American Mineralogist, Volume 59, pages 709-718, 1974
BiaxialColor Centersin AmethystQuartz
F. HlsseNl, eNo ArvrN J. CoHBll
Department of Earth and Planetary sciences, uniuersity ol pittsburgh,
Pittsburgh, pennsyluania I 5260
Abshact
Anisotropy of the characteristic absorption bands in a natural amethyst crystal from Brazil
is demonstrated by the dependenceof the value of the absorption coefficient upo,n the direction
of the electric vector of incident light within the 4.13-0.95 eV region, for differently oriented
sections of the crystal. The absorption indicatrix is used for interpreting the observed optical
absorptions. A new absorption band in the 6.20 eV region is reported. A complex absorption
band in the 5.60 eV region, usually attributed to a transition related to ligand-cation charge
transfer in Fe"* compounds, is clearly resolved into at least three bands.
The biaxiality and pleochroism of amethyst is explained as due to the existence of strongly
anisotropic color centers, related to iron, of orthorhombic or lower symmetry. Upon bleaching
the amethyst color, either optically or thermally, the biaxiality disappears. This anomalous
biaxiality and pleochroism observed in amethyst quartz is a property oi the color center itself
and not a property of the c-quartz Crystal structure.
fntroduction
The color of amethyst quarlz has been attributed
to color centers related to iron impurity. Berthelot
(1906) recognizedthis as a radiation color; it is
bleachedby heating the crystals at -400"C, but
restoredupon re-irradiation by gammarays, X-rays,
or other ionizing radiation. A detailed review of the
optical and other properties of amethyst-quartzincluding literature referencesthrough 1960 was publishedby Frondel (1962).
Basal (0001) sectionsof amethyst-coloredc-quartz
crystals display a weak anornalousdichroism and
biaxial characterwhich is in direct conflict with the
holotrigonal symmetry of the ,rquartz'structure and
thus has led to numeroussuggestionsthat the symmetry is lower than holotrigonal. Melanckolin and
Tsinober (1936) and othersattemptedto associate
the biaxialty of amethystwith Brazil twinning. The
anisotropythey observedin the amethystErn spectrum led Barry and Moore (1964) and Barry,
McNamara, and Moore (1965) to attribute its
biaxiality to non-random substitution of FeB* ions
in the equivalentsilicon sites.In the presentinvestigation, optical and thermal bleaching experiments
removed the observedanisotropy,optical biaxiality,
'Present address,
Materials Engineering and physical
SciencesDepartment, American University in Cairo, Cairo,
United Arab Republic.
and pleochroism from a basal section of synthetic
amethystcrystal, which then became very close to
uniaxial. The anisotropy of the optical absorption
spectrumof amethystwas observedearlier by Cohen
(1956) using plane polarizedlight.
The view of Barry and co-workers that the
biaxiality in amethystis due to the anisotropicdistribution of Fe3* ions in positions substitutional for
silicon in the .-eu?rtz structure is no completely
implausible.flowever, they attributed the homogeneity of the electronspin resonanceand the removal of
biaxiality after heating at about 650"C to a redistribution of FeS*ions. It is unlikely that the substitutional Fe3* ions can diffuse in the ,teuafiz
structure at 650'C and be evenly redistributed in
positionssubstitutionalfon silicon withomtdisruption
of the structurebecauseof the covalentnature of the
cation-oxygenbonds in quartz. To account for the
homogeneity of the electron spin resonance and
the removal of biaxiality and pleochroismobserved
a.fter heating, we have to assumethat either the
Fe3* ions resppnsible are not substitutional for
silicon, or the anisotropy observedis also related to
an interstitial impurity in conjunction with the substitutional Fe3* ions (Hassan, l97O). The biaxiat
nature of ar4etfrystis without doubt associatedwith
the color-prodpcingcenters,and any explanationfor
the anomalous biaxiality has to account for this
association. The role of iron in producing the
7W
F. HASSAN, AND A. T. COHEN
7to
SylvaniaType Dsr halogenlamp as the light source
was mounted in place of the standard (visible)
source.All absorptionspectrawere measuredin an
atmosphereof pure dry nitrogen gas to eliminate
water
from the optical path and thus water bands
Experimental
from the near infrared as well as to minimize as
Materials
much as possible the absorption in the ultraviolet
region causedby molecular oxygen. A curve anaexperiments
anisotropy
for
the
The specimenused
natural
lyzer at Mellon Institute, CarnegieMellon Univerzone
a
of
was cut frorn a deeply colored
R-1454
Pittsburgh, Pennsylvania,was used to resolve
No.
sity,
Brazilian amethystcrystal, Specimen
History,
the
bands.
Natural
from the National Museum of
Washington,D. C. This specimenwas in the form of
MeasurementsUsing Polarized Light
a rectangular block whose two pairs of polished
In the visible from 3.09 eV and near infrared
faces permitted the anisotropic absorptionmeasureup to 0.95 eV, the light was polarized by
the
regions
mentsto be madeperpendicularto both the c and
of
43 zrXeS(Fig. 1). Later, thinner wafers were cut means polaroid J-film. For the near ultraviolet,
from the samespecimenin order to make measure- a Glan prism was used as the polarizer with cutoff
ments in the higher absorbanceregion in the deep at approximately4.13 eV. Both the polaroid film
ultraviolet. The specimenswere polished using sil- and the prism were fitted into specially constructed
icon carbide powdersof various grain sizes,and the holders allowing their free rotation around 180".
final polish was done by alumina lapping powders' The dichroic ratio calculated,is defined as:
The thicknessof the polishedspecimenswere measktto, / kt<ut
ured with a Doall micrometer having a metric
where z4 and B arc directions of maximum and minvernier scale.
amethystinecolor in d-quattzas well as the nature of
the color centers are given in the following paper
(Cohen and Hassan,t974).
imum absorption coefficients respectively.
Optical Absorption M easurements
Light from the monochromator is slightly polaAll absorptionspectrawere measuredat room tem- rized, by internal reflection, and, in addition' the
perature us air using a Cary double beam recording response of the photocells is sensitive to the direcModel 14R, serial number 1564. tion of the electric vertor of the incident light (E).
spectrophotometer,
A Cary Model I47l25O light housingcontaininga When quantitatively comparing results from different
o3-oxis
c-oxis-
Eil.
FIc. 1. Orientation and dimensions of the Brazilian
amethyst specimen used in the absorption experiments'
A and B show the normal incidence of polarized rays on
planes cut perpendicular to the c axis and to the as axis,
respectively.
specimens and different laboratories, it is necessary
to determine the polarization introduced by the
instrument and the sensitivity of the photocell' Since
the present experiments were performed on the same
specimen with the same polarizers and spectrophotometer, measurements of these variables were
found unnecessary. The reproducibility of the optical
system was checked by deterrnining the absorbance
in a given direotion, then rotating both the polarizet
and the sample through 90'. The absorption spectra
wers found to be consistent. This shows that the
response of the photocell is the same in the two
directions of the electric vector of the incident light
within the experimental error. In the present investigation the polarizers were always rotated so that
any error due to the nonuniformity of the coloring
material would be eliminated. The procedure followed in recording the absorption spectra in polarized light was as described by Cohen and Smith
(19s8).
Tl,respectrumobservedwhen the electricvector
BIAXIAL
COLOR CENTERS IN AMETHYST
frAVELENGTH IN AI{GSIROTTS
to.o
in this specimenand consistsof three or more bands
rather than the one band reported previously for
amethyst quartz. The absorption bands are ligandcation electronicchargetransfertransitionsof Or- -+
Fe3. (Cohen and Hassan, l97A), and their clear
resolution here is due to the extremely small iron
content (83 ppm). The complexity of the absorp
tion in this region (6.2-5.6 eV) is attributed to the
existence of Fest in more than one site in the
d-qtl:artzstructure (Cohen and Hassan, 1.974). Figure 3 shows the details of the optical absorption
spectra of the sample in the visible and near infraTesre
Frc. 2. Absorption spectra of Brazilian amethyst for normal incidence on planes cut (A) parallel to a positive
rhombohedral face (r), (B) perpendicular to the aa axis,
(C) perpendicular to the c axis. Measurements were taken
using normal light.
(E)
of the incident
light is parallel
71r
QUARTZ
1. Position of Absorption Band Maxima
Natural Amethyst Crystal, Normal Light*
Band
DeslgnatLon
1-o
2-B
to the optic axis
c, is called the z-polarizedspectrum.The o-spectrum
is observed when E is perpendicular to the optic
axis. Other directionsof E are marked on the appropriate spectra in the figures. The slightly polarized
Iight of the spectrophotometersystemnot intentionally polarized by use of polaroid film or polarizing
prism is referred to as normal light.
Orlentation
of
Speclmen
3-y
4-6
5-E
Chonnels in tQuartz Crystals
A model of the .u-qtJattzcrystal structure reveals
the often describedchannelsparallel to the c axis
and those parallel to the a axis. In addition. other
channelsthrough the structure run pelpendicularto
the positive rhombohedral (r) faces and, in the c-a
axial planes, at 45" on both sides of the c axis
(Hassan,I97O).
6-n
Anisotropic Optical Absorpfion Bands of a Natural
Brazilian Amethyst-QuartzCrystal
g- **
Figure 1 shows the dimensions,orientation, and
interrelationshipsof the plane-orienteddirections in
the crystal studied.The resultant absorptionspectra
at room temperatureexhibit a number of overlapping
bands-labelled ,a, p, 8, g . . . (Fig. 2)-with absorption maxima located as summarizedin Table 1.
The band at 6.2 eY has not beenpreviouslyreported
in the literature.That at 5.6 eV is clearly resolved
9-K compLex
*
**
*'t*
Energy ln
eV
lc
6.L4
Il"
ll'
l"
ll"
6.26
6,20
5.63
5.63
tl
5.60
f.
5.15
llr
llc
in a
4.96
lc
3.54
ll"
ll'
3.54
3.54
fc
3.023
tl
llc
3. 023
Il'
f"
ll"
ll"
l-e
ll.
Is
ll.
ll.
3. 023
2.285
2.27
2.40
1.65
1.60
1.30
***
***
No. R7454
Stnithsonian rnstitution,
A ueak botd possiblg^&te to a spin forbLddc-n
transition of the Fez+ ion.
Broad cotnplee absorption in tlp. 1.50-L.00
eV region.
F. HASSAN, AND A. T. COHEN
712
WAVELENGTHIN AI{GSTROMS
6@0
5000
thermal bleaching, on a basal section of natural
amethyst.
Anisotropy of Absorption in the BaswlPIane
Figure 4 shows the polarized absorption spectra
for the main band (d band) in the visible region
centered at 2.285 eV with the incident light along
the c axis. Curve 1 in Figure 4 refers to a measurement with E vibrating parallel to the direction of
u
o
z
@
N
@
E
o
@
WAVELENGTH
--a6oo.-- IN ANGSTROXS
799q
6000
6
o
d
lr.l
Frc.3
"t..J;il-,ilT;
XT'J'XlIi,stspecimen
r.50
d
-.{
illustrated in Figure 2 showing the details of the visible and
near infrared regions using normal light,
lrJ
lt
lrJ
-90
red regionsbefore cutting from it the thinner wafers
used in measurementsillustrated in Figure 2. Thin
wafers were necessarybecause of the high light
absorbancein the ultraviolet region.
It is clear from studyingFigures 2 and3 that the
shapes,the absorption coemcients,and the energy lrJ
values at the peaksof the absorptioncurves depend oz
on the orientation of the sectionunder investigation' P r.oo
o
an
,
The Amethyst Color qnd Bioxialily
\
/ i/\'.,
i
\
l
l
i
i
\
.
\
the
(0.167
from
thick)
cut
'
.
\
cm
A basal section
ft\
l/ !/ !/ -, <i t f i iN r
\ \'..\
specimenand exhibitinga biaxial angle -10' (9\tt /i/"r
11') washeatedin air. The amethystcolor remained
essentiallyunchangedto 280oC, a very slight fading
was first noticeableat 300oC, and at 390'C the
The biaxial
amethystcolor completelydisappeared2.
n3o
to
exposure
after bleaching.Upon
anglebecame
started
X-rays, a faint but clear amethystinecolor
\\. \
developingin the first few minutesof irradiation and
O r i c n l o t i o nl c
-oz
-Er
\ l '.-\
\
becamedeeperduring further exposure(1.5 hours).
--l,lormol lighl
inThe d band, the main band in the visible region,
or ploncpoloritcdlishl
Il-n5*,8f".t
creasedin intensity as depth of the amethystcolor
increased (Cohen and Hassan, 1974). The biaxial
3.o
," ,..fi8o* uo.r,
ENERGY
PHoroN
angle increasedto -10o after recoloration. Optical
Frc. 4. The polarized absorption spectra of Brazilian
bleaching, using light from a Hg-Xe lamp whose
in the visible region for light normally incident
amethyst
peak intensity is at 2537 A, had an effect similar to
plane-polarized with
d)
'Both d and bands also disappeared. A detailed study
3
of the fading characteristics upon heating and growth curves
of the amethyst bands upon re-irradiation with X-rays has
been done by Hassan (1970).
on a basal section if (A) the light is
E parallel to a,, (B) normal, or (C) plane-polarized with
E perpendicular to a. The insert spectrum illustrates the
change in optical density of the main band with changing
orientation of E.
BIAXIAL
COLOR CENTERS IN AMETHYST
713
QUARTZ
Tl^il,n 2. hoperties of the Optical Bands in a Natural Amethyst Crystal
NorMl
Incldence
on Plate
Cut
e
I.
Banal Mxinun norml
(eV)
ltght
3. 5 4
Orientation
E at MxiDu
absorptlon
of
*60o fron a,
*30o fron a,
n
lo-
3,48
parallel
Ic
3.02
*60'
la^
s
K
conplex
I.
to c
Orlentatlon
of
E at EiniEun
absorptlon
-30o fron aa
-60" fron aa
perpendlcular
Angle between
nax. and nin.
(o)
absorptton
90'
90"
to c
Dlchorolc
ratio
kn"*/knf,
1.11
0.10
90'
1. 1 3
1. 4 0
0,o2
0.04
fron aa
-30o from a,
90'
r.25
+30o fron d3
-60o fron a,
90'
L.24
90'
L.92
para11el to c
perpendlcular
to c
-30o fron a,
Energy ShlfL
of band
(eV)
uxinun
2.28
+600 fron 43
lo^
2.27
parallel
lc
1,30
*70o frou a,
+20o fron 43
L.20
0.018
la^
1.30
*45" froo c
parallel
1.10
0.02
to c
perpendicular
maximum absorptioncoemcient,crtrue2 is in normal
light, and curve 3 is with E vibrating parallel to the
direction of minimum absorption coefficient. The
integratedabsorption in the d band is greater when
E vibrates in the direction of the a2 axis than when
it vibratesperpendicularto it in this specimenorientation. The magnitudeof the anisotropyis given by
the value of the dichroic ratio in Table 2. The insert
spectrum shows how the absorbanceof the d band
changeswith changeof the orientation of the electric
vector of the incident radiation (E). The insertspectrum was obtained by rotating E at the wavelength
of the maxirnumof the d band in normal light (2.285
eV) in the a plane where the dBaxis is the 0o orientation. Data were taken every 5'. A maximum
absorpion for E at *60' and a minimum for E at
-30" from a3 wztSobserved.
Curve 1 in Figure 5a is the spectrum with E vibrating perpendicularto the 41 axis, curve 2 is the
spectrum in normal light, and curve 3 is the spectrum with E vibrating parallel to the cr axis. The
anisotropy of the ( and q bands in the near ultraviolet is illustrated in Figure 5 (b and c, respectively). The 7 band has absorption maxima for E
at +60o and at *30o to referencedirection a3 as
well as two minima at -30o and -60". The polarizationorientationat +600 and -30o is the sameas
that observed in the adjacent d band, and could
possibly be due to overlappingof thesebands. The
other polarizationorientation observedat *30" and
-60o from the referencedirection might also correspond to that of the very close ( band.
The broad absorptionin the vicinity of 1.3 eV in
Figure 6 was studied in the same way. This band
is attributedto the 5T2o(uD) uEo(5D) spin-allowed
90.
to c
to c
90'
0.04
1,06
0.00
transition of Fe2*in an interstitialoctahedralenvironment. The absorption in this region is quite complex. It has been previously observed that this
absorption is split into two componentsin a more
ionic environment(Cotton and Meyers,1960). This
splitting is consideredto be due to a dynamic JahnTeller effect in the excited 1Eo state. In the q\artz
structure it was found to be composednot only of
bands having maxima at 1.34 eV and 1.265 eY,
but also severalminor bands (Fig. 6) using different
orientationsof the electric vector of polarizedlight.
Figure 7 shows the polarized absorption spectra
of Brazilian amethyst in the near infrared region.
Curve 1is the spectrumwith E of the incident radiation vibrating parallel to the direction of the experimental maximum, curue 2 is the spectrum with E
vibrating in the direction of the exhibited minimum,
and curve 3 is in nonnal light. An approximate
gaussianresolution of the spectra suggeststhat the
absorption in the 1.3 eV region consists of three
different bands with peaks at about 1.45, 1.28, and
l.l4 eY in normallight (Figure 8a). The bandshave
different anisotropiesand their energyvalues at the
peaks shift for different orientations of the electrio
vector of the polarizedlight. Figures 8b and 8c show
the resolvedpolarizedspectrameasuredin the direction of maximum and minimum absorption coefficients respectively.The band around 1.67 eV in
normal light (Fig. 8a) alsocontributesto the absorp
tion in this spectral region. The existenceof more
than one band makes this optical region very complex, and for this reason the band is labelled
xcomprex.
The anisotropy it exhibits, as illustrated in
the insert of Figure 7, is actually a summation of
the anisotropiesof severalbands.
F. HASSAN. AND A. I. COHEN
714
WAVELENGT}IIN ANGSTROMS
3500
40@
4500
(b)
(o)
(rrg \
I
u,=
0.35
j
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1 bond
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z
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VOLTS
ENERGYIN ELECTRON
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Frc. 5. (a) The polarized absorption sp€ctra of Brazilian amethyst in the near ultraviolet;
(b) and (c) illustrate the change in the optical density of the 3 and the 4 bands by changing
orientation of E.
It is clear from the observationsdescribed.that anisotropic. An absorption maximum is observed
the absorption of light normally incident on a basal when E vibrates along a direction 45" on either side
section of an amethyst-coloredo,-qvartzcrystal is of the c axis (Fig. 9). Absorption minima are obhighly anisotropic.Similar orientationsin uncolored served when E vibrates, either paral[el or perpendicular to the c axis. The direction of maximum
eqtJartz are isotropic.
absorption conforms with the direction of channels
Anisotropy ol Absorption Parallel to the c Axis
in the c-quartz structure (Hassan, l97O).
The anisotropy of light absorption in the bands
Discussionof Results
describedabove was also studied in a specimencut
The experiments described above indicate that
perpendicularto the a axis. Data for the visible and
the near infrared regions were obtained, and the the absorption bands in amethyst a-qaartz are
anisotropic. The anisotropic absorption of planeresults aro shown in Table 2.
The anisotropy of the {, .,7,and d bands in the polarized light by the absorbing centers indicates
samespecimenshowsa maximum absorbancewhere that the wave function of at least one of the states
E vibrates parallel to the c axis, and a minimum involved in each center has directional properties.
when E vibrates perpendicular to the c axis. The It was noticed that thesedirectional propertieshave
r(omprex
band although exhibiting a weak absorption a marked relationship to the c-.quartzstructure in
in the orientationparallel to the c axis is still strongly regard to both the channels and specific crystal-
BIAXIAL COLOR CENTERSIN AMETHYST QUARTZ
bands was found to be dependenton the direction
of the electric vector of the incident radiation, on
the orientation of the section under investigation,
and on the amount of absorptionpresentin adjacent
bands.The energyshift of the absorptionbandswith
polarizedlight was calculatedand is givenin Table 2.
The dichroic ratios for several of the bands
observedcalculated from the expressionpreviously
defined are given in Table 2.
U VELEXGTI|ril INGSTR(IS
9@o
to@
715
Absorption Indicatrix
The absorptioncoefficient is defined by Blakney
and Dexter (1954) by the following expression:
:
2rqh2cW;1
-r
,oro, 1^' "^
where h,ois the energy of the quantum of light incident on a crystal of volume V, dielectric constant
ka, and index of refraction 27,ard where 116l, is the
squareof the amplitude of the vector potential asso-
rr,
|J
2
o
G
9 0.60
q
o
EOO
WAVELENOTH IN ANGSTROTIi
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PIOTOII EiERCY N Er.€CTROIULTS
FIc. 6. The polarized absorption spectra of Brazilian
amethyst in the near infrared. Dotted curve refers to the
absorption in normal light, the remainder of the curves
refer to different orientations of E as indicated. The
referencedirection is the a" axis.
lographic directions. The bands differ in anisotropy
and in most cases have directions of absorption
maxima and minima at right anglesto each other
(Table2).
The energyat the absorptionpeak of the different
talr
\
;\
c:
Orl.Olollon lc
El + 70' trcn o!
---E1+
20' trut| o!
--lloflrrl lhhl
E'Ehct?h
Y.cla of phr
90lol
r.50
t.oo
PT|oTot{ENERGY
IN ELECTROI
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Ftc. 7. The anisotropic absorption of Brazilian amethyst
in the near infrared region. The insert q)ectrum illustrates
the change in the optical density of the r".*pr"" bands with
changing orientation of E.
F. HASSAN, AND A, I. COHEN
716
(o)
o.50
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o
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o.40
r.50
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PTIOTON ENERGY IN ELECTROI{ VOLTS
Frc. 8. An approximate gaussian resolution of the summation absorption curves of Brazilian amethyst in tle near
infrared region illustrated in Figure 7. (a) Spectrum in
normal light; (b) spectrum in the direction of minimum
absorption coefficient; (c) spectrum in the direction of
maximum absorption coefficient.
ciated with
the radiation
field of the quantum.
Zay
using the same source of radiation. Therefore, at a
given wavelength, the absorption coefficient is directly related to the index of refraction.
In order to illustrate how the absorption coefficients of the optical bands in amethyst vary according to the vibration direction of the electric vector
of the incident light and to learn about the orientation of the absorbing centers, an orderly frame was
constructed and will be referred to as the absorption
indicatrix. It is used here as a method for interpreting the observed optical absorption. In isotropic
media, the absorption coefficient will not change with
the vibration direction of the electric vector of the
incident light at a given wavelength. The interaction
of the highly ordered atoms with the light waves
would result in cancellation of any directional effect.
Consequently, all the vectors relating the absorption
coefficient to vibration directions are of equal length,
and the expected isotropic absorption indicatrix is
a sphere. Basal sections of ,cu-Quartz,being perpendicular to the optic axis, are isotropic. However,
within a basal section of amethyst-quartz the tips of
the vectors (absorption coefficients) for the d band
do not fall along a circle, as might be expected, but
along an ellipse. Thus light travelling along the c
axis in an amethyst section, is resolved into two
polarized components vibrating at right angles to
each other. The difference in magnitude, (one is a
minimum, ko1, and the other is a maximum fto2), indicates a directional efiect that results from either
a change in the atornic arrangement (crystal symmetry) or the presence of some foreign configuration
different from the regular one. Knowing that no
major deviation from the ,rquartz structure has been
detected in amethyst, we can conclude that the
anisotropy observed in the optical absorption in the
0 band is due to the presence of a foreign configuration having a directional electron density. The (
band in amethyst, having a different energy value
than the d band, also shows similar anisotropic
behavior but has a different polarization orientation,
its maximum and minimum differing in position from
those for the d band.
For light vibrating parallel to the c axis, the
absorption coefficient ker in both the d and ( band
peaks was found to be greater than in any other
orientation observed.
is the probability per unit time that the quantum is
absorbed by the crystal, and the rate at which
the 0 and ( Bands
energy is absorbed per unit volume is given by Proposed Symmety for
(h^/V) \ryfi AI the describedexperimentswere
The crystallographic axis, c, can no longer be
performed on the samepolished wafer of amethyst, called the optic axis in regard to the color center,
BIAXTALcoLoR cEwrpns rN AMETHvSTeuARTz
717
IN ANGSTROMS
WAVELENGTH
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8
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r)
ld
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-.J.
o
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(of I .265 eV)
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-Normol
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VOI-TS
PHOTONENERGYIN ELECTRON
o
90
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t80
Ftc. 9. (a) The anisotropic absorption in the near infrared region of Brazilian amethyst for
normal incidence of light on a section cut perpendicular to the a" axis. (b) The pleochroism
Kcompr€x
band at different orientations of E, at the two different energies indicated.
sincelight vibrating along its direction (in the 2.285
and 3.54 eV regions) is resolved into more than
one component. The values of the absolption coefficientsin the principal directionsko1, ka2, and ko
can be describedfor three mutually perpendicular
vibration directions where ko, + ko, + kzr. These
results suggest symmetry no higher than orthorhombic for the color center or centersresponsible
for the absorpion in amethyst in the visible and
near ultraviolet regions since the centers are of a
biaxial nature, with the crystallographicc axis as
their acute bisectrix.
This work thus explainswhy the symmetryobtained
from optical measurements
diftersfrom the symmetry
obtainedby X-ray measurements.
The anisotropyof
these color centers is attributed to the existenceof
a directional electron density, which conforms with
the crystallographicdirectionsor the channelsin the
rquartz structure. In the basal sectionof the specimen investigated,the electrondensity (in the d band
region) is oriented in the direction of the az axis.
This observationsupports the idea that a selective
substitution for Sia*by Fe3. is the actual precursor
of the color center (Bany and co-workers, 1964,
1965). However, in a section cut perpendicularto
Conclusions
the as axis, the electron densitiesof severalcenters
The anomalous biaxiality arid pleochroism ob- absorbingin the (, '0, and,ftcomprex
band regions are
served in amethyst crystals are attributed to the oriented in the directions of the existing channels
existenceof color centerswith symmetry no higher in the structure, which indicatesthat interstitial imthan orthorhombic. Bleachingthe amethystsections purity is involved in the processof the color center
either thermally or optically resultsin the disappear- production.A detailedstudy of the nature of the iron
ance of both the d and the ( bands responsiblefor centers in a-qvartz is discussedin the following
the iimethyst color. The basal section then appears paper (Cohen and Hassan,1974).
to be very close to uniaxial. The amethystinecolor,
biaxiality, and pleochroismreappearupon exposure
Acknowledgments
to ionizing radiation. It is therefore obvious that it
The authors would like to thank the National Museum of
is the optical color centers that depart from the
Natural History for the loan of the amethyst quartz crystal
normal quartz symmetry, not the quartz structure used in this
work. One of us (F. H.) is grateful for a
as a whole (as suggestedby Pancharatnam,1954) Sigma Xi research award in support of this work while she
that is different in amethystthan in other ,a-quartz. was a graduate student at the University of Pittsburgh.
7r8
F, HASSAN, AND A. I, COHEN
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Manuscript receiued, lanuary 10, 1972; accepted
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