Article 1 Halvorsen.indd

132 Pages·2011·8.25 MB·English

Volume 30 No. 1/2
January/April 2006
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The Gemmological Association and Gem Testing Laboratory of Great Britain
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The Journal of Gemmology
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Assistant Editors: M J O’Donoghue, P G Read
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Vol 30, No. 1/2, January/April 2006 ISSN: 1355-4565
11
The Usambara eff ect and its
interaction with other colour
change phenomena
Asbjørn Halvorsen
NORPLAN, Underground Construction Technology Division, Ski, Norway
Email: [email protected]
Abstract: The Usambara effect, colour change with change in path length of light
through a material, is found to interact with the alexandrite effect, colour change with
change in spectral distribution of light. This article provides insight into the interaction
between the Usambara effect and other colour change phenomena. In colour change
studies of the past more focus has been placed on the alexandrite effect, but old studies
also show awareness of the Usambara effect. This contribution provides a review of
previous work and updating of earlier interpretation of this effect in the light of new
observations. Epidote and kornerupine are introduced as new colour change minerals,
and the Usambara effect is discussed in synthetic alexandrite and chlorophyll.
Keywords: absorption-modified dispersion, alexandrite effect, concentration effect,
fluorescence, thermochromy, Usambara effect
Introduction
“Let us rather recognise that even relatively concluded that this depended primarily on
simple observations can still provide us with the path length of light through the stone,
unexpected and awe-inspiring phenomena.” modified by type of illumination and varying
This statement by Dr Kurt Nassau in the with pleochroic directions. This effect
preface to the 2001 edition of Physics and seemed to be hardly known to gemmologists.
Chemistry of Color referred to the Usambara Nassau responded to the article by Halvorsen
effect and some other new discoveries. The and Jensen (1997a) with a Letter to the Editor
Usambara effect is named after the verdant (Nassau, 1997). He advised that this effect
Usambara Mountains, south of the Umba was well known in the field of organic
area in Tanzania, and was introduced by dyes and expressed his surprise that it had
Halvorsen and Jensen (1997a,b). They had not been observed previously in minerals.
studied a peculiar colour change from green Further, he expressed expectation that
to red in chromiferous tourmaline from now, when attention had been drawn to
Nchongo in the Umba area (Figure 1) and these effects, they would very likely also be
© Gemmological Association and Gem Testing Laboratory of Great Britain ISSN: 1355-4565
2
Colour change phenomena
in minerals – previous
investigations
Colour change effects are reversible
effects where radical changes in colour
(hue) of a mineral are observed as result
of environmental changes. A common
attribute for most colour change minerals is
transmission spectra with two pronounced
transmission peaks, i.e. dichromatic spectra.
In this study, the following causes of colour
change are discussed:
l Change in spectral composition of light
– alexandrite effect
l Change in path length of light through a
material – Usambara effect
l Change in temperature – thermochromy
l Change in the direction of light in relation
to the optic axes of an anisotropic mineral
Figure 1: A green UE tourmaline turns red when – pleochroism
placed on top of another green UE tourmaline. l Change in concentration of light absorbing
impurities – concentration effect
observed in other minerals. In a colorimetric Throughout this paper UE is used as
study of the same tourmalines, Liu et al. abbreviation for Usambara effect and AE for
(1999a) suggested that the Usambara effect is alexandrite effect.
a complex phenomenon, including effects of
Alexandrite effect (AE)
both path length and type of illumination.
The optical behaviour of the Nchongo Gem minerals changing colour between
tourmalines is extreme. After having studied daylight and incandescent light have been
a cut stone from this locality, Nassau (pers. known and appreciated for centuries. Such
comm., 2002) commented: “I cannot think colour change was first observed in a mineral
of any other stone that would show such found in the emerald mines at Tokowaia in
clear-cut contrasting colours.” This colour the Urals (Wörth, 1842; Kokscharow, 1861).
change behaviour is also found in some According to Gübelin and Schmetzer (1982)
other minerals and man-made materials, and other recent authors, alexandrite was
e.g. plastics (Nassau, 2001). Most striking discovered in 1830. Of the older authors, only
is maybe the similarity with colour change Wörth (1842) seems to inform on when this
found in chlorophyll – the main colorant in happened: the mineral was first found in the
nature. Tokowaia mines in 1833. This was claimed to
The aim of this paper is to place the have happened on the birthday of Alexander
Usambara effect in the wider context of Nikolajewitsch, later Tsar Alexander II,
other colour change phenomena and to and the Finnish explorer and mineralogist
update understanding of it by description Nils Nordenskiöld suggested that the new
and discussion of new observations. Much of mineral be named alexandrite. The colours
this is based on a detailed study of Nchongo were described as emerald green in daylight
tourmaline, but first there is a review of and reddish violet in candlelight which were
previous investigations of colour change linked by Nordenskiöld to the red and green
phenomena. Russian military colours.
J. Gemm., 2006, 30, 1/2, 1-21
3
The name alexandrite was also used by that the new tourmaline variety was a perfect
Wörth, who examined the new mineral counterpart to alexandrite; they also claimed
in St Petersburg in 1833 and found it to be that previously only alexandrite was known
chrysoberyl, a beryllium aluminium oxide for such colour change. Gustav Rose had
(Wörth 1842: Kokscharow, 1861). Kokscharow donated samples of Ural chromian tourmaline
gave Gustav Rose the credit for the first to the Museum of Natural History in Berlin
comprehensive crystallographic description in 1829, but did not observe the colour change
of alexandrite (i.e. chrysoberyl). Rose (1842) behaviour in these. The relevant passage of
described in detail the mineral deposits in Cossa and Arzruni (1883), translated from
the Urals. In an emerald green chrysoberyl he German, is:
found dichroism to be influenced by type of “Two minerals that have been unknown till
light: in transmitted light a hyacinth colour now deserve attention: a beautiful emerald
was observed in one direction in the crystal. green chrome mica and a deep green chrome
This colour was seen only in strong sunlight tourmaline. The latter was certainly seen,
or candlelight, not in normal daylight. Wörth collected and described by G. Rose, but
(1842) described observations done in overcast the true nature of this remained however
daylight: green colour in one direction and hidden for this sharp observer. Prof. Websky
red perpendicular to this. He found that by recognised at the first glance that this mineral
changing the proportions of green and red in contained chrome. This was also confirmed
the light, the colour of the mineral changed. by the observation of the beautiful dichroism,
When illuminated by candle light or by a low till now only known for alexandrite, and the
sun, the red rays dominated the green rays pronounced partial transparency for certain
and the observed colour was red. Both Rose parts of the spectrum. This property was
and Wörth concluded that the chrysoberyl especially clear by use of lamp light; these red
was coloured by chromium oxide. Chemical rays were transmitted nearly unimpaired in
analyses by Awdejew (1842) showed a the tourmaline, which appeared intense ruby-
chromium oxide content of 0.36%. red.”
Another mineral known for colour The British Museum also received a
change (in addition to strong pleochroism) sample of the Ural chromian tourmaline from
is tourmaline. Bank and Henn (1988) Arzruni. Dunn (1977) examined this sample,
reported change from green in daylight a dravite with a Cr O content of 5.96%, but
2 3
to brownish red or red in artificial light in did not comment on any colour change
tourmalines from Tanzania, and although behaviour.
their searches for previous examples of According to Gübelin and Schmetzer
tourmalines with the AE had proved fruitless, (1982), White et al. (1967) introduced the
such tourmalines from the Urals had been term alexandrite effect. This denotation had,
described a century earlier by Cossa and however, already been used by Neuhaus
Arzruni (1883). This tourmaline was found (1960), who presented typical absorption
in chrome-iron deposits near Sysert, south spectra for several chromium-containing
of Yekaterinburg, associated with emerald, minerals, all with two absorption maxima,
uvarovite, demantoid and a new mineral, and classified them (Figure 2) as:
chrome mica. The detailed description l Red group, with absorption maximum I at
includes results of chemical, crystallographic <~540 to ~570 nm and maximum II at <~390
and optical analyses. In daylight, prismatic to <~415 nm, mainly oxides.
crystals of tourmaline were yellowish brown l Green group, with maximum I at >~580
when illuminated parallel to the optic axis to >~650 nm and maximum II at >~420 to
and blue green perpendicular to this. In >~460 nm, mainly silicates.
incandescent light the colours were orange l Transition group, including AE minerals,
reddish brown to ruby red and weak green, with maxima in general located between
respectively. Cossa and Arzruni suggested the maxima of the red and green groups.
The Usambara eff ect and its interaction with other colour change phenomena
4
Figure 2: Chromium-containing minerals classified as three spectral groups according to Neuhaus (1960),
with location of absorption maxima I and II for the three groups.
In 1970 Crowningshield reported a rare requirement of at least 20° calculated absolute
garnet from Tanzania, which was found to hue-angle change between different light
be an isomorphous mixture of pyrope and sources in the CIELAB colour space. This
spessartine, and appeared blue-green in concept was further elaborated by Liu et al.
daylight and purple-red in incandescent light. (1999b).
However, garnets with green to red colour
change were not as rare as Crowningshield Usambara effect (UE)
suggested, as similar garnets had been Chrysoberyl and tourmaline
found in the Czech Republic (Fiala, 1965) and
Norway (Carstens, 1973). Early awareness of the UE is found in
In a study of colour change in man-made the descriptions by Haidinger (1849) and
crystalline chromium compounds, White Kokscharow (1861) of Ural chrysoberyl.
et al. (1967) found the AE in ruby with 20 wt% Haidinger observed the trichroic colours using
CrO, appearing pink in incandescent light a dichroscope loupe. In daylight, variations of
2 3
and green in daylight. Schmetzer et al. (1980) green were observed, and under lamplight he
described the AE in chrysoberyl, garnet, described colour 1 (lightest tone) as orange-
corundum and fluorite. Bank and Henn (1988) yellow, colour 2 (darkest tone) as emerald
mentioned that such colour change was also green and colour 3 (medium tone) as reddish
known in alexandrite, garnet, corundum, violet. A remarkable observation was that the
spinel, zircon, fluorite, kyanite and diaspore. latter colour was dichromatic, i.e. having two
Bernstein (1982) described the AE in monazite colour maxima. In thin sections colour 3
from North Carolina, showing yellow-orange was green with an addition of violet, and
in daylight, reddish-orange in incandescent in thick sections reddish violet. Haidinger
light and pale green in fluorescent light. recorded the strange phenomenon of colour 2
He also listed other AE minerals, including also being green in lamplight, i.e. not
coquimbite in addition to those listed by Bank influenced by change in illumination, but
and Henn (1988). being dominated by the red with a lighter
Liu et al. (1994, 191) described the AE tone (see Discussions with Dr Kurt Nassau
as “change in colour appearance with below). Also Cossa and Arzruni (1883) seemed
differences in lighting” and stated that they to observe the UE. Their main concern in
had observed approximately 40 gem minerals regard to colour change in the Ural tourmaline
displaying this behaviour. They introduced was change in pleochroic colours with type
four categories of colour change that all met a of illumination. They also described how the
J. Gemm., 2006, 30, 1/2, 1-21
5
pleochroic colours varied from thin to thick sections red. The depth of the red seen
sections of the mineral, with reference to code through a Chelsea filter, as well as the strength
numbers in Radde’s International Colour Scale. of dichroism, varied in proportion with the
Regrettably it has not been possible to trace depth of the green. In a dark green stone, the
illustrations or explanations for these colour thinner sections showed strong dichroism in
codes. light and dark green colours, while thicker
The early researchers apparently had a good sections showed light and dark red dichroic
understanding of the factors affecting colour colours. Under a SW UV lamp the tourmaline
change in minerals, and of the interaction showed mustard yellow fluorescence and
between these. Wörth (1842) concluded that under high intensity light a dim red glow.
chromium oxide was the colouring agent in Webster did not comment on the influence
alexandrite. He had found that a solution of of type of illumination on the colour change
chromium in hydrochloric acid or in sulphuric in these tourmalines. In 1997 Halvorsen
acid transmitted both green and red rays and and Jensen were not familiar with Webster’s
had demonstrated for other researchers how observations, but their observations (op. cit.,
the colour in crystallised bodies (presumably 1997a) were very similar. Based on spectral
laboratory-grown crystals) coloured by analyses, and in agreement with chemical
chromium, was affected by pleochroism. analyses by Basset (1955), Webster concluded
According to Wörth, Nordenskiöld later found that the vanadium content exceeded that of
the same properties in the Ural chrysoberyl. chromium, and that the vanadium content in
Wörth also reported in detail how the dark stones was higher than in light stones.
pleochroic colours are influenced by type of Basset did not describe colour change in the
illumination. tourmaline. This is understandable in view of
In a paper on colouring in minerals, the apparently poor quality of sample material,
Kennard and Howell (1941, 407) used the as indicated in his description. Bank and
term ‘polychromatism’ with reference to “the Henn (1988) referred to Webster (1961), but did
fact that the hue and saturation of the colour, not mention any colour change as a result of
in isotropic as well as anisotropic materials, changes in path length. Also Crowningshield
are dependent on both the concentration of (1967) found that some very small Tanzanian
the absorbing substance and the depth or chromian tourmaline samples appeared bright
thickness of the medium traversed.” As an red under the Chelsea colour filter. On the
example of a strongly polychromatic mineral, basis of an absorption spectrum he inferred
they mentioned ferric oxide (hematite), with the presence of chromium. Crowningshield
hues ranging from yellow through red to also did not observe any colour change, but
nearly black. They did not mention colour his stones were very small. In a detailed
change caused by change in illumination. description of many gem minerals from the
Polychromatism was the term used by Webster Umba valley, Zwaan (1974) described a sample
(1994) to describe green to red colour change of emerald-green tourmaline from the Umba
in chrome alum solutions, caused by either mine showing red in transmitted incandescent
increase in concentration or sample thickness. light but did not further explore the
Haidinger (1849) and Kokscharow (1861) also phenomenon. How can this pronounced colour
used this denotation for a UE type of colour change behaviour have avoided further focus
change in alexandrite. in recent publications on gemstones from this
Webster (1961) described tourmalines from region? UE tourmalines also occur in the John
the Gerevi Hills, Tanzania, which appeared Saul ruby mine, Taita-Taveta District, Kenya
red under a Chelsea colour filter. In a thorough (Simonet, 2000) which is located just north of
description of the optical behaviour and the Umba area of Tanzania, about 95 km NNW
physical properties of this mineral, he used the of Nchongo. Emerald-green tourmalines with
term ‘dichromatism’ to describe the property similar colour change behaviour have also
such that in transmitted light, thinner sections been found in Madagascar (M.S. Krzemnicki,
of a dark stone appeared green and thicker pers. comm., 2004).
The Usambara eff ect and its interaction with other colour change phenomena
6
Garnet
of pyropes, Fiala (1965) found that the
Fiala (1965) described change from green colour changed from orange-red to violet
in daylight to red-violet in incandescent light with Cr O content increasing from 1.15% to
2 3
in pyropes from peridotites in the Ceské 6.54%. Pyrope garnets showing an AE had
Stredehori Mountains, close to Trebenice previously been described from peridotites
in the Czech Republic. The green colour found on the island of Otterøy, close to
in daylight was seen especially in small Molde, Norway, changing from violet in
fragments – an observation indicating daylight to wine red in incandescent light
presence of the UE. C. Simonet (pers. comm., (Hysingjord, 1967 and 1971). Then, Carstens
2000) observed the UE in garnets from Taita (1973) found that a more chromium-rich
Hills, Kenya. He found that this type of pyrope from the same location changed
colour change was much stronger in garnets from blue-green in daylight to wine-red in
than in tourmalines from this area, especially incandescent light. This pyrope had a Cr O
2 3
in daylight, and that many stones showed content of 6.22%, while the one with colour
both AE and UE. In the trade, gem dealers change from violet to wine red had 3.72%.
he had met described the UE in many stones Carstens analysed absorption spectra for
as dichroism and talked about dichroism in the green chromium-rich pyrope and a red
garnets. chromium-poor pyrope (1.50% Cr O ), finding
2 3
Manson and Stockton (1984, 200) studied a a pronounced shift towards the red end of
selection of garnets exhibiting colour change the spectrum for the two absorption peaks,
between incandescent light and daylight as well as for the minimum between these,
equivalent illumination and observed that: with increased Cr O content. He concluded
2 3
“While all the stones show some change in that the change from green to red in these
colour between incandescent and daylight or garnets, being essentially solid solutions of
fluorescent illumination, most also display a pyrope, almandine and uvarovite, occurred
different colour when light is passed through at about 6-7% Cr O . Orgel (1957) reported a
2 3
the stone, as compared to internally reflected similar colour change in laboratory-grown
light from the same source.” They referred rubies which remained red with up to 8%
to the latter as colour shift and suggested: Cr O , turning progressively more and more
2 3
“Colour shift does not occur with a change green with increasing Cr O content. This
2 3
in illumination, but rather with the relative effect was also described by Thilo et al. (1955)
amounts of light (from a single source of and by Nassau (2001).
illumination) that a viewer observes either
Thermochromy
(1) passed through the stone or (2) internally
reflected by a gemstone. The former reveals This colour change phenomenon is related
the stone’s body colour, the latter requires the to crystallographic lattice distortions caused
viewer and illuminant to be on the same side by change in temperature. Kenngott (1867)
of the gem, so that the internal reflections described colour change to green by heating
(which represent the reflected colour) may be of red corundum from Sri Lanka, and
observed.” reversion to red by cooling. Thermochromy
The observations by Manson and Stockton in corundum and spinel has been thoroughly
are similar to observations concerning UE discussed by Weigel (1923) on the basis of
tourmalines and are discussed below. detailed spectral analyses and he found that
the colour change in ruby was influenced
Concentration effect by pleochroism. Thilo et al. (1955) concluded
that the colour change with temperature in
Colour change with change in ruby is momentary and strictly reversible.
concentration of light-absorbing impurities They studied the colour behaviour of samples
is covered by the denotation ‘polychromatism’ of synthetic corundum with variation in
(Kennard and Howell, 1941). In his study chromium content and measured the grey
J. Gemm., 2006, 30, 1/2, 1-21
7
point temperatures, at which a grey colour material thickness (or path length) to 2z will
was observed on the transition between red reduce the intensity of transmitted light to
and green. The lower the chromium content, a fourth. A crucial condition in regard to
the higher was the grey point temperature. the UE is that the increase in absorption is
With a Cr O content of 5%, the grey point higher for the higher frequencies than for
2 3
temperature for ruby was found to be of the the lower. With increased path length, the
order of 600°C. They also found that the grey intensity of the red transmission is increased
point changed with type of light. At room relative to the intensity of the green, and
temperature corundum with 17% Cr O was thereby the balance between the green and
2 3
grey in daylight and red in incandescent red transmissions is shifted towards red,
light. These findings were analysed by Orgel a shift that can be observed as relatively
(1957) who described this optical behaviour sudden. The sensitivity of the human eye is
of ruby as anomalous. Based on experimental highest in the green sector, for wavelengths
data on chromium-containing solids, Poole between 500 and 510 nm (Kuehni, 1997), so
(1964) suggested that thermochromy is a if light transmitted through a mineral has
general property of solids containing Cr3+ transmission peaks of the same intensity in
ions. He assumed thermochromy to be the green and red, the mineral will appear
present in chromium-containing minerals, green. With a minor change towards red in
including ruby, emerald, uvarovite, fuchsite the balance between the transmissions in the
and chrome diopside. Carstens (1973) found green and red, a mineral might still appear
that his red pyrope with 1.50% Cr O turned green, but if there is sufficient increase in red
2 3
green when heated to about 200°C, while a transmission, a sudden change to red may be
violet pyrope with 3.72% Cr O turned green observed (Poole, 1964).
2 3
at about 150°C. Neuhaus (1960) described
Discussions with Dr Kurt Nassau
thermochromy in chrome alum solutions.
Extensive discussions between Nassau and
The Usambara eff ect
the author covered various aspects of the UE
and, concerning its definition, Nassau (pers.
Basic idea
comm., 1998) suggested:
Characteristic for UE is colour change as “It is necessary here to distinguish
response to change in the light transmission between a colour change that involves a
path length. A green UE tourmaline may change in hue (or dominant wavelength
turn red in transmitted light when placed or chromatic colour, etc.) and one that
on top of another green UE tourmaline does not do so. The term Usambara effect
(Figure 1). When light is transmitted through is obviously intended to apply only to a
a coloured mineral, certain frequencies are change involving hue. Most materials will
absorbed and the remaining frequencies change colour without a change in hue when
combine to give the mineral its colour. either the concentration of a light-absorbing
Change in path length of transmitted light impurity or the light path length is increased.
will affect the spectral power distribution. If there is a change in the field (crystal or
An increased path length will result in a ligand), then the hue will change with the
general increase in absorption. The Lambert concentration, but not with the path length.
law, I = I *exp(-a*z), defines the degree of Only when at least one transmission band
(z) 0
absorption for a given path length z, where I extends significantly out of the visible region,
0
is the intensity of light entering the material, as at the red end of the spectrum with your
and I is the intensity of the light after tourmaline, will there be a change of hue
(z)
passing through a thickness z of material with either concentration or path length. It
with absorption coefficient a. If the intensity seems to me that only under these conditions
of light is reduced to half on passing through should the former be termed “concentration
material with thickness z, an increase in dichroism” in dyes or the concentration effect
The Usambara eff ect and its interaction with other colour change phenomena
8
of colour-producing impurities in minerals. (Figure 7). Electron microprobe analyses were
Similarly, only the latter should be termed carried out by C.J. Stanley at the Natural
the Usambara effect.” History Museum, London, using a Cameca
In a discussion about causes for observed SX50. Fluorescence was measured by J.
colour changes in UE tourmaline, Nassau Kihle, Institute of Energy Technology, Oslo,
(pers. comm., 2000) commented: Norway, using a customised Olympus BX-
“One interesting question which your 61 microscope at 10x magnification, fibre-
discussion has raised in my mind is why it is optically connected to a modified Edinburgh
so rare to see more than one of the pleochroic Analytical Instrument CD900 excitation/
colours from a gemstone (without turning emission spectrometer, excitation wavelength
it or using a polariser): andalusite with both set to 405 nm.
reddish and greenish colours is exceptional Gemmological instruments utilised by
– some alexandrite also shows this effect, but the author include a Krüss refractometer,
less strongly, some zoisite with yellow flashes OPL dichroscope, dichromatic colour filters:
in the green, and green to almost black in Chelsea filter with transmissions in the
tourmaline (not exactly a colour change). deep red and in the yellow-green (570 nm).
Even then, why do only some pleochroic Hanneman Aqua filter with transmissions in
materials show only some of their colours deep red and in blue-green (490 nm). Fibre
on being turned (without a polariser)? Ruby optic lamps had 20W and 150W tungsten
shows both its colours, but iolite shows only halogen light sources. A Nikon Coolpix 990
two of its three colours. I believe that I now camera was used to record images.
know the answer: the reason lies in the fact
Change in path length of transmitted light
that the ordinary ray colour is seen in all
orientations. If the o rdinary ray colour is Observations
intense and saturated, it will then hide the
I a The colour of a green UE tourmaline,
other colours in those orientations where one
illuminated from behind, shifts towards
would expect to see them.”
red when studied through green or blue
This explanation is consistent with that
UE sheets of plastic. The same happens
given by Wörth (1842) and by Haidinger
when the tourmaline is studied through
(1849) for their observations concerning
solutions of certain types of green and
alexandrite.
blue dyes for colouring Easter eggs.
New observations, investigations and I b A certain orange-yellow sapphire placed
on top of a green UE tourmaline turns
interpretations
deep red in transmitted light.
The observations described below relate I c Normally a green UE tourmaline appears
to UE tourmalines from Nchongo, unless red when studied through a Chelsea
otherwise specified. They are grouped colour filter, and an even stronger red
according to the cause (tentative or proven) through a Hanneman Aqua filter. In a
for the observed phenomena: colour change pale green UE tourmaline, the colour of
due to change in path length of transmitted light, a thicker part of the stone may turn red
thermochromy, dispersion and visible-light- under a Chelsea filter, but not the thinner
induced fluorescence. Discussion and suggested parts. The same effect is seen in tsavorites
explanations follow the observations. from Kalalani, Umba area, which show
Spectral analyses were carried out by more red in the thicker than in the
K.A. Solhaug, Agricultural University of thinner parts when studied through a
Norway, Ås, using an Ocean Optics SD Chelsea filter.
2000 spectrometer (Figures 3, 10, 15 and 17) I d An emerald green kornerupine crystal
and L.O. Björn, Lund University, Sweden, from Nchongo, Umba area, shows a
using an Optronics 754 spectroradiometer peculiar pleochroism.
J. Gemm., 2006, 30, 1/2, 1-21