温泉科学（J. Hot Spring Sci.），66，89-106（2016）

原著

Artificial Chemical Weathering of Plagioclase by
Acidic Hydrothermal Solutions
─ The Effect on Chemical Composition of Hot Spring Water ─
Shoichi Kobayashi1）*, Yumi Baba1）, Akiko Kanayama1）,
Jun Naohara2）, Takabumi Sakamoto3） and Koichi Kitaoka1）(Received Aug. 5, 2016, Accepted Sep. 21, 2016)

酸性熱水溶液による斜長石の風化変質に関する実験的研究
─温泉の化学成分への影響─
小林祥一1）*，馬場友美1），鑛山明希子1），猶原
要

順2），坂本尚史3），北岡豪一1）

旨酸性熱水溶液による主造岩鉱物である斜長石（灰曹長石と曹灰長石）の人工風化実験を行い，岩石・鉱物の科学的風化作用の温泉水の化学組成への影響を調べるために以下の実験を行った．斜長石試料は切断後片面を研磨し用いた．開放系に改良したソックスレー抽出器を用い，
pH 4 に調整した塩酸，硫酸，硝酸および蒸留水にて 50℃，150 mL/day で，最長 600 日間の様々な期間晒すことを行った．溶脱元素は ICP-MS で検討し，結晶表面の化学組成，形態変化，生成物は偏光顕微鏡，SEM, EPMA, XRD, TEM によって検討した．実験時間の増加に伴って斜長石表面には不規則な割れ目などが，また（Ca＋Na＋K）/（Si＋Al）比の減少が見られた．
溶脱のしやすさを示すモル比（溶脱した各元素の量/未処理の斜長石中の各元素の量）は，Mg, Ba, La, Ce, Y が比較的高い値を示し，Fe, Mg, Zn, Mn および 3 価の希土類（Ce, La,
Nd, Sm）は反応の初期では高い値を示すが実験時間の増加によって減少することを，また構造中のフレームやその隙間を埋める斜長石の主成分 Si, Al, Na, Ca の溶脱は実験期間を通してほぼ一定であることを示した．灰曹長石および曹灰長石中から 525 日間に溶脱した主元素 Si,
Al, Ca, Na の量は顕著である．以上の結果は，斜長石から溶脱したこれら元素は，温泉水の化学組成形成の一因である可能性を示した．
キーワード：泉質，灰曹長石，曹灰長石，人工風化実験

1)

Faculty of Science, Okayama University of Science, 1-1 Ridai-cho, Kita-ku, Okayama 700-0005, Japan.
1）
岡山理科大学理学部 〒700-0005 岡山市北区理大町 1-1．*Corresponding author：E-mail kobayashi@das.ous.ac.jp, Tel & Fax 086-256-9704
2)
Faculty of Engineering, Okayama University of Science, 1-1 Ridai-cho, Kita-ku, Okayama 700-0005,
Japan. 2）岡山理科大学工学部 〒700-0005 岡山市北区理大町 1-1．
3)
Faculty of Risk and Crisis Management, Chiba Institute of Science, 3 Shiomi-cho, Choshi, Chiba 2880025, Japan. 3）千葉科学大学危機管理学部 〒288-0025 千葉県銚子市潮見町 3 番．

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小林祥一，馬場友美，鑛山明希子，猶原順，坂本尚史，北岡豪一

温泉科学

Abstract
In order to predict the effects of chemical weathering of rocks and minerals by acidichydrothermal solutions on chemical composition of surface water such as hot spring water,river water and seawater, artificial chemical weathering of plagioclases, which is the major
constituent mineral of various rocks, has been conducted chemically and mineralogically.
Oligoclase from Minas Gerais, Brazil and labradorite from Chihuahua, Mexico were exposed
to HCl, H2SO4 and HNO3 solutions at pH 4, and distilled water at 50℃ and 150 mL/day forvarious times up to 600 days in an open system using an improved Soxhlet extraction
apparatus. The plagioclase plates with 10 mm×8 mm×5 mm in size were used for theexperiment. The leached sample solutions were analyzed for major elements such as Si, Al,
Fe, Ca, Mg, Mn and Na, and minor elements such as Ce, La using ICP-MS. Morphologicaland chemical changes on the mineral surfaces were studied by SEM, EPMA and micro­
scopic techniques. Alteration products were examined using TEM and XRD.
The SEM images of the artificial weathered oligoclase and labradorite surfaces showedthat many irregular fractures and etch pits developed on the minerals. The (Ca＋Na＋K)/(Si＋Al) molar ratio for oligoclase and labradorite surfaces decreased with an increase inleaching time. Molar ratio, which is calculated by dividing the cumulative total mole ofreleased each ion by the mole of the each element in the unaltered plagioclase, varied
depending on the experimental period. The ratios of the elements such as Mg, Ba, La, Ce
and Y were relatively high, whereas the ratios of Na and Ca of the major elements were
low. At an early stage, after 6 days, the molar ratios of Fe, Mg, Zn and Mn, and trivalentCe, La, Nd, and Sm are relative high. But at a later stage, after 525 days, the ratios are
reduced significantly. The ratios of major elements in framework and cavity of plagioclase
structure such as Si, Al, Na and Ca are almost constant in the early and later stage. The
amount of leaching Si, Al, Ca and Na, which are major elements in both oligoclase andlabradorite, shows the high value though the molar ratios of the elements are low. The
dissolved ions from plagioclase by hydrothermal solution are likely to be one of the sourcesof ions in hot spring water.
Key words : spring quality, oligoclase, labradorite, artificial weathering

1.

Introduction

Minerals and rocks are altered by chemical weathering in the Earth’s crust near the surface.
In the chemical weathering process, the mineralogical components are modified naturally by
dissolution, oxidization, and hydration resulting from contact with geothermal water, rainwater,
oxygen, and carbon dioxide. The alteration is controlled by temperature, chemical compositionand pH of hydrothermal solution. Dissolved chemical compositions in hot spring waters byreaction between hydrothermal solution and rocks and minerals have been reported previously(e.g. Hurwitz et al., 2010 ; Yoshiike, 1995).
In order to predict the effect of chemical weathering by acidic water on chemistry ofground water and seawater, the authors have conducted artificial chemical weathering of rocks
in an open system using an improved Soxhlet extraction apparatus. In Kobayashi et al. (1993),
the artificial chemical weathering of granitic rock was simulated chemically and mineralogicallyusing HNO3 solution at pH 4 for 189 days. As a follow-up study, Kobayashi and Sakamoto (2001)
extended the reaction time to 780 or 824 days using HNO3, H2SO4 and HCl at pH 3 or 4, and also
distilled water in an attempt to further simulate natural environmental conditions. In Kobayashi
et al. (2005) the artificial chemical weathering of gabbroic rock was also reported. These90

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experiments were conducted on condition of acidic hydrothermal solution at 50℃. Hada (1999)
used a circulating system of distilled water with carbon dioxide and air at pH 4.2-4.4 and 22℃ tostudy the chemical weathering of granodiorite.
The chemical weathering process of the rock is extremely complex because of not only the
degree of weathering of each constituent mineral is different but also the dissolved ions fromweathered minerals react easily with weathered products and/or other unweathered constituent
minerals. Moreover, both quantitative and qualitative data of dissolution ions from eachconstituent mineral in the works using granitic and gabbroic rocks could not be confirmed
because it is difficult that the source of these dissolution ions could be specified accurately.
Therefore, it is strongly important to examine the rock and its individual minerals separately toconsider the reaction process for weathering of whole rocks by hydrothermal solution.
On the other hand, many experiments on weathering and dissolution experiments of rockforming minerals, especially plagioclase, have been conducted. For example, the dissolution rateof minerals has been obtained experimentally (e.g. Holdren and Berner, 1979 ; Schott et al., 1981).
Lasaga (1984) evaluated the mean lifetime of 10－3 m radius crystals of various minerals at 25℃
and pH 5, based on the dissolution rate of the minerals. The weathering rates and relatedparameters of plagioclase are previously summarized in White and Brantley (2003). However,
the experimental conditions in these studies differ greatly from those in natural weathering
conditions, as many of them were operated using plagioclase of 38-1000 µm in grain size at pH
3-7 under condition that is not open system.
Distribution map and the physical chemistry of Japanese spring waters are shown by
Geological Survey of Japan, AIST (2005). Hot spring water is divided into acidic (＜pH 3), weakly
acidic (pH 3-6), neutral (pH 6-7.5), weak alkaline (pH 7.5-8.5) and alkaline (＞pH 8.5) spring waterby hydrogen ion concentration. According to the Standard Method of Analysis for Mineral
Springs, they are categorized, based upon their temperature, into cold (＜25℃), tepid hot (2534℃), hot (34-42℃) and high-temperature (＞42℃) springs. According to Geological Survey ofJapan, AIST (2005), many of hot springs belongs to weak alkaline to alkaline. The most of these
springs are cold or tepid hot types at 25 to 35 degrees. On the other hand, high temperature
hot spring is limited to acidic or weakly acidic types because they distribute near the volcanoes.
It is considered that the reaction of acidic hydrothermal solution and plagioclase is easy toproceed at higher temperature, and the effect on the hot spring water is high.
Consequently, in order to predict the effect of acidic hydrothermal solution on chemistry of
surface water such as hot spring water, ground water and sea water, we have been conducted
artificial chemical weathering for polished sample plates of several millimeters square ofoligoclase and labradorite feldspars under an opened system. These minerals are a kind ofplagioclase and the main rock-forming minerals. The artificial hydrothermal solutions are H2SO4,
HNO3 and HCl at pH 4, and distilled water for reference. We have examined mineralogical andchemical changes on plagioclase surface and alteration process concerning dissolved ions.

2.

Material and Experimental Methods

Oligoclase and labradorite from Gerais in Brazil and Chihuahua in Mexico were used in this91

小林祥一，馬場友美，鑛山明希子，猶原順，坂本尚史，北岡豪一

温泉科学study, respectively. The chemical composition of the bulk of fresh oligoclase and labradorite wasanalyzed by X-ray fluorescence (XRF, Rigaku RIX-2100) and ICP-MS (Analyzed by Actlabs Co.,
LTD., Canada), and shown in Table 1. For example, since general formula of albite is shown as
NaAlSi3O8, each analysis has been recalculated on the basis of the 8O (O : oxygen). The albite(Ab), anorthite (An) and orthoclase (Or) molecules of oligoclase and labradorite are Ab73.6An22.3Or4.1
and Ab39.0An59.4Or1.6, respectively. Plagioclase compositions are usually expressed in terms of thetwo end-members albite and anorthite, AbxAn100-x. The series is divided into albite, oligoclase,
andesine, labradorite, bytownite and anorthite at An mole percentages of 0-10, 10-30, 30-50, 5070, 70-90, 90-100 respectively.
Hydrothermal solutions flow through opening of some rocks including plagioclase, but theflow rate is not only constant but also irregular such as a geyser. In this work, we assumed thesolutions have flowed and stagnated. Therefore, an improved Soxhlet extraction apparatus wasused in this study (see the details in report of Kobayashi and Sakamoto, 2001). Figure 1 shows aschematic illustration of the experimental apparatus. Oligoclase and labradorite plates were cut
from a fresh block with a diamond blade and polished to provide specimens. The dimensions ofeach plate were about 10×8×5 mm. Forty plates weighing 33-40 g were put in the extraction
chamber of four apparatus and four other apparatus without mineral specimens were used ascontrol. The extraction chamber was heated with a mantle heater at 50℃ during the experiment.
HCl, HNO3 and H2SO4 solutions at pH 4, and distilled water for reference were prepared.
Usually, HNO3 content in hydrothermal solution is very low ; it also was used for reference. The
four fresh solutions, which were stocked in the reservoir tank, were dripped on a specimen
covered by a Teflon sheet in each apparatus using a roller pump with the flow rate of 150 mL/
day. The Teflon sheet is put so that the each solution is not directly exposed on the same
Table 1

Chemical composition of oligoclase and labradorite* Total Fe as Fe2O3

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sample. Each solution reacted with the platy
specimen in the extraction chamber. The
reactions ran until about 50 mL of solution
could be collected in the extraction chamberof the apparatus. Then, the leached solution
was collected by siphoning into a container.
The leached solution was never return to theextraction chamber.

The procedure wasrepeated three times a day.

This can becompared to natural conditions where asporadic gush of spring water such as ageyser could occur. Namely, the process issimilar to combining flow method and batch
method. The artificial chemical weathering
was conducted for various times up to 600
days. Therefore, each total dripping of threekinds of acidic solutions and distilled water isabout 90,000 mL for 600 days, respectively.
Artificially weathered sample plates weretaken out of each apparatus and comparedwith the starting material using microscope,
X-ray diffractometer (XRD ; Rigaku RINT2500V). Chemical analyses of the plate surface
were performed on an energy-dispersion

Fig. 1 Schematic illustration of experimental
apparatus.

spectrometer and scanning electron microscopewith energy-dispersion spectrometer (SEMEDS system ; JEOL JSM-5410LV SEM equipped with a JEOL JED-2140 EDS). Accelerating
voltage and specimen current were kept at 15 kV and 0.15 nA on the Faraday cup, respectively.
The beam diameter was 5 µm. Standards used were quartz (Si), corundum (Al), rutil (Ti),eskolaite (Cr), hematite (Fe), periclase (Mg), manganosite (Mn), wollastonite (Ca), jadeite (Na),adularia (K). The ZAF method was used for corrections. Alteration products were picked byhand from the surfaces of each mineral and examined with transmittance electron microscope(TEM ; JEOL JEM-2010F).
Samples of the leached solution, which was stored in the container over a 3-day period, werecollected every 3 days for the first 30 days, and then at 30-day intervals between 30 days and
150 days, and at 45- or 90-day intervals for the last 600 days. Major and minor ions in theleached sample solutions were determined using an inductively coupled plasma mass
spectrometry (ICP-MS ; SII. SPQ-9700). The dissolution degree of each element is shown by
molar ratios calculated from the cumulative total mole of released ions divided by the mole ofindividual elements in the starting materials of untreated oligoclase or labradorite (see appendix).

93小林祥一，馬場友美，鑛山明希子，猶原

3.
3. 1

順，坂本尚史，北岡豪一

温泉科学

Results and Discussion

Leached sample solution
The total amount of major and minor ions released from the oligoclase and labradorite as afunction of leaching time up to 525 days are shown in Figures 2 and 3, respectively. The
released ions are shown by molar ratios (see appendix) as mentioned above. The molar ratios ofeach ion in the leached solution from labradorite are relative high compared with that ofoligoclase.
Figure 2 shows the molar ratios of major (a) and minor (b) ions in the leached solutions of
H2SO4, HNO3 and HCl at pH 4 and distilled water for oligoclase as a function of leaching time. K,
Mg and Fe, which are minor elements of plagioclase, are relatively high compared with Na, Ca,
Al and Si, which are major elements in plagioclase. The ratios of Ba and Zn are also highcompared to other ions. Figure 3 shows the ratios of major (a) and minor (b) ions after leachingwith acid solutions and distilled water for labradorite. The ratios of Mg, Fe, K, Zn, Ba, Sm, andTb are relatively high. On the other hand, the ratios of Na, Al, Ca and Si are relative low as wellas these in oligoclase. Matsui et al. (1977) determined the partition coefficients between phenocryst
and groundmass, and demonstrated partition coefficient versus six-fold coordinated ionic radius
diagram of plagioclase. It is obvious from the diagram that the most suitable cation is Sr to fit
the cavity, which is the space provided by the aluminum-silicate framework. The ionic radius ofmajor cations in plagioclase, Na and Ca ions, are a little smaller than Sr, but the partition
coefficient have considerable high. Therefore, it is consider that Na, Ca and Sr are easy toremain in the cavities within the framework. The molar ratio of Sr in Figure 3 is also relative
low. Although the molar ratios of ions generally decrease gradually with increasing theduration, the major elements such as Na, Ca, Si and Al are dissolved at a nearly uniform rate.
The dissolution degree of rare element (e.g. Mg and Fe) in the plagioclases extremely decreases
after 350 days. Afifi et al. (1985) explain that dissolution of the various minerals, particularly
those containing easily leachable cations such as Mg, are difficult to assess because of possible
precipitation or incorporation of these cations in the alteration products. Thus, it is consideredthat the apparent dissolution degree becomes slow. It is also showed that the reactivity isdifferent depending on a kind of the acid solutions. Its order for oligoclase seemed to be HCland HNO3＞H2SO4＞distilled water for minor elements though major elements as Na and Ca are
not so clear (Fig. 2). It seemed that the order for labradorite is HNO3 and H2SO4＞HCl＞distilledwater in many elements (Fig. 3). The molar ratio of the each element in the acidic leachedsolutions is shown to be high compared with distilled water. It can be seen that the pH of thesolution flowing in the earth’s crust also is one of the factors in determining the spring quality.
From the same experimental results of granite using HNO3 solution at pH 3 (Kobayashi and
Sakamoto, 2001) and 4 (Kobayashi et al., 1993), the influence of the leaching amount of plagioclase
is estimated to increase as the pH is low.
Figure 4 shows the relation between the molar ratios of each ion resulting from dissolutionof oligoclase and labradorite only after 6 (REX ･ 6) and 525 (REX ･ 525) days, and the ionic radius.
Smith and Brown (1988) reported coordinations of Na and K in feldspars usually vary between
five-fold and nine-fold. Moreover, Downs et al. (1996), and Megaw et al. (1962) indicated that Na,94

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Fig. 2 Molar ratios of each ion in HCl, HNO3, and H2SO4 leached solutions at pH 4, and distilledwater to oligoclase versus duration.  Molar ratio of Mg*＝Mg/10, Fe*＝Fe/10, Ba*＝Ba/2 and
Zn＝Zn/2.  a) Major ions, b) minor ions.

95小林祥一，馬場友美，鑛山明希子，猶原順，坂本尚史，北岡豪一

温泉科学

Fig. 3 Molar ratios of each ion in HCl, HNO3, and H2SO4 leached solutions at pH 4, and distilledwater to labradorite versus duration.  Molar ratio of Mg*＝Mg/5, Fe*＝Fe/5 and Zn*＝Zn/2.  
a) Major ions, b) minor ions.

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Fig. 4 Molar ratios of the elements released at 6 (REX ･ 6) and 525 (REX ･ 525) days of oligoclase(a) and labradorite (b) against ionic radius.

Ca and K are five-fold, six- or seven-fold and nine-fold coordination, respectively. K having large
ionic radius is an appropriate element in the cavity of alkali feldspar rather than plagioclase. On
the other hand, the ionic radius was changed by a coordination number. In this figure, the ionicradius expediently used the value of six-fold coordination number that had been indicated by
Shannon and Prewitt (1969). The molar ratios clearly vary with the ions. At an early stage,
after 6 days, the ratios of trivalent ions such as Ce, La and Sm having an ionic radii near 100 pm,
which occupied cavities within the framework, are relative high. But at a later stage, after 525
97小林祥一，馬場友美，鑛山明希子，猶原順，坂本尚史，北岡豪一

温泉科学days, the ratios are reduced significantly. The ionic radii of these ions are similar to Na and Ca,
which are typical elements in the cavity of plagioclase. In addition, some small size ions ofaround 70 pm such as Fe, Mg, Zn and Mn, which occupied cavities within the framework, also
showed a similar change. The ratios of Na, Ca and Sr having the most appropriate ionic radiusto the size of cavity as indicated by Matsui et al. (1977) are almost constant in the early and laterstage. Moreover, it seems that the ratio of trivalent Al ion in labradorite, which is one of theconstituent elements of the framework, is increased in the ratio of 525 days. From the above
results, it is concluded that the dissolution of elements from plagioclase proceeds rapidly fromtrivalent ions and small size ions in the cavities within the framework by an exchange reaction
with H3O+ ions in the early stages, and then the dissolution of divalent and monovalent ionswhich occupy commonly such as Na, Ca and Sr in the cavities are generated successively, andfinally the dissolution of Al from the framework is occurred in the later stages of the reaction.
3. 2

Artiﬁcially weathered sample plates

3. 2. 1

Surface morphology

The surface of labradorite plates became cloudy after 120 days and a part of themdisintegrated into small fragments during the experiments. The most altered parts of theFig. 5 SEM images of untreated oligoclase (a), and artificial weathered oligoclase after
581 days by artificial weathering using HCl (b), HNO3 (c) and H2SO4 (d) solutions ofpH 4, and distilled water (e).

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labradorite exfoliate by scratching with fingernail after 600 days. The alteration increased withan increase in reaction time. In contrast, the oligoclase surface showed almost no change.
Figures 5 and 6 show SEM images of surfaces for oligoclase and labradorite before and after(581 days) the experiments, respectively. Numerous irregular fractures formed in the labradorite
crystal reacted with H2SO4 and HNO3. Dissolution proceeded along cleavages and cracks in the
crystals. The dissolution of labradorite by the H2SO4 and HNO3 solutions is remarkable whencompared with the HCl and distilled water. The surface of oligoclase is not readily altered by
the acid solutions although etch pits were confirmed on the surface by HNO3 solution.
3. 2. 2

Chemistry of mineral surface

The chemical change on oligoclase and labradorite surfaces was examined by EDS. Figure
7 shows the change in (Ca＋Na＋K)/(Si＋Al) molar ratio for plagioclase surface as a function ofleaching time. The results suggest that the main elements constituting labradorite are easily
released by HNO3, H2SO4, HCl and distilled water at an early stage up to 200 days. After thistime, the change in ratio with an increase in leaching time gradually becomes smaller. These
data suggest that the leaching of Ca, K, Na proceeded at the initial stage of the weathering on
the original mineral surfaces, and these were controlled by secondary formed products. In
Figure 7b, the (Ca＋Na＋K)/(Si＋Al) molar ratios of labradorite for three kinds of acidic solutions

Fig. 6 SEM images of untreated labradorite (a), and artificial weathered labradorite
after 581 days by artificial weathering using HCl (b), HNO3 (c) and H2SO4 (d)solutions of pH 4, and distilled water (e).

99小林祥一，馬場友美，鑛山明希子，猶原順，坂本尚史，北岡豪一

Fig. 7 Changes in (Ca＋Na＋K)/(Si＋Al)
molar ratio for oligoclase (a) and labra­
dorite (b) surfaces during artificial
weathering by acid solutions.  Each bar
shows the distribution range of data.

温泉科学

Fig. 8 Photomicrograph showing thefrontal part of the oligoclase (a) andlabradorite (b), which is in directcontact with pH 4 HNO 3.   Crosspolarized light.

are similar up to 120 days, but after then, it is seemed that the ratios by HNO3 and H2SO4
treatment are smaller than one of HCl. Therefore, the reactivity order, which estimated by theratio at 581 days of three kinds of the solutions and distilled water, was HNO3 and H2SO4＞HCl＞
distilled water. On the other hand, there was only a little change shown in the chemistries ofthe oligoclase surface by the three acidic solutions and distilled water (Fig. 7a). The orders ofchemical changes on oligoclase surface for kinds of the solutions were H2SO4＞HCl and HNO3＞
distilled water.
The depth dimension of the sections of altered oligoclase and labradorite sample plates,
which were cut and polished in the direction of the right angle to the surface, were observed
using a microscope, SEM, and EDS. The frontal part of plagioclases after 581 days treatment
with the HNO3 solution did not show birefringence clearly (Fig. 8) and showed a weak contrast
in the SEM image (Fig. 9a). The altered part of labradorite is slightly thick compared with it ofoligoclase. The element distribution maps of Na, Al, Si, K and Ca in crystals of oligoclase andlabradorite show that the decrease of these elements were observed in near the front (Fig. 9b-f).
From the photomicrograph and SEM image, it can be seen that plagioclase is changed toalteration products of up to 5 µm in thickness, such as a leached layer or hydration layer by theacid solution at an early stage of weathering.
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Fig. 9 SEM and X-ray images of element distribution in oligoclase and labradorite.  
a) SEM image, and b) Na, c) Al, d) Si, e) K and f) Ca images.

3. 2. 3

Altered products

TEM image of the altered products from the labradorite surface is shown in Figure 10. The
morphology of the products shows a typical two-dimensional flake of 100-200 nm in length. EDS
detected mainly Si, Al and Ca in the altered products, but Na was hardly detected (Fig. 10b). On
the other hand, the relative intensities of Al and Ca are considerably lower than those in unalteredlabradorite (Fig. 10c). The formation of smectite from plagioclase is often observed in natural101小林祥一，馬場友美，鑛山明希子，猶原順，坂本尚史，北岡豪一

Fig. 10 TEM image (a) and EDX spectra of the weathered productsof labradorite by pH 4 H2SO4 solution after 410 days (b), and EDX
spectrum of unweathered labradorite (c).

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weathering (e.g. Sakamoto and Kobayashi, 1997 and 1998). Therefore, from the TEM and EDS
information, the alteration product is likely to be Ca-smectite. Because altered product isextremely small amount, it could not be identified by XRD.
In the present study, the formation of altered products was not so remarkable in this work
using individual mineral of oligoclase and labradorite though a little altered product onlabradorite surface was detected. However, no altered product was detected from the oligoclase.
In contrast to this, previous studies (e.g. Kobayashi and Sakamoto, 2001) have reported theexistence of altered products on the surface of oligoclase in granite. From this it may be inferredthat the altered product such as smectite was formed by addition of elements (e.g. Mg and Fe),
which dissolved from coexisting biotite in granite.
3. 3

Effect of plagioclase-acidic water interaction on chemistry of spring water
Geological Survey of Japan, AIST (2005) summarized 4,536 hot and mineral springs in Japan.

The data for each spring was compiled into tables consisting of name, location, temperature,amount of discharge, spring quality and pH. Generally, it is considered that hot springs are
divided into 4 types of neutral type, acidic type, carbonate type and alkaline simple type.
Among them, weakly acidic springs which is conducted in this work, are over 250 such as Appispring in Akita prefecture, Ubako spring in Kanagawa Prefecture, Shinyu spring in Kagoshima
Prefecture, and so on. Springs are seen under various geological environments such as plutonic
rock, volcanic rock, sedimentary rock and metamorphic rock areas. On the other hand, plagioclasesuch as oligoclase and labradorite, which consist mainly of Na, Ca, Al and Si, is one of theessential minerals in these rocks. For example, plagioclase is included 25 to 60% in the modal
value in igneous rocks. According to Geological Survey of Japan, AIST (2005), Na, Ca, Al and Si
as cation are included in all types of the hot spring water, as well as acidic type.
Although the dissolution degree of each element was shown by molar ratios as mentioned
above, in order to know the actual leaching amount from plagioclase, the amount of dissolved
major and rare earth ions (REE) during 525 days is shown in relation to the ionic radius (Fig. 11).
Si, Al, Ca and Na, which are major elements in both oligoclase and labradorite (Table 1), areshown the high value though the molar ratio of the ions are low as shown as Figures 2 and 3.
The values of REE showing high molar ratio at an early stage is also low because REE content
in plagioclase is originally low. These elements, especially Na and Ca, are major elements inplagioclase though pyroxene, amphibole and biotite are also slightly included. Al is also major
element in plagioclase though it is included in amphibole and mica of rock-forming mineral. Si iscontained in all of rock-forming minerals. Therefore, major elements in the cavity and frameworkof plagioclase are one of the important sources for the hot spring water in chemical weatheringby acidic water under the earth surface.

4.

Conclusions

In order to predict the effects of interaction between acidic hydrothermal solution and rocks
and minerals on spring quality, artificial chemical weathering of plagioclases composed in almost
rocks in earth crust has been investigated chemically and mineralogically. The mineralogical103小林祥一，馬場友美，鑛山明希子，猶原順，坂本尚史，北岡豪一

温泉科学

Fig. 11 Amount of dissolved major and rare earth elements during 525 days of oligoclase(a) and labradorite (b) against ionic radius.

and chemical changes of oligoclase and labradorite surfaces, and the molar ratio of ions released
from the each plagioclase by the pH 4 acidic leached solutions differed from the reaction with
distilled water. At an early stage, the molar ratios of Fe, Mg Zn, Mn, Ce, La, and Sm in leachedsample solution, which occupied cavities within the framework, are relative high. But at a laterstage, the ratios are reduced significantly. The ratios of major elements in framework and cavity,
such as Si, Al, Na, Ca, K, and Sr, are almost constant in the early and later stage. The amount ofleached Si, Al, Ca and Na having the most appropriate ionic radius to the size of cavity, which
are major elements in both oligoclase and labradorite, are shown the high value though themolar ratios are low. The SEM images of plagioclases surface showed that many irregular
fractures formed by dissolution developed on the minerals. (Ca＋Na＋K)/(Si＋Al) molar ratios
for oligoclase and labradorite surfaces decreased with an increase in leached time.
The dissolved ions from plagioclase by acidic hydrothermal solution are likely to be one of
the sources of ions in hot spring water of the acidic or weakly acidic types. The results obtained
are expected to give the fundamental chemical information that is useful in consideration of thespring quality, source and the path of the hot spring water.
(This study was presented at the 67th Annual Meeting of the Balneological Society of Japan on
September 6, 2014.)
Acknowledgments
We would like to thank Prof. E. Sukedai of Okayama University of Science for help with
TEM techniques. We also wish to express our gratitude to Prof. N. Sasaki, Editor in Chief of the104

第 66 巻（2016）

酸性熱水溶液による斜長石の風化変質に関する実験的研究

Journal, and two anonymous reviewers for their insightful and constructive comments. This
work was supported in part by a Grant-in-Aid for the Scientific Research (No. 24540522) from the
Ministry of Education, Science, Sports and Culture, Japan.
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温泉科学

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Appendix : The molars of released elements (REX ･ d) in the leached solution calculated from
REX ･ d＝(V ･ CX ･ d)/AX,
where d, V, CX and AX are experimental period, volume of leached solution, ion concentration ofx and atomic weight of x, respectively. V is 450 mL because samples of the leached solution,
which was stored in the container over a 3-day period, were collected every 3 days. For instance,
REX ･ d of Na ion at the first 3 days in the leached solution is
RENa ･ d＝(450 ･ CNa ･ 3)/ANa.
The cumulative total mole (CTMX ･ D : D＝total reached time) for the first 30 days is calculated
from CX ･ d (d＝3, 6, 9, 12, 15, 18, 21, 24, 27, 30) as follows :CTMX ･ 30＝REX ･ 3＋REX ･ 6＋………＋REX ･ 30.
After then, leached solution was collected at 30-day intervals between 30 days and 150 days, andat 45- or 90-day intervals for the last 525 days. For instance, the CTM60 of Na after 60 days is
calculated using ion concentration in the leached solution of 60 days, it is assumed that leachedion concentrations from 30 days to 60 days are the same as them at 60 days as follows :CTMNa ･ 60＝CTMNa ･ 30＋(450 ･ CNa ･ 60)･(60-30)/3/ANa.
The molar ratio which means the dissolution degree of each element was calculated from the
CTMX ･ D divided by the mole of individual elements in the unaltered plagioclase.

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