O10H6F2. Mineral. Mag., 30, locality are completed. 569-573. Miyake, H. (1965), Study on genesis of skarns and the related minerals. J. Sci. Hiroshima Univ., References Ser. C, 4, 395-428. Gross, S. (1977),The mineralogy of the Hatrurim Murdoch, J. (1955 a), Scawtite from Crestmore, formation, Israel. Bull. Geol. Surv. Israel, 70, California. Amer. Mineral., 40, 505-509. 1-80. Murdoch, J. (1955 b), Bultfonteinite from Crest- Heller, L. and Taylor, H. F.W. (1956),Crystallogra- more, California. Amer. Mineral., 40, 900-904. phic data for calcium silicates. pp 37-38, H. M. Numano, T., Kusachi, I. and Hanazawa, M. (1976), S.O. London. Determination of fluorine in rocks and miner Kusachi, I., Henmi, C. and Henmi, K. (1980), 10A als. Bull Sch. Educ. Okayama Univ., 45, 47-55 tobermolite from Fuka, the Town of Bitchu, (in Japanese). Okayama Prefecture. J. Miner. Soc.Japan, 14, Parry, J., Williams, A. F. and Wright, F. E. (1932), On 314-322 (in Japanese). bultfonteinite, a new fluorine-bearing hydrous Kusachi, I., Henmi, C, and Henmi, K. (1983),Occur- calcium silicate from South Africa. Mineral. rence of skarns at the Fuka Dorogiwa outcrop, Mag., 23, 145-162. the Town of Bitchu, Okayama Prefecture. Pluth, J. J. and Smith, J. V. (1973), The crystal struc Bull. Sch. Educ. Okayama Univ., 64,171-182 (in ture of scawtite, Cal (Si6O18)(CO3• 2H20. Japanese). Acta Cryst., 29, 73-80. Kusachi, I., Numano, T. and Henmi, K. (1971), Tilley, C.E. (1930), Scawtite, a new mineral from Contact minerals from Kushiro, Hiroshima Scawt Hill, Co. Antrim. Mineral. Mag., 22, 223 Prefecture (3) foshagite, hillebrandite and -224.

岡山県備中町布賀産含大江石脈

草 地 功・逸 見 千代子・逸 見 吉之助

岡 山 県 川 上 郡 備 中 町 布 賀 に は ゲ ー レ ン石 ・スパ ー石 ス カ ル ンが 発 達 して い る。 そ の うち 道 路 際 露 頭 と 称 され て い る露 頭 の ス パ ー 石 ス カ ル ン 中 に 大 江 石 を 含 む 輻1-3cmの 細 脈 を 見 出 した 。 共 生 鉱 物 と し て は 大 江 石 の 他 に,バ ル トフ ォ ン テ イ ン石,ス コ ー ト石,ゾ ノ トラ石,方 解 石 が あ る。 大 江 石 と ス コ ー ト石 は スパ ー石 ス カ ル ン 形 成 後 の 割 目 充 填 鉱 物 と し て 初 生 的 に 生 成 さ れ,脈 中 で は 中 心 部 の ス コ ー ト石帯 の 両 側 に,各 々 が1-3mmの 幅 で 交 互 に2条 ず つ 配 列 す る 対 称 的 な 帯 状 構 造 を 示 して い る。 バ ル ト フ ォン テ イ ン 石,ゾ ノ トラ石 は 大 江 石,ス コ ー ト石 生 成 後 の 作 用 に よ り,ス コ ー ト石 か ら二 次 的 に 生 成 され た も の で あ る。 この 論 文 は 大 江 石,パ ル トフ ォ ン テ イ ン石,ス コ ー ト石 に つ い て の 化 学 組 成,X線 粉 末 回 折 値,光 学 的 性 質 お よ び 物 理 的 性 質 を 求 め る と と も に,大 江 石 を 含 む 脈 の構 成 鉱 物 の 生 成 過 程 を 考 察 した 。

主 な 地 名 の 日本 語 表 示

Bitchu,備 中; Fuka,布 賀; Dorogiwa-roto,道 路 際 露 頭; Mihara,三 原; Kushiro,久 代 。