。ーivine Sand fr。m Kawashiri Beach in Kag。shima, Japan

45
01ivine sand五〇m Kawashih Beach in Kagoshima, Japan
Tbmoaki MATSUI 求
(Received 13 October, 2000)
Abstract
The olivine sand Hom Kawashiri Beach was studied by chemical and X-ray
analyses･ At least six groups of olivine are mixed in the olivine sand in this reglOn,
and they are classined according to the composition of the silicate一melt inclusions
in them. The composition of the olivine sand ranges Hom Fo70 tO Fo75, and the
sum of the Mn2SiO4 and Ca2SiO4 COntent is less than 1 (mo1 班). These crystal
chemical data for the olivine sand not only provide basic mineraloglCal information
about it, but they also help us to estimate the orlgln Of it.
Introduction
Kawashiri Beach is situated approximately 3 km east of Kaimon-d負ke volcano,
approximately 50 km south of Kagoshima City, and measures about 2 km in length
(Fig･ l)･ Beach sand in Kawashiri is coTpOSed of quartz, plagioclase･ olivine･ pyroxene･
amphibole, magnetite, hematite and several kinds of rock血agments (Fig･ 2a)･ The
olivine sand in question is deposited as sediment especially on the surface of the
backshore.
The mineral name olivine is used to call members of the solid solution between
forsterite (Fo), Mg2SiO4 and fayalite (Fa), Fe2SiO4. The (Mg,Fe)-Olivines are common
and important rock-forming minerals, and are particularly characteristic of ultrabasic
and basic Igneous rocks. Their composition is a good indication or the differentiation
stage of the parent magmas in which they crystallized･ With respect to the physical
求 Depanment of Geology, Faculty of Education, Kagoshima University, 20-6 Korimoto
1-chome, Kagoshima, 890-0065, Japan･
脱児島大学教育学部研究紀要自然科学編第52巻(2001)
Fig.1 Location of Iくawashiri Beach and samp一ing point of the olivine sand
a
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Fig.2 Beach sand in Kawashiri. a:mass of the beach sand on the surface of the
backshore, b:the olivine sand picked up from the mass of the beach sand.
properties observable under the stereoscoplC ZOOm microscope, olivines have no cleavage, and the samp一es rich in Fo content are transparent and omen have a beauti請l
greenish to yellowish colour. These characteristic features make the oliVine sand easily
distinguishable in the sediments on the beach･ Therefore, Olivine sand is a good
teaching material for the general public言ncluding elementary school and junior high
school students, who are trylng tO understand the rock-forming mineral cycle in nature.
The cycle illustrates various geologlCal processes that act to transform one rock
ｨ
ｨ
ｨ爾
Olivine sand from Kawashlri Beach in Kagoshima, Japan
47
type into another･
It has been well-known that the orlgln Of the olivine sand in Kawashiri Beach
is nom ML Kaimon-dake volcano, but it has not been connrmed by chemical and
X一〇ay analyses. The pumOSe Of this work is to investigate and record the chemical
and structural characters of the olivine sand Hom Kawashiri Beach.
Experimental procedure
Befbre the experiments, Olivine sand sampled was washed in distilled water by
ultrasonic cleaning tO desalt it. The olivine sands which were polished to a thin section
were observed under a polarizlng microscope･ Powder X-ray di冊action patterns for
mineral ident綿cation were measured on a Rigaku X-ray di組･actometer with Ni-nltered
CuKa radiation (Mini flex). The program UnitCell (Holland & Redfern, 1997) was
used for rennement of the unit-cell parameters of the olivine crystal in onhorhombic
symmetry･ Chemical analyses were performed with an electron-probe microanalyzer
(EPMA), JEOL JAX-8621･ ZAP on-line mll matrix conections were applied to quantitative analyses (Reed, 1996)･
Results and discussion
The olivine sands Hom Kawashiri Beach are about I mm in diameter on average,
and some specimens reach a maximum size of about 4 mm in diameter (Fig. 2b).
Many crystals are of a pale yellow to greenish yellow colour, and are o龍en abundant
in black spherical inclusions. The olivine sand also shows a wide variation in its
shape,
which
mainly
depends
on
the
degree
of
roundness･
However,
it
is
di縦cult
to discriminate the sand grain with its orlglnal shape before erosion Hom one with
the roundness a請er 紅iction･ The most euhedral crystal typically exhibits a shape
of forsterite (Deer et al., 1992), which is sunounded by well-developed (110), (021),
(010), (001) and (101) (Fig, 3).
In thin sections, the olivine sands are almost colourless or slightly yellowish in
colour. All crystals of the olivine sand are homogeneous in the interference colours
with high bireHingence･ There are no cleavages in the crystals･ The black spherical
鹿児島大学教育学部研究紀要自然科学編第52巻(2001)
48
inclusions mentioned above are present as opaque inclusions in thin sections･ It is
worthy of note that there are some glass inclusions in almost all grains of the
olivine sand. For the sake of convenience, samples of the olivine sand were classined
into nve groups on the basis of the characters observed under the stereoscopic ZOOm
microscope and the polarizing microscope (Fig. 4).
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`.軍佇%B
一
1､1葛-
ｲ钁
亨も./.
ヽ一〇 .〟
●: 白騙ｶ
Fig.3 Characteristic externa一s of oiivine crystaL a: euhedrai oIivine sand from
Kawashiri Beach, b: idea一 morpho一ogy of forsterite crysta一, modified after
Deer et aI.(1992).
01ivincsand缶omKawashiriBeach
No 僞uhedral? Yes
No 亦
Yes
ﾖ
&7F
Dark t PaleDark
認 &ｶ li
' 劔Pale
ﾆ3 i 剩F
I
II
Ill
ﾖﾓ
&ｶ
'
b
ﾆS
IV
Fig.4 Scheme for the assignment of the number for samp一es on the basis of
observation under the stereoscopic zoom microscope and the po一arizing
microscope.
OLivine sand fnom Kawashiri Beach in Kagoshim, Japan
49
Ⅹ･ray analyses and the cell parameters
Identincation of me olivine sand was made by X一重ay powder di鮒action. Using
the PC-program UnitCell (Holland & Redfem. 1997). unit cell parameters for mass
or the olivine sand were obtained by the least square refinement or 25 reflections
(Table I). me space group Pbnm with orthorhombic symmetry was used in the
re血ement
of皿s
study.
The
Fb
(mo1
%)
content
is
me
mem
Value
for
57
samples
of me olivine sand, wmch was obtained by using EPMA deschbed later. The cell
p紬ameterS and Fo contents were comp孤ed with synmetic fbrstehte (Schwab 皮 Kunster,
1977) and hyalosidehte血om the Skae唯aard intmsion (Smith, 1966). Because Fez+ and
Mg孤e not the same size (Shannon, 1976), change in composition result in change
in cell p紬ameterS.
Tab一e.1 Comparison between celi parameters and Fo (forsterite) Contents for the
olivine sand from Kawashiri Beach. The data for forsterite and hyalosiderite
are referenced.
smple皿ne
Fo
(mol
Forstehte" 1
%) a
(A) b
00 4.7540 1
(A) C
0.
1
97
(A) V(Å')
1 5.9806
01ivhe smd"` 74(1 ) 4.768(2) 10.284(4) 6.01 7(3) 295.0(2)
Hyalosideme``` 53
･5 4･784 1
0･3
1
8 6･027
●: SyndIetic fbrsterite (Schwab 皮 K櫨nster, 1977)･
.: mis study. Fo (moI %) is de mean value ror 57 samples of the olivine samd･
章軍事: Hyalosidehte缶om me Skae坪山h血戦ion (Smm, 1 966)･
Chemical analyses
Representative microprobe analyses for the six kinds of olivine血om Kawashih
Beach 紬e
glVen
in
lbble 2.
Because,
in
the
olivine
of
group
Ill,
it
was
proved
that two虹nds of glass inclusion, opaque and transparent, always existed separately
in the din:erent crystals, it was subdivided into two groups, IIIa and IIIb, respectively.
Five elements, Si, Fe, Mn, Mg and Ca were detected by EPMA in every olivine
smd crystal examined. Calculated end-members are Mg2SiO4, Fe2SiO4, Mn2SiO4 and
Ca2SiO4. The last occurs very rarely in nature with olivine stmcture (Bhdge, 1966);
it is used here for the sake or convenience･ The Fo content in the samples range
鹿児島大学教育学部研究紀要自然科学編第52巻(2001)
50
from 70 to 75 (mol %), and the components of Mn2SiO4 and Ca2SiO4 are Present
in very small amounts. Thus olivine sands 血Om Kawashih Beach are chemically
con血med as chrysolite.
Back scattered electron image of the olivine sand by EPMA is shown in Fig 5.
Two kinds of conoded inclusions, bhght and dark, can be seen in these plCtureS･
Tab一e,2 Representative microprobe ana一yses of the oiivine sands from Kawashiri
Beach.
I
II
IIIa
IIIb
IV
V
SiO2 37.35 38.28 38.97 38.96 38.80 38.33
FeO` 25.60 24.39 22.73 24.70 23.33 23.55
胡no
0.46 0.48 0.36 0.49 0.53 0.40
MgO 35.39 37.58 38.83 37.81 38.68 37.95
CaO
0.10 0.13 0.15 0.12 0.11 0.14
Total
(wt.
%) 99.10 100.86 101.04 102.08 101.45 100.37
Nmber ofcations on the basis or4 0
obs. cal°.`` obs. cal°.`` obs. cal°.●` obs. cal°."" obs. cal°.`" obs. cal°.i"
s IF eMMgc a皿
1.0030.998 0.9971.002 1.0030.996 1.0020.999 0.9991.000 0.9991.000
0.5720.572 0.5310.532 0.4890.490 0.5310.532 0.5020.502 0.5130.514
0.0100.010 0.0110.010 0.0080.008 0.0110.012 0.0120.012 0.0090.008
1.4091.410 1.4591.458 1.4891.490 1.4501.450 1.4841.484 1.4741.474
0.003 0.004 0.0040.004 0.0040.004 0.0030.004 0.0030.002 0.0040.004
2.9972.994 3.0023.006 2.9932.995 2.9972.997 3.0003.000 2.9993.000
Designation by olivine end-members
0
4
2
5
7
(mol
3
Total
7
0
Ca2SiO4"`` 0.2 0.2 0.2 0.2 0.
7
4 5 0
7 2
4
2 ｣ 6
く J 5 4
2
︻ 1 2
4
一ヽ一 ∠ 0 5
Mn2 S iO4
0 0 0 0
Fe2SiO4
7
Mg2 S iO4
1 0.2
%) 99.8 100.2 99.6 99.9 100.0 100.0
*: Total irons were calculated as FeO.
筆録: Calculated on the basis of end-member
t筆書: Ideal end-member for me sake ofconvenicncc.
Olivine sand fnom Kawashiri Beach in Kagoshima, Japan
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Fig.5 Back scattered e一ectron images of the olivine sand from Kawashiri Beach.
Except for samp一e ii, ai一 samp一es bearing silicate一meit inc一usions are showed.
PhotograPhs of a. b. C. d and e correspond to the samp一e of group L
IIia, IIib, IV and V, respective一y. Oi: oiivine, SMI: S掴cate一melt inc一usion,
Hem: hematite.
鹿児島大学教育学部研究紀要自然科学編第52巻(2001)
52
Table.3 ReBreSentative microBrObe analyses of silicate - melt inclusions in the olivine
sand from Kawashiri Beach.
I
IIm
IV
V
●
5 7 0 /
l ヽ ) t t
l ヽ一 7 -
1 2 一 O
匿｢ ‖
柳駆設
7 0 0 3
1
0 ′ 0 1
5
1 0.77 9.40
5 7 0
1
o o ′ D
l ヽ ) o O
l ヽ J
O o 7 1 - - -
-
Fee"
4 ･ -
鵬02
0 ノ 5 0
4
Also,
伽9896
SiO2
IHa
9.70 5:1妙 7.52
MhO
0.25 0.22
0.
MgO
4.01 3.27
4.57 2.98 254
Coo
8.19 9.30
8.
Na20
2.42 2.93
2.90 2L卵 1.86
K20
0.43 0.43
0.5
Total 92.65 93.
1
1
8 01L20 02
1
39 8.95 7.68
1 045 0.64
4 94.63 94.脇‥ 93.96
*: Total irons were calculated as Fee.
The bright one corresponds to black spherical inclusion which is o駄溜Ⅳed as opaque
inclusions under the polarizing microprobe･ Judging from cnemicaA ∝膿宣pOSISIOn and
atomic ratio, the bright inclusion seems to be hematite. Amぬ鉦i盤c皿sion was found
to be a silicate-melt inclusion, and their chemical compos弛o鵬are in Table 3. In
addition to Si, Al, Ti, Fe, Mn, Mg, Ca, Na and K in the tabk. comsi飯めIe amount
of P, S and Cl aLre detected on the qualitative analyses. If these s髄cate一皿由t inclusions
in the olivine sand from Kawashiri Beach are pnmary, day provide infomation
about the conditions of parent silicate magma, and the environH愉nt dlat血合host olivine
crystal surrounding them grew in.
C onclusions
The results of the study deschbed here, suggest me fbllowlng COnClusions:
1)
At
least
six
groups
of
olivine
are
mixed
in
the
olivine
sand 血om
Kawashih
Beach, and they are classi鯖ed according to the silicate-melt inclusions in them･
2) me composition of the olivine sand ranges血om Fo70 tO Fo75, and the sum of
Olivine sand fnom Kawasht'ri Beach l'n Kagoshima, Japan
53
the Mn2SiO4 and Ca2SiO4 COntent is less than 1 (mol %).
3) The opaque inclusion that is present in every olivine sample五〇m Kawashih Beach
seems to be hematite.
This new work on crystal chemistry of the olivine sand From Kawashiri Beach
not only provides basic mineraloglCal inゎ-ation about it, but also helps us to estimate
the ongln Of it.
Acknowledgements: The author wishes to thank M. Kimata (Institute of Geoscience,
University of Tsukuba) for his helpful suggestions･ Thanks are due to N･ Nishida
(Chemical Analysis Center, University or Tsukuba) for guidance in electron microprobe
analysis. The author is grateml to the Chemical Analysis Center, Universlty Of
Tsukuba, for electron microprobe analysis data.
Rererences
Bhdge, T. B. (1966): Bredigite, lamite and y dicalcium silicates五〇m Marble canyon･ Ame′∴ Min･, 51,
1766-74.
Deer. W. A., Howie, R_ A, Zussman, I. (1992): An inlr10duclion to the rock-forming minerals. 2nd ed.
Longman, China, 696p･
Holland, TJ. B. 皮 Redfem, S. A. T. (1997): UnitCell - a nonlinear least-squares program for cell parameter
rennement implementlng regression and deletion diagnostics･ ∫ Appl. CIySt., 30, 84.
Reed, S. J. B. ( 1996): Electron microprobe analysis and scanning electron micnoscopy in Geology･ Cambridge
Univ. Press, Cambhdge, 20lp.
Schwab, R. G. &陥nster, D･ ( 1977): Prazisionsgitterkonstantenbestimmung zur Festlegung Rentgenographis-
cher BestimmungskJven fu I Synthetische Olivine der Mischkristallreiche Forsterit-Fayalit. Neues Jahrb.
Min., Mh., 205-215.
Shannon, 良. D. (1976): Revised e飾ective ionic radii and systematic studies of interatomic distances in
halides and chalcogenides. Acta CIySらA32, 751-767.
Smith, ∫. V. (1966): X-ray emission microanalysis or rock-forming minerals II: Olivines. ). Geol., 74,
1-16.

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