岩石 鉱物 鉱床 学 会誌 64巻2号, 1970年

FORMATION OF VERMICULITE AND KAOLIN MINERAL FROM HORNBLENDE

KATSUFOSHI ToMITA, HIROSHI TATEYAyIA* and NOBORU OBA

Institute of Earth Sciences, Faculty of Science, Kagoshina University, Kagoshima, Japan.

Abstract: Vermiculite and kaolin mineral showing a single crystal pseudomorph of hornblende are found in the hornblende andesite distributed in Iriki-cho, northern part of Kagoshima Prefecture. The vermiculite was formed from hornblende and then the kaolin mineral was formed from the vermiculite by weathering. Zoning due to successive argillization is observed in the crystal.

INTRODUCTION Vermiculite occurs as an alteration product of mafic minerals in an ultramafic body which has been intruded by small dikes and sills of acidic rock, usually pegmatites. It is accompanied by other alteration minerals such as antigorite, chrysotile, chlorite, talc, pyrophyllite, biotite, and amphi boles. Occurrences of vermiculite have been attributed both to hydrothermal and to supergene origins by different workers (Bassett, 1959; Kulp, 1954). Bassett concluded that the vermiculite at Libby, Montana was formed by the supergene alteration of biotite which was formed from augite under hydrothermal condition. A number of investigators have reported the alteration of micas to vermiculites (Brown, 1953; athaway, 1954; Rich and Obershain, 1955). Vermiculite is often found in sediments as a weathering product of biotite. It is uncommon that vermiculite is found from non micaceous minerals, but Weaver (1958) reported vermiculite derived from nonmicaceous minerals such as volcanic material, chlorite and hornblende from marine sediments. The vermiculite in Triki is a weathering product of hornblende and has a pseudomorph of a single crystal of hornblende. The end product of the weathering is relatively pure kaolin mineral. It is concluded that hornblende altered to vermiculite, and that the vermiculite altered to kaolin mineral by weathering.

* Present address; Geological and Mineralogical Institute, Tokyo University of Education. (Manuscript received, March 19, 1970) Formation of vermiculite and kaolin mineral from hornblende 65

OCCURRENCE

In the southern part of Iriki-cho, hornblende andesite is widely distributed.

This rock is thought to be Pliocene in age. Weathered hornblende occurs as a local development in an outcrop of the andesite. Samples were collected from fresh hornblende to altered hornblende in sequence. Samples are divided into three types by the degree of argillization. Mineral phases observed in the samples due to weathering may be summarized as follows: (1) Single phase of hornblende. (2) Three phases of hornblende, vermiculite and kaolin mineral.

In this specimen partial alteration of a single crystal of hornblende to vermiculite and kaolin mineral is observed. Unaltered hornblende can be distinguished in the innermost part of the crystal by direct observation. (3) Two phases of vermiculite and kaolin mineral. Surface of the specimen of this type is composed almost entirely of kaolin mineral.

The color of the unaltered hornblende is blackish gray and the vermiculite is neutral gray with a golden luster. The color of the kaolin mineral is golden white.

MINERALOGICAL DATA

A most altered specimen in this area was selected and studied.

X-ray analysis

Laue photograph of the specimen is shown in Fig. 1 together with those of parent hornblende and intermediate alteration product of the hornblende.

The Laue photograph of the most altered specimen shows a pattern of kaolin

mineral and vermiculite. That of parent hornblende shows typical hornblende pattern and that of a crystal of intermediate stage shows patterns of hornblende, kaolin mineral and vermiculite. All three phases may coexist in the crystal.

X-ray powder diffraction data of the unaltered hornblende are listed in

Table 1. X-ray powder diffraction patterns of the specimen after various treatments are shown in Fig. 2. A 7.2A peak of kaolin mineral and a small

14.9A peak of vermiculite are observed in the pattern of natural state of the

specimen. The 7.2A peak disappeared upon heating at 700•Ž. The 14.9A peak

contracted to a 10.2A on heating to 300•Ž for one hour and did not expand

when treated with ethylene glycol. The specimen heated at 60•Ž for one hour

shows an interstratified structure. The specimen is probably intimately mixed

with vermiculite and its dehydrated form. The formation of the interstra

tified structure is due to partial dehydration of the vermiculite (Gruner, 1934).

Rhoades and Coleman (1967) succeeded in forming an interstratified structure

from vermiculite by potassium sorption.

Similar structure is commonly observed in weathered biotite (Bassett, 1959) 66 Kat3uto3hi Tomita, Hiroshi Tateyama and Noboru Oba

Fig.1. Laue photographs of fresh hornblende (a), intermediate altered hornblende (b) and most altered hornblende (c).

Ｔ able 1. X-ray powder diffraction data for hornblende. 入(CuKa)=1.5418 A Formation of vermiculite and kaolin mineral from hornblende 67

Fig. 2. X-ray powder diffraction patterns of the most altered specimen after various treatments.

and formation of an interstratified structure from mica by potassium release

was reported by Tomita and Sudo (1968a, b).

This most altered specimen is kaolin mineral with small amount of vermi

culite. A-axis rotation photograph of the kaolin mineral is shown in Fig. 3.

It confirmed that the specimen is relatively well-crystallized kaolin mineral

composed of metahalloysite. Rotation photograph technique revealed that transformation of c-axis of hornblende to a-axis of kaolin mineral occurred.

Differential thermal analysis

Differential thermal analysis curve of the specimen was taken with an automatic thermal analyser. Differential thermal analysis curve of the specimen run under atmospheric pressure and at a heating rate of 10•Ž per minute shows two endothermic peaks (Fig. 4). A peak at 586•Ž is due to dehydroxylation of structural water of kaolin mineral. The peak temperature is a little lower than that of well-crystallized lcaolinite. An endothermic peak at about 146•Ž with a shoulder is due to dehydration of the kaolin mineral and 68 Katsutoshi Tomita, Hiroshi Tateyama and Noboru Oba

Fig. 3. A-axis rotation photograph of the kaolin mineral.

Fig. 4. Differential thermal analysis curve of the most altered specimen.

the vermiculite. The specimen shows the usual SiO2 -Al2O, (spinel) exothermic peak at 960•Ž.

Infrared absorption spectra

Powdered specimen for infrared absorption analysis was prepared by the

Nujol paste method. Absorption spectra were obtained using the Shimadzu IR

6 type spectrometer. Figure 5 shows absorption bands of the specimen. The spectrum shown in the figure is characterized by the presence of absorption bands with peaks at 3600, 3400 and 1640cm-1. Absorption bands between Formation of vermiculite and' kaolin mineral from hornblende 69

Fig. 5. Infrared absorption curve of the most altered specimen.

3700 cm-1 and 3600cm-1 are due to OH of kaolin mineral and vermiculite (Scholze and Dietzel, 1955; Roy and Roy, 1957). A broad absorption band at about 3300-3500cm-1 is due to the existence of interlayer water and an absorption at 1640cm-1 is also due to adsorbed water. These absorption bands are of vermiculite. The result revealed that the infrared absorption spectra of the specimen overlap those of vermiculite and kaolin mineral.

Chemical analysis A chemical analysis of the specimen is listed in Table 2. The chemical composition is similar to that of well-crystallized kaolinite. Higher content of H2O (-) than that of well-crystallized kaolinite is due to small amount of

Table 2. Chemical composition of kaolin mineral. 70 Katsutoshi Tomita, Hiroshi Tateyama and Noboru Oba vermiculite and to the fact that the kaolin mineral contains a metahalloysite like mineral. Higher contents of Fe2Os+FeO (4.08%) and MgO (3.07%) than those of normal kaolinites are due to vermiculite.

DISCUSSION AND CONCLUSION

In the southern part of Iriki-cho, hornblende andesite is widely distributed. In an outcrop of the andesite altered hornblendes are observed. In the andesite biotite could not be found. Intermediate altered hornblende contains hornblende, vermiculite and kaolin mineral. In the crystal hornblende is still preserved in the innermost part and vermiculite was formed surrounding it. Kaolin mineral was formed on the surface of the crystal. Kaolin mineral is also observed along the twinning plane of the hornblende. Most altered hornblende shows elongated hexagonal flake and it contains kaolin mineral and small amount of vermiculite. Kaolin mineral is observed on the surface of the crystal. The elongated hexagonal flake looks like a separated piece of the hornblende cut parallel to the (100) plane of the hornblende. In the Iriki area many hot springs are distributed and it is possible that the andesite was washed by a little acid solution in rainy season. Argillization of the hornblende is considered to have progressed gradually. In this area vermiculite is believed to have been formed by hydration and desilication of the hornblende. In this case transformation of c-axis of hornblende to a-axis of the vermiculite occurred. Kaolin mineral was formed from the vermiculite by desilication.

ACKNOWLEDGEMENTS

The writers wish to thank Professors T. Hase and M. Nakatani of the Chemical Institute of Kagoshima University, who provided convenience for the use of infrared spectrometer. Deep gratitude is also expressed to all staffs of the Institute of Earth Sciences, Kagoshima University, for their valuable comments on this study. The present study has been supported by a Grant in Aid for Scientific Research from the Ministry of Education.

REFERENCES

Bassett, W.A. (1959), The origin of the vermiculite deposit at Libby , Montana. Am. Min., 44, 282•`299.

Brown, G. (1953), The dioctahedral analogue of vermiculite. Clay Min . Bull., 2, 64-69.

Gruner, J.W. (1934), The structure of vermiculites and their collapse by dehydration. Am. Min., 19, 557•`574.

Hathaway, J.C. (1954), Studies of some vermiculite-type clay minerals . Formation of vermiculite and kaolin mineral from hornblende 71

Proc. Third Nat. Conf. on Clays and Clay Minerals. 74-86. Kulp, J.L. and Brobst, D. A. (1954), Notes on the dunite and the

geochemistry of vermiculite at the Day Book dunite deposit, Yancey County, North Carolina. Econ. Geol., 49, 211•`220. Rhoades, J. D. and Coleman, N. T. (1967), Interstratification in vermiculite and biotite produced by potassium sorption. I. Evaluation by simple X-ray diffraction pattern inspection. Soil Sci. Soc. Am. Proc., 31, 366 - 372.

Rich, C.I. and Obershain, S. S. (1955), Chemical and clay mineral properties of a red-yellow podzolic soil derived from muscovite schist. Soil Sci.

Soc. Am. Proc., 19, 334-339. Roy, D.MI. and Roy, R. (1957), Hydrogen-deuterium exchange in clay and

problems in the assignment of infrared frequencies in the hydroxyl region. Geochim. Cosmochim. Acta, 11, 72•`85. Scholze, H. and Dietzel, A. (1955), Infrared study of clay minerals. Naturiviss., 42, 575. Tomita, K. and Sudo, T. (1968a), Interstratified structure formed from a

pre-heated mica by acid treatments. Nature, 217, 1043•`1044. Tomita, K. and Sudo, T. (1968b), Conversion of mica into an interstratified mineral Reports of Faculty of Kagoshima Univ., No. 1, 89•`119.

Weaver, C.E. (1958), Potassium fixation by expandable clay minerals. Am. Min., 43, 839•`861.

摘 要

角閃石か らのバ ー ミキュライ トとカオ リン鉱物の形成

富田克利 ・立 山 博 ・大 庭 昇

鹿 児島県 入 来 町 南 部 の 角 閃 石 安 山 岩 中 の 角 閃 石 が 風 化 に よ っ て 角 閃 石 の 仮 晶 を呈 し な が ら パ ー ミキ ュ ラ イ トと カ オ リ ン 鉱 物 に 変 化 して い る の が み られ た 。 あ ま り変 質 を うけ て い な い 角 閃 石 は 中 心 部 に 角 閃 石 が あ り,そ の ま わ り に バ ー ミキ ュ ラ イ トが で き て お り,表 面 が カ オ リ ン 鉱 物 こ変化 し て い る.風化 が 進 ん だ 角 閃 石 は伸 び た六角板状 を呈 し,角閃 石 の 壁 開面｛100｝ に平行 に 割 れ た 形 を 示 し て い る｡そ れ ら はバ{一 ミ ュ ライト と力オリ ン鉱 物 に 変 っ て お り,中 心 部 に バ ー ミキ ュ ラ イ}が あ り,表 面 と劈 開 面 の 部 分 が カ オ リ ン鉱 物 に

変 化 して い る.最も風 化の進 んだも の で は角閃 石 のc軸がカオリン鉱物の のa軸に 変 換 し て い る 。