AmericanMineralogist,Volume65,pages1119-1129,1980
Axinite mineral group in low-graderegionally metamorphosedrocks in southem
New Zealand
leN J. PnrNcre AND YosuKE KAwAcHI
Geology Department, Univerity of Otago
Dunedin, New Zealand
Abstract
In southern New Zealand, minerals of the axinite group are widespread in vein assemblages in regionally metamorphosed rocks of the prehnite-pumpellyite, pumpellyite-actinolite, and chlorite zone greenschistfacies.Fe, Mg-axinites, approachingend-memberferroaxinite in composition, along with quartz and often prehnite, pumpellyite, iron-rich
epidote, and chlorite fll veins in spilitized volcanic and greywackelithologies. Tinzenite and
more cofilmonly manganaxinites occur in quartz veins in nearby femrginous and manganiferous cherts. Chemical analysesof vein axinites are presented,as w€ll as analysesof four
porphyroblastic ferroan manganaxiniteswhich occur as rock-forming minerals at widely separated localities.
Compositional variability in published axinite analysesalong with those of this study can
be attributed partly to formation temperatures.A low-temperature miscibility gap may exist
in the axinite group. Tinzsnile or manganaxinite and ferroaxinite are stable in low-grade
metamorphic rocks of appropriate compositions, whereasferroan manganaxinites and manganonn ferroaxinites occur in somepegmatites,skarns, and regionally metamorphosedrocks
which equilibrated at more elevated temperatures.
For a wide lange of burk compositionsin biotite and garnet zone greenschistfacies and
amphibolite facies rocks of southern New Zealand, tourmaline is the only observedborosilicate phase. At metamorphic conditions typical of thesegrades,axinite minerals would be restricted to relatively Ca-rich lithologies by a reaction of the form:
3 ferroaxinite + 2 chlorite + 2 albite * 5 quartz
: 2 tourmaline * 4 epidote + 2 actinolite * 5 water
Introduction
spreadas being mainly due to host rock composition.
The general formula for the axinite group proposedby Lumpkin and Ribbe (1979)is:
Minerals of the axinite group have long been recognizedas typical phasesin manganese-ore
deposits,
skarns, and pegmatitesand associatedhigh-temper- [(Mn,Fe'*,M g,Zn Al)(Car-,Mn ) (Alz-,Fef*)]]I
Ozaki (1970,1972\examinedthe
atureenyironments.
(oH,_-o-x(Brs*--Al,)I'o3ol
relationship between mode of occurrence of axinite
and chemical composition. He distinguished five dif- where w < 1, r ( l, y 11 l, z 11 l, and VI and IV
ferent categories:manganeseand femrginous ore de- represent co-ordination of the cations. Sanero
posits,pegmatites,contactmetamorphicand metaso- and Gottardi (1968) have clarifed the nomenmatic ore deposits,regionally metamorphosedrocks, clature and defined the end members ferroaxinite
and veins in igneous and sedimentary rocks. When [Fe2*CarAlrBSioO,r(OH)]and manganaxinite
axinite analysesfrom each occurrenceare plotted on [Mn'*CarAlrBSLO,r(OH)]. The name ferroaxinite is
a Mn-Ca-Fe diagram the manganesecontentsof ax- applied to thoseaxiniteswith Ca > 1.5and Fe ) Mn,
inites in each rock type are shown to decreasein the and manganaxinitewhere Ca > 1.5 and Mn > Fe.
above order, although considerable overlap is ob- Tinzenite includes those axinites with Ca < 1.5 and
served(Fig. l). Ozaki interpreted this compositional Mn > Fe which approach the empirical composition
ffi03-.00/.x/
80/ I r 12-l l 19$02.00
ll19
I 120
PRINGLE AND KAWACHI:
AXINITE
\
l.
manganese and ferrugrnous
ore deposits.
2. pegmatites
3. contact netamorphic and
metasomatic ore dePosits
4,
regional
metamorphic
rocks
5. veins in igneous and
rocks.
sedinentary
30
lln
/
Irc \
Fig. l. Triangular Mn-Ca-Fe diagram after Ozaki (1972), showing compositional variations of axinites in different [thologies.
MnrCaAl,BSLO,s(OH).Saneroand Gottardi (1968),
followed by Ozaki (1969, 1972) and Lumpkin 6d
Ribbe (1979), showed that substitution in the group
is principally in two series,one from ferroaxinite to
manganaxinite, the other fron manganildnite towards tinzenite.
Axinite in low-grade regionally metamorphosed
rocks was first reported by Kojima (194) in the
Sambagawametamorphic belt in central Japan. Here
axinite occurs in a stilpnomelane-bearing band in
greenschist.The only extended account ofaxinite as
a fock-forming mineral in schistsis by Nureki (1967),
who described and partially arralyzedmanganaxinite
from transitional blueschist-greenschistfacies rocks
of the Sangunmetamorphiczonein soutlwest Japan,
where it occurs as inclusion-studded porphyroblasts
and as clear grains in nearby quartz-albite-ildnite
veins. The manganaxinite-bearing assemblagescontain stilpnomelane * chlorite + epidote + albite +
qrraftz + muscovite + calcite + sphene + opaque.
Examples of axinite-bearing veins in regionally
metamorphic rocks are more numerous. Simonen
and Wiik (1952) describe ferroaxinite-quartz-calcite
veins in amphibolites and basic intrusive rocks in
Finland. In a review of axinite oocurrencesin the
Norwegian Caledonides,Carstens(1965)emphasized
the association of epigenetic axinite-bearing veins
with metabasite lithologies. Worldwide, over forty
records of axinite from veins in regionally metamorphosed rocks of grads higher than zeolite facies have
been reported. The majority of these are in rocks
6pafaining mineral assemblages
characteristicof the
amphibolite facies and lower grade.
Recent workers on low-grade metamorphic rocks
in southernNew Zealand have recognizedminerals of
the axinite group at various localities in several distinct terranes(Coombset al., 1976)flanking the axis
of the Haast Schist(e.9.Mason, 1959;Manserghand
Watters, 1970;Read and Reay, l97l; Andrews et al.,
1974; this study) (Fig. 2). Four occurrenoesare as
rock-forming minerals, others are confined to veins,
but all are restricted to rocks of the chlorite zone
gteenschist facies, pumpellyite-actinotte facies, and
prehnite-pumFellyite facies. We suspectthat such
occurrencesare relatively abundant and that minerals of the axinite group are more coilrmon in regionally metamorphosed sequencesthan has generally
been appreciated.
Field relations and petrography
Torlesseterrane and adjacentHaast Schistterrane
These terranes consist of Late Paleozoic to Mesozoic greywackes and argillites together with'rare
cherts, spilitized pillow basalts, limestones and conglomerates, and their schistose derivatives. Axinite
PRINGLE AND I(AWACHI:
AXINITE
tl2l
Fig. 2. Axinite localities in the southern half of the South Islant
(a) Lord Range(Andrewset al-,1914);(b) Pertl
axinite occurrences:
(d) Humboldt Mountains (Bishop et al.,1976)-Dots are localitiesof
PareoraGorge (OU a672); (3) Kirkliston Mountains (OU 33912)
35524,15597,35526,35523,35528);(5) Hawkdun Range (Gradv,
28n$;Q) Akatore(Readand Reay, l97l) (OU 25686)'
35595,35596,28558,33366,
minerals along with other low-grade metamorphic
phasesoccur in veins at numerous locatties. Axinite
porphyroblasts, however, are more restricted in distribution.
Near Dansey Pass, thick axinite-bearing quaftz
veins cut a schistosemetachert in chlorite zone greenschist facies rocks of textural zone IIIA (Bishop,
1972). TtLe axinite, a manganaxirdte (OU 25336),'
coexists with hematite and manganoan brunsvigite
which contains 4.04.3 weight peraent MnO. Minor
tourmaline, along with spessartineand muscovite, is
presentin the host rock but not in the axinite-bearing
veins. Elsewherein the chlorite zone, subhedral manganaxinite porphyroblasts(OU 35528)studdedwith
minute inclusions predominantly of quartz and hematite occur within stilpnomelane-chlorite-epidote laminas in a fine-grainedhematitic metachert.Also, an
impure marble contains small grains of ferroan manganaxinite (OU 25335) mantled by manganoan
brunsvigite. Other phases in this rock include prisI Sample numbers are Geology Department, UniversitY of
Otago catalogue numbers.
matic tourmaline, manganoanstilpnomelane,hematite, and abundant calcite'
At lower grade, in pumpellyite-actinolite facies
rocks, veins containing quartz and pale brown manganaxinite (OU 35527) occur in massivehematitic
metacherts. Pinkish brown and pale purple ferroaxinites are cofilmon constituentsof anastomosingveins
in nearby metabasitelithologies (OU 35523' 35524'
35525,35526,35597).The latter veins vary in thicknessfrom a fe* mm to severalcm and contain, in addition to ubiquitous quattz, variable amounts of calcite and iron-rich epidote (Psru-Psr.).lron-rich
epidoteis often concentratedasborder zones'l-3 cm
wide in thicker ferroaxinite-bearing veins. Albite,
pumpellyite, chlorite, asbestiform actinolite, sericite,
pyrite, and sphene are common accessoryphases'
Mg-pumpellyite containing 2.5 weight percent MgO
coexistswith ferroaxinite in OU 35526.
Manganaxinites in veins from two other localities
were analyzed. In the northern Malvern Hills, 70 km
west of Christchurch, buff-brown manganaxinite
(OU #1676)fills thin veinlets along with quartz in a
rhodonite-pyrolusite-bearing metachert. The host
tr22
PRINGLE AND KAIIACHI:
rock forms part of a 600-m-thick sequenceof me_
tabasites, hematitic and manganiferous metacherts,
and marbles metamorphosed to prehnite_pump_
ellyite facies.
A large boulder of pumpellyite-bearing metachert
in river gravels of Station Stream, which drains the
eastern Kirkliston Mountains, contains veins up to
several cm thick of quartz and yellow-brown man_
ganaxinite (OU 33912). Red and green metacherts
are important constituents in a metavolcanic se_
quenc€ cropping out nearby, and probably the boul_
der is locally derived.
Other analyzed axinites from vein assemblagesin
Torlesserocks are from tl.e Hawkdun Range (Grady,
1968), where pale brown ferroaxinite (OU 21067,
21069, 21070) along with quartz and prehnite oc_
cupiesveins cutting prehnite-pumpellyite faciesme_
tagreywackes.Axinite-bearing veins in similar lith_
ologies have been described from the perth River
(Mason, 1959)and from near Lake Aviemore (Man_
sergh and Watters, 1970).These samplesdiffer from
others in this study in fsing remote from metabasite
and,/or cherty lithologies.
Porphyroblastic manganaxinites occur together
with qlartz, ferrostilpnomelane,minor manganoan
ripidolite, and sericite in a prehnite-pumpellyite
faciesmetachert (OU 4672) from the Lower pareora
Gorge (Fig. 2). The porphyroblasts comprise six percent of the rock, are typically wedge-shapedcrystals
up to 300 pm diameter,and are studdedwith minute
AXINITE
inclusions of quartz and hematite which causea pink
to pale brown coloration of cores in some grains
(Fig. 3).
The Torlesse rocks pass gradationally into the
Haast Schist terrane with an increasing development
of penetrative schistosity. Metamorphic grade in
thesetransitional areasis commonly of pumpellyite_
actinolite facies. It is near the marginal fringe of the
Haast Schist terrane that axinite-bearing assemblages
are widespread. Axinite has not been reported from
schists of biotite zone or of highsl metamorphic
grades in New Zealand.
Caplesterrane
The CaplesGroup flanking the southwesternmargin of the Haast Schist terrane consistslargely of volcanic arc-derived sediments with minor spilitic volcanics,limestones,and cherts.Theserocks form part
of the somplexly deformed Te Anau Assemblageof
Turnbull (1977).
Axinite-bearing qluartz veins occur widely near
Lake Wakatipu (e.g. Kawachi, 1975; BishoB et aI.,
1976).Fenoaxinites from severallocalities were analyzed. In each casethey are from veins in mafic volcanogenic rocks of prehnite-pumpellyite or pumpellyite-actinolite facies. Associated minerals, as in
the ferroaxinite-bearing assemblagesof the Torlesse
terrane, include qu,afiz, pumpellyite, epidote, chlorite, calcite, and albite.
A Mg-pumpellyite coexisting with ferroaxinite in
Fig' 3' Manganaxinitc porphyroblasts in oU 4672. Minute inclusions of quartz and hematite ar€
concentrated in grain cores. The
fne-grained groundmassconsistsof quart4 ferrostilpnomelane, manganoan aliOotite, and minor spessartine.
Scale bar is 250 pm.
PRINGLE AND KAWACHI:
AXINITE
tt23
OU 28558 contains 3.1 weight percent FeO*' and necessary.Using this method most analyzedferroaxi3.40 weight percent MgO, and is typical of pump- nites have no or low FerO, content (Table 2). Howellyite compositions in rocks of the higher-grade ever, for most manganaxinitesFerO. content repreparts of the pumpellyite-actinolite facies (Kawachi, sentsa significant amount of the total iron. Likewise,
1975).The composition of coexistingplagioclaseis wet-chemically determined compositionsof mangaalmost end-memberalbite (Ab", ') and calcite in the naxinites listed by Ozaki generally have higher
same vein is close to pure CaCOr. Optical data for Fe'*: Fe'* ratios than ferroaxinites (Ozakt, 1972).
these phasesin other ferroaxinite-bearingvein as- MnO contents of manganaxinitesin Table I range
semblagessuggestsimilar compositions.Epidotesare from 7.19 to 16.25weight percentand FeO * FerO,
all iron-rich, as indicated by their high birefringence rangesfron 4.29 to 0.48 weight percent.Total (Fe +
Mn + Mg) of ferroaxinites closely approaches2.00
and yellow to pale greenpleochroism.
per formula unit and there is a negative correlation
porphyroblasts
Although ferroan manganaxinite
betweentotal Fe and Mg, an oppositetrend to Fe(OU
associgreen
33366)
sericiticschist
occur in pale
in
Mg variations seenin analyzedmanganaxinites(Fig.
Mt.
Nicholas
pillowed
near
metabasalts
ated with
yet
4a). With increasingMg content,Mn in manganeseno
(Turnbull,
1974),
as
the Thomson Mountains
rapidly from tinzenite(Fig. ab).
phasesdecreases
rich
veins
in
this
identified
in
manganaxinites have been
show limited Mg-Mn subhowever,
porphyroblasts
Ferroaxinites,
terrane. The ferroan manganaxinite
higher
more variable Mg conand
despite
stitution,
percent
rock
are
dotted
of
the
and
comprise5.5volume
plot
within a restricted
analyses
Ferroaxinite
quartz
albite.
tent.
and
mainly
of
with minute inclusions
join
a Ca-Mn-Fe tri+
Mg)
on
the
Ca-(Fe
near
field
lozenge-shaped
idioblastic
The crystals are com-monly
within the comfall
(Fig.
5).
They
diagram
and
angular
maximum
dimension
pm
in
grains lessthan 200
low-grade,refrom
other
axinites
positional
range
of
quartz,
along
with
Pumpellyite,
are rarely twinned.
Fig. l)
(compare
terranes
metamorphosed
minor
gionally
detrital
and
phengite, albite, chlorite, sphene,
ferroaxiof
formula
ideal
the
approach
closely
and
mineral
assemblage.
epidote,completesthe
Along strike to the southeast,on the coast, com- ilte.
Tinzenite from Akatore (Table 1,25686\ contains
parable rocks known as the Tuapeka Group also
more MnO and lessCaO than our anaHere,
considerably
Haast
Schist
terrane.
into
the
gradenorthwards
and approachestheoretical tinmanganaxinites
lyzed
yellowish
(OU
25686)
tinzenite
near Akatore Creek,
magnesium contents
Although
composition.
zenite
quartz,
rhodonite,
with
along
occupies thin veinlets
pyroxmangite, and apatite in an akatoreite-bearing are negligible,someof the analysesare calculatedto
contain up to 0.33 weight percentFerOr. In contrast,
manganiferousmetachert(Read and Reay, l97l).
tinzenite ftsrr finzens, Switzerlandhas 1.59weight
Mineral chemistry of tinzenite and axinites
percentFerO, (Milton et al.,1953).
Porphyroblastic manganaxinite crystals from the
water
have
for
and
boron
elements
except
Major
terrane in the Lower Pareora Gorge are
Torlesse
computer-controlled
a
JEoL
JxA-5A
analyzed
by
been
zoned (Fig. 2), and minute quartz
standards,
compositionally
Analysis
conditions,
microprobe.
electron
(<l pm) concentratedin
inclusions
hematite
deprocedure
similar
to
those
and
are
correction
and
quality
inhibit
of microprobe analythe
grain
cores
(1973).
and
Coombs
Nakamura
by
scribed
Representativeand averagedtinzenite and axinite ses. Typical analyses from the core and rin of a
analyseslisted in order of increasingmanganesecon- single porphyroblast are given in Table 1. For the
tent are given in Tables I and2. Cationic proportions core analysis, oxides and structural formula are reare calculatedon an anhydrousand boron-freebasis calculated assuming a3.70 percent qrLurtzimpurity.
(O : 28), and water and B"O, contentsare estimated This was estimated by reducing weight percent SiO,
in stoichiometric amounts OH : 2.0 and B : 2.0. to the averagevalue of other analyzed manganThe tendency for the totals of most analysesto ap- axinites. Porphyroblast cores are enriched in MnO
proach 100 percent supportsthe validity of this cal- and depletedin total iron, AlrOr, and CaO relative to
culation. The amount of Fe"O, in the analyses has porphyroblast rims. Unzoned porphyroblastic manganaxinitesfrom DanseyPass(OU 25335)have combeen calculatedon the basisof (Al'" + Ti + Fe'*; :
4.000,after assigningAl to fill tetrahedralsiteswhere positions closely similar to the coresof the Pareora
mineral. Porphyroblastic manganaxinites from near
2 FeO+and Fe2O3*will be usedin this paper for total iron calcu- Mt. Nicholas in the Caplesterrane (OU 33366),although studded with quartz and albite inclusions
lated as FeO and total iron calculated as Fe2Or respectively.
PRINGLE AND KAWACHI:
AXINITE
Table l. Average and selectedanalysesof tinzenite a1d manganaxinites
Specm
i en
Numbeo
rf
analyses averaged
si02
ri02
At^0C^
n
(a)
.J
F e O( a )
Mn0
Mgo
Ca0
Na20
Kzo
T ot a l
t i n z e n it e
0u256860uzs3360u44676
ou33er20u35527
0il3!31['-tlJliiru
43.3
0.02
18.2
0 .l 0
0.09
20.6
0.08
]3.3
0.03
0.00
95.72
0U44672
.NFA
76
(b)ll
E]
42.6
0.07
I 6. 9
0. 7 4
0.00
13.12
0.30
18.9
0.03
0.01
92.67
"2"3
Hr0 (a)
6.24
'I
.61
6.il
si
FE
8.04
0 .0 0
3.98
0. 0 2
0.01
I .08
0 .0 t
3 .7 8
0.1I
0. 0 0
Mn
5 .2+
211
1.58
42.5
0.03
]6.5
'l.96
0.75
13.12
0 . 19
17.6
0. 0 2
0.03
92.70
42.7
0.05
17.1
1.23
0.74
13.14
0.14
17.9
0.03
0.02
93.05
42.2
0.01
17.s
0. 4 5
0.65
11.72
0.30
18.8
0.02
0.02
91.67
43.6
0.00
16.6
2. 1 8
0.32
11.37
0.46
18.3
0.03
0.03
92.89
4 2. 0
0.04
]6.9
I .64
2.06
ll.31
0.62
18.6
0.02
0.01
93.20
6.07
1.57
6.12
1.58
6.07
'l.57
6.14
1.59
6 . 10
rtm
4 6 .3
0 . 17
15.9
42.6 +z-t
0 . 1 8 0. 0 1
17.3 t8.6
0 . 7 4 0 .0 0
? lo
2.44(") I .99
1 0 . 5 3 1 1 . 4 4 8.78
0.18
0.20 n??
16.4
1 7. 8
t9.5
0.04
0. 0 4
0.01
0.04
0 . 0 4 0. 0 5
o? 17
92. 00
YZ. J5
0U33366
42.0
0 . 1r
I8.1
0. 0 0
4.29
7. 1 9
u. f,o
19.2
0. 0 5
0.08
9l.58
6.t0
l.s8
6.20
I.60
6.10
1.58
.l.82
8.09
0.03
3.87
0.I|
0.32
'l.84
0 .l 8
0.06
7.98
0.00
4 .l 0
0.00
0.50
1.39
0.09
7.98
0.02
4.05
0.00
0.68
l.16
0.16
?A?
?On
t.?6
Atomic proportions, 0 = 28
Ti
AI
Fe-'
Mn
0.02
z.05
Na
K
total
Analyst
0. 0 1
0.00
17 07
YK
8.12
0.00
3.71
0.28
0.12
2.12
0.05
8.09
0.01
3.82
0 .t 8
0.12
2.11
0.04
J.OU
J.bJ
0 .0 9
3.84
0.01
0.00
I8.03
0.01
0.01
18.02
IJP
IJP
o.u3
0 .0 0
J. YJ
0.06
0.10
lao
8.23
0.00
3. 6 9
0.31
0 .0 5
,l.82
70q
0 .0 t
3.79
0.23
U.JJ
0.01
0.01
18.02
0.09
3. 8 4
0. 0 1
0.01
1 7. 9 8
0.13
3. 7 0
0.01
0.01
I7.95
0.01
0.00
I 8 .l 4
IJP
IJP
IJP
IJP
? 70
(a)
Calculated(see text).
(b)
4 4 6 7 2 ( c o r e ) r e c a l c u l a t e d a s s u m i n ga n e x c e s so f 3 . 7 0 %S i 0 r .
Total Fe as FeO.
(c)
aligned subparallel to schistosity, are also compositionally unzoned.The Fe: Mn ratio of a typical analysis (Table l, OU 33366),however,is slightly higher
than Fe: Mn ratios of porphyroblastic axinites northeast of the Haast Schist axis.
In general, porphyroblastic axinite compositions
are intermediate between m'nganaxinites and ferroaxinites from nearby metamorphic vein assemblages.This relation5hiFis shown in Figure 5.
Mn distribution between coexisting chlorite and
axinite varies consi$tently from negligible and minor
MnO contents in chlorites frorn ferroaxinite-bearing
0.01 0.00
0.0r
0.01
17.97 17.98
IJP
IJP
?Ol
0.02
0.02
I8.00
IJP
samples to several weight percent MnO in chlorites
associatedwith manganaxinite.Rangesfor phasesin
five samplesare listed in Table 3.
Discussion
Axinite compositionand metamorphicgrade
In southern New Zealand the common association
of Mg-ptrmFellyite with axinite minerals and the restricted occurrence of axinite to rocks of low metamorphic rank suggest that temFeratures and pressures of axinite formation are unlikelv to have
PRINGLE AND I(AWACHI:
tr25
AXINITE
Table 2. Average and selectedanalysesof ferroaxinites
|,,,!,
Jts!v
0u355250U2l070 0u355230U355240u355960U35s970u287340U2l067 0u2l069 0u355260U28558
I
N u m b e or f
analyses averaged
si02
Ti02
Al^0^
ZJ
(")
Fe^o^
a5
(")
Feo
Mn0
Mso
Ca0
Naro
Kzo
TotaI
B ^ 0 ^' ' '
a5
Hz^ 0 ( a )
1
44
t4
7
JJ
43.0
0.0]
18.8
42.7
0. 0 2
17.s
42.8
0.03
'18.4
42.4
0.01
18.5
42.2
0.0]
18.7
43.2
0.04
17.9
0.00
0.00
0.00
0.00
0. 0 0
0.46
I .96
l.ll
'l.33
8.9]
,l.08
'I
.50
20.0
0. 0 2
0.01
92.57
8.78
0.98
].36
20.1
0. 0 3
0.02
92.50
9.26
0.72
L20
20.1
nd
nd
92.19
9.61
0.59
1.21
20.1
nd
nd
92.42
7.55
0.46
2.40
20.1
0.03
0' 0 2
92.16
6.17
t.60
6.]8
1.60
6.22
1.61
0'12
0'09
0'07
43.0
0.00
18.2
42.5
0.02
I I .9
42.6
0.04
18.6
0.28
0.00
0.00
0.00
0.00
0.75
6.87
6 . ]5
t -zt
| .zo
8. 2 4
1.25
l 39
2 0. 0
0.04
0.0]
9r.90
2 0. 3
0.01
0.01
92.13
6.14
'L59
6.ll
'l.59
8 .3 7
I .95
'|
lE
'19.6
0. 0 2
0. 0 1
92.70
o,al
t.ol
1.58
1.97
20.1
0.02
0.02
92.14
l.5l
l.l9
20.0
nd
nd
92.45
6.21
I .6r
6.20
l.6l
tqE
2a1
20.2
20.1
nd
nd
nd
n0
9 2 . 13
92.23
D.az
'I
.61
o.40
1.62
447
42.9
0. 0 5
18.4
42.1
0.0]
18.3
0.02
'I
8.0
+J. J
l0
h.al
o.4l
I .61
l.6l
Atomicproportions,0 = 28
si
tl
AI
?+
Fe''
fe
Mn
Mg
LA
Na
K
TotaI
Analyst
8.088.027.g47.g37.s68.067.s37.987.997'967'918'05
0.000.000.000.0]0.000.000.000.0]0.000.000.000.0.1
3.964.004.164.084.]03'894.064.044.054.094'133'93
0.040.000'000'000.000.,1.10.000.000.000.000.000.07
1.3ll.l3l'30].070.951.30].41].39].371.451'5]1..18
0']6
0.]7
0.]8
0.20
0.20
0.20
0.24
0.25
0.3]
0.320.550.330.7]0.800.390.370.420.380.340.340.67
3.g24.024.004.033.984'044.093.994.024.044.044.01
o'0]
0.00
0'02
0.01
0.ol
0.00
0.00
0.00
0.01
0.00
'17.95
18.03 17.gg ]8.01 18.04 18.0]
17.gg lt.s7
IJP
IJP
IJP
]JP
IJP
( a ) C a l c u la t e d ( s e e t e x t ) .
exceededthe upper stability limit of Mg-pumpellyite.
Recent experimental work by Schiffman and Liou
(1977) and Plyusina and Ivanov (1978) have established the upper stability limit of Mg-pumpellyite to
between340" and 390'C over a P(fluid) rangeof I to
8 kbar. Temperatures well in excess of these undoubtedly prevailed during axinite formation in
many pegmatitesand skarns reported elsewhere(e.9.
Mozgova, 1964).
Original locality descriptions of the analyzed axinites listed by Ozaki (1972), together with our data,
reveal that the composition of axinites in regionally
metamorphosed terranes varies systematically with
metamorphic grade. The host lithologies of Ozaki's
category 5 (Fig. l) invariably contain mineral assemblages typical of the prehnite-pumpellyite and
YK
YK
YK
0'ol
0'01
17.99
18.00
18.02
0'01
0'01
18.01
Y K ,I J P
IJP
IJP
YK
nd = not determined.
pumpellyite-actinolite facies, whereas rocks of category 4 have been regionally metamorphosed to
greenschistand lower amphibolite facies. Similarly'
manganeseand femrginous ore deposits soil4ining
manganaxinite or tinzenite (category I of Ozaki,
1972\ have been affected by low-grade Sambagawa
metamorphism. Axinites in rocks sf highsl-fs6ps1ature paragenesessuch as skarns and pegmatites are
commonly ferroan manganaxinites or manganoan
ferroaxinites intermediate in composition. between
ferroaxinite and tinzenite end members. In general,
ferroaxinite and tinzenite are stable in low-grade
rocks but becomemore Mn- and Fe-rich respectively
with increasing metamorphic grade.
This trend is compatible with our observations.
Porphyroblastic manganaxinites have compositions
t126
PRINGLE AND KAWACHI:
00.5
Mg
1
00.5
I
Mg
Fig. 4. Fe ls. Mg (a) and Mn vs. Mg (b) plots of reprcsentative
and average tinzenite (solid square), manganaxinites (open
circles), and ferroaxinites (solid circles).
intermediate between ferroaxinites and manganaxinites and tinzenites from nearby veins (Fig. 5),
and it is unlikely that temperatures and pressuresof
axinite-bearing vein formation have surpassedthose
of their host rocks. Furthermore, the chemical zoning
AXINITE
detectable in porphyroblastic axinites n OU 4672
may reflect axinite growth accompanying increasing
metamorphism. Compositions of grain cores plot
near the field of vein nanganaxinites on the Ca-Mn(Fe + Mg) triangular diagram (Fig. 5). However,the
distinctly FeO*-, MgO-, and CaO-enriched and
MnO-depleted rims of the samegrains are more oentrally located on the figure.
Banno and Matsui (1979,p.68) showed that the
stability temperatures of intermediate members of
any mineral solid solution series depend on differencesin ionic radii of substitut:ngions. From Banno
and Matsui's data, differencesin ionic radii between
the divalent ions Mn, Fe, Mg, Mn, Ca suggestthat in
axinite a continuous solid solution between axinite
and tinzenite exists only at moderately elevated temperatures typical of amphibolite facies conditions. At
lower temperatures a miscibility gap in the axinite
mineral seriesis likely to occur irrespective of bulk
rock composition.
Low-temperature miscibility gapsare known to ocmins14ls,suchas the
cur in other manganese-bearing
rhodochrosite-calcite isomorphous series(Goldsmith
and Graf, 1957) and the pyralspite garn€t group
(Miyashiro, 1953).In both thesegroupsthe break in
prehni te
u m p e lI y i t e
tinzenite
\
istecile.
a c t r n ol r t e
Ca
urmali ne
,' ct o
hlorite
tu-Mt
end
rrrenrber
end
nrDer
f er ro axinite
nanqan
axinit
3tn.
/
|
,/l
,,'Manganaxinite:
|
.Fer
'Fe.n91
Fig. 5. Ca-Mn-Fe (Fe + Mg) triangular diagram, showing compositional variations of analyzed axinites.
PRINGLE AND KAWACHI:
Table 3. Ranges in MnO contents for coexisting axinite and
chlorite
Specinen
Nunber
MnO axinite
MnO chlorite
ou28558
0.28 -
0.85(7)
0.0s - 0.r0(4)
ou35523
r.03 -
1.5r(4)
o.22 - O.29(3',)
ou25335
10.45 - 12.19(s)
ou44672
core
rim
II.44
8.78
2.s6 - 3.02(3)
ou25336
11.60 - 14.41(6)
3.99 - 4.32(3)
The nunber of analyses
brackets.
z.+J
-
z.Yz\J)
of each phase are enclosed
by
AXINITE
rane, tourmaline is the only boron-bearing phaserecosnized.
Mineral assemblages6palnining axinite and/or
tourmaline are portrayed on ACF-B2O, diagrams in
Figure 6. We proposethat in rocks of low metamorphic rank (chlorite zone greenschistfacies and
lower), the mineral assemblagesepidote-axinitechlorite and epidote-tourmaline-chlorite limit axinite and tourmaline occurrencesto rocks of restricted composition (Fig. 6a). Tourmaline can occur
only in relatively aluminous rocks (e.g. metapelites),
whereas axinite may be present in relatively calcareous and mafic lithologies (e.9. metabasalts).
Rocks with bulk-rock compositions within the area
axinite-epidote-tourmaline-chlorite in Figure 6a
may contain both axinite and tourmaline.
The elimination of membersof the axinite group
from rocks of more aluminous compositions during
progressive metamorphism may be represented by
equations such as:
solid solution disappearsat high temperatures. The
compositional fields of epidotes and piemontites enlarge with increasing temperatures (Miyashiro and 33Fe,
.rM&.roCaoALBrSi.O'o(OH)'
Seki, 1958).However,in this case,the valency states
ferroaxinite
of Fe and Mn (Fe'* and Mn3*) differ from those in
+ 19.5(Mg,Fe)o.rAl,
urSi,u'O1o(OH).
axinite, and it is the Mn'* content of the minerals
ripidolite
which increaseswith increasing temperature.
+ 22NaAlSi,O,+ 93SiO2+ 54.750.2
quartz
Host rock compositionalcontrols
albite
Axinite-bearing veins in low-grade, regionally
metamorphosed rocks in southern New Zealand
commonly cut metabasaltsand associatedlithologies
such as metacherts, volcanogenic metasediments,or
argillites. The axinites have formed in areas remote
from igneousintrusion and are clearly not of contactmetamorphic paragenesis.This relationship suggests
that the boron necessaryfor axinite formation must
have been derived mainly from metabasalticrocks or
associatedmetacherts or argillites under the conditions of prehnite-pumpellyite, pumpellyite-actinolite, and chlorite zone greenschist facies metamorphism.
In regionally metamorphosed rocks throughout
southern New Zealand tourmaline intermediate in
composition between dravite and schorl [NaFe
Mg"ALBrSLOr,(OH,F)olis widespreadas an accessory rock-forning phase and is locally abundant in
manganiferous metachertsand marbles as well as in
veins. However, in prehnite-pumpellyite, pumpellyite-actinolite, and chlorite zone greenschistfacies
rocks the association of tourmaline with axinite is
rare. The two minerals coexist in only one sample
(OU25335). In rocks metamorphosedbeyond chlorite zone greenschistfacies in the Haast Schist ter-
: 22NaFeMgrALB'SLOr'(OH)o
dravite-schorl
+ 3TCarNrFeSirO"(OH)
epidote
| 29CarMgrFerSi'Orr(OH),+ l9.5HrO
actinolite
Or, alternatively, without change in oxidation state:
+ 5MgrAlrSiO'(OH).
3FerCa*ALBrSi,O3o(OH),
amesrte
ferroaxinite
+ 2NaAlSi,Os+ 5SiO' quaftz
albite
2NaMg,ALB3SLO"(OH). + 4CarAl,Si,O,r(OH)
zo$lte
dravite
+ 2CarMgrFe,SirOo(OH),+ 5HrO
actinotte
Since the reactions are not univariant, due to variable MgO/FeO ratios in chlorite and amphibole
phases,they may proceed over a range in temperature and pressure.Thus, in rocks with metamorphic
rank equivalent to or higher than biotite zone greenschist facies, tourmaline can occur in lithologies with
a wide range of compositions whereas axinite is restricted to more Ca-rich rocktypes (Fig. 6b).
PRINGLE AND KAWACHI:
AXINITE
b
Fig. 6. ACF-B2O3 quaternary diagrams, sssgming excess SiO, and H2O are compl€tely mobile (see text for explanation). (a)
Compositional relationships between the borosilicates axinite (Ax), tourmaline (Tm), danburite @b), datolite (Dt), and dumortierite
@m) and metamorphic phasestypical of the prehnite-punpellyite, pumpellyite-actinolite, and chlorite zone greenschistfacies. (Cc calcite, Pr : prehnite, pu : pumpelMte, A6 : actinolite, Ep = epidote, Chl = chlorite). (b) Mineral assembl,ages
reported in the biotite
and garnet zones,greenschistfacies and the amphibolite facies. Hornblende (Hb) is the stable amphibole phase.Othcr abbreviations as
in 6a.
Within the high-grade part of the Haast Schist terrane, rock compositions appropriate for axinite formation are rare and axinite has not been reported.
On the other hand, tourmaline is widespreadin feldspathic and pelitic schists(e.g. Reed, 1958;Mason,
1962).
We have observedthe assemblagetourmaline-calcite in marbles from the Torlesse,Caples, and Haast
Schist terranes. For these rocks the interpretation of
mineral assemblageson ACF-BrO, diagrams cannot
be applied, since SiO, is commonly not an excess
component.
Datolite and danburite may be stable in Ca-rich
lithologies, provided that sufficient boron is available. Vein assemblages
consistingof datolite * calcite + q\aftz t laumontite + heulandite have been
described in Torlesse'greywackesfrom the central
North Island (Manion, 1974).However, datolite and
danburite have not been recorded in regionally metamorphosed rocks of southern New Zealand, and
their paragenetic relationships with other metamorphic phases are unclear. Dumortierite will be restricted to very aluminous lithologies. It has been recognized in a sheared porphyritic andesite dyke at
Karikari Peninsulia,Northland (Black, 1973), where
it is intergrown with epidote and tourmaline.
Acknowledqments
The authors thank Drs. A. Reay, M. Ozaki, I. Turnbull, and
W. A. Watters for kindly providing specinens for this study, and
ProfessorT. Sameshimafor information regarding borosilicate localities in the North Island New Zealard. Discussions on initial
drafts of the manuscript by ProfessorD. S. Coombs and Mr. J. Y.
Bradshaw led to signifcant improvements. Thanks dso to Miss
E. A. Gray (typing), Mr. J. M. Pillidge (polished thin sections),
and Messrs. E. L McKenzie and A. S. Cosgrove (microprobe
maintenanc€).
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tt29
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