Frost, Ray L. (2006) A Raman spectroscopic study of selected minerals of the
rosasite group. Journal of Raman Spectroscopy 37(9):pp. 910-921.
Accessed from http://eprints.qut.edu.au
Copyright 2006 John Wiley & Sons
A Raman spectroscopic study of selected minerals of the rosasite group
Ray L. Frost• •
Inorganic Materials Research Program, School of Physical and Chemical
Sciences, Queensland University of Technology, GPO Box 2434, Brisbane
Queensland 4001, Australia.
Minerals in the rosasite mineral group namely rosasite, glaucosphaerite,
kolwezite, mcguinnessite, nullaginite and pokrovskite have been studied by Raman
spectroscopy at 298 and 77 K and complimented with infrared spectroscopy. The
spectral patterns for the minerals rosasite, glaucosphaerite, kolwezite and
mcguinnessite are similar to that of malachite implying the structure is the same as
malachite i.e. monoclinic. A comparison is made with the spectra of malachite. The
symmetry of the carbonate anion in the rosasite mineral group is C2v or Cs and is
composition dependent. Two (CO3)2- symmetric stretching modes are observed for the
rosasite minerals at 1060 and 1090 cm-1. Two hydroxyl stretching modes are
observed for the rosasite mineral group. The position of these bands is determined to
be a function of the hydrogen bond lengths. Hydrogen bond distances for rosasite are
calculated as 2.867, 2.799 and 2.780 Å whereas for pokrovskite the distances are
3.280 and 2.999 Å. The effect of lowering the temperature from ambient to 77
K results in a decrease of the hydrogen bond distances by 5%. Multiple Raman bands
are observed in the 800 to 850 cm-1 and the 720 to 750 cm-1 regions and are attributed
to ν2 and ν4 bending modes confirming the reduction of the carbonate anion in the
Key words: glaucosphaerite, kolwezite, mcguinnessite, nullaginite, rosasite,
hydroxycarbonate, Raman spectroscopy
The carbonates are a group of over 60 naturally occurring minerals containing
the essential structural building block (CO3)2-. Most of these minerals are relatively
rare and often in association with other building blocks such as hydroxyls, halogens,
sulphate, silicate, phosphate, etc. The common simple rock-forming carbonates can be
divided into three main groups: 1) the calcite group, 2) the dolomite group and 3) the
aragonite group. Peter Williams reports that whilst metal substitution in azurite is
extremely uncommon, such is not the case for malachite 1. In minerals related to
malachite, ions identified together with Cu(II) are: Zn(II), Co(II), Ni(II) and Mg(II).
The most common congener of malachite is rosasite. No single crystal study of
rosasite has been forth coming. Powder diffraction studies of rosasite suggest the
mineral is triclinic 2
The rosasite mineral group are monoclinic or triclinic hydroxy carbonates with
the general formula A2(CO3)(OH)2 or AB(CO3)(OH)2 where A and B is cobalt,
copper, magnesium, nickel and zinc 3. The chemical composition of these minerals
• Author to whom correspondence should be addressed (email@example.com)
means that the minerals are highly colourful, often green to blue. Minerals in the
rosasite group are related to the mineral malachite 4,5. Minerals in this group include
rosasite [(Cu,Zn)2(CO3)(OH)2] 6-8, glaucosphaerite [(Cu,Ni)2(CO3)(OH)2] 9-11,
kolwezite [(Cu,Co)2(CO3)(OH)2] 12, mcguinnessite [(Mg,Cu,)2(CO3)(OH)2] 13-16, and
nullaginite [(Ni)2(CO3)(OH)2] 17-19. Apart from rosasite the minerals are rare
secondary minerals. Besides the chemical composition, the structural relationships
between these minerals are demonstrated by the similarity of their powder diffraction
patterns 20. A significant feature of these minerals is their microcrystalline fibrous
habit. This characteristic precludes in most cases single crystal studies. The space
group symmetry and cell parameters are mainly derived from powder pattern
indexing. Apart from that of malachite, no other structural determinations are
available for the rosasite minerals. Rosasite as with the other minerals of this group
form spheroidal aggregates in extremely thin fibrous crystals. Rosasite forms in the
oxidation zones of zinc-copper deposits. It is found typically as crusts and botryoidal
masses or nodules. Crystals are fibrous and found in tufted aggregates. Rosasite may
be associated with aurichalcite, smithsonite and hemimorphite.
Raman and infrared spectroscopy have been used to investigate carbonates
including azurite and malachite 21,22. A detailed single crystal Raman study has been
undertaken 5,21. Goldsmith and Ross reported the infrared spectra of azurite and
malachite 21. However the vibrational spectroscopy of minerals of the rosasite group
has not been undertaken. No infrared spectra of the minerals of the rosasite group
have been forthcoming 22-25. An infrared stretching vibration of the hydroxyl unit of
azurite was observed at 3425 cm-1, whereas two bands were reported for malachite at
3400 and 3320 cm-1. The observation of two bands for malachite suggests coupling of
the hydroxyl stretching vibrations 5. This coupling was not observed for azurite 5.
Azurite and malachite form the basis of pigments in samples of an archaeological or
medieval nature 26-29. It is not known whether minerals of the rosasite group have
been used in pigments for paintings and other art works of archeological significance.
However the colour of the minerals suggests that this is a real possibility especially
for rosasite. However a complete Raman spectroscopic analysis is not required as
individual Raman bands are simply used to identify the mineral pigments in the
samples. Malachite has a characteristic intense band at ~430 cm-1 and for azurite an
intense band at ~400 cm-1. The deformation modes of azurite were reported at 1035
and 952 cm-1 and at 1045 and 875 cm-1 for malachite. 22,30 Thus even though the two
carbonate minerals have the same space group, the molecular structure of the minerals
is sufficiently different to show infrared bands at slightly different wavenumbers.
Differences between the spectra of malachite and azurite may be explained by the
molecular structure of azurite being based upon a distorted square planar arrangement
compared with a distorted octahedral arrangement about the copper in malachite.
The symmetric stretching bands of carbonate for azurite and malachite were
observed at 1090 and 1095 cm-1. Goldsmith and Ross report the infrared bending
modes of carbonate at 837 and 817 cm-1 for azurite and at 820 and 803 cm-1 for
malachite 21. Two ν3 modes were observed at 1490 and 1415 cm-1 for azurite and at
1500 and 1400 cm-1 for malachite. The observation of these two bands shows a loss
of degeneracy. Such a conclusion is also supported by the observation of two ν4
modes at 769 and 747 cm-1 for azurite and 710 and 748 cm-1 for malachite. The
vibrational spectroscopy of these two minerals is complicated by this loss of
degeneracy. Schmidt and Lutz report some vibrational spectroscopic data 31. Two
infrared bands at 3415 and 3327 cm-1 were observed for malachite. Although the
Raman spectra of the mineral brochantite [Cu4(OH)6SO4] have been reported, the
Raman spectra of malachite and azurite were not 31.
In this work I report the vibrational spectroscopy of minerals of the rosasite
group and relate the spectroscopy to the mineral structure.
The minerals, their formula and origin used in this study are listed in Table 1.
Selected minerals were obtained from the Mineral Research Company and other
sources including Museum Victoria. The samples were phase analysed by X-ray
diffraction and for chemical composition by EDX measurements.
X-ray diffraction (XRD) patterns were recorded using CuKα radiation (n =
1.5418Ǻ) on a Philips PANalytical X’ Pert PRO diffractometer operating at 40 kV
and 40 mA with 0.125° divergence slit, 0.25° anti-scatter slit, between 3 and 15° (2θ)
at a step size of 0.0167°. For low angle XRD, patterns were recorded between 1 and
5° (2θ) at a step size of 0.0167° with variable divergence slit and 0.5° anti-scatter slit.
Mineral samples of the rosasite were coated with a thin layer of evaporated
carbon and secondary electron images were obtained using an FEI Quanta 200
scanning electron microscope (SEM). For X-ray microanalysis (EDX), three samples
were embedded in Araldite resin and polished with diamond paste on Lamplan 450
polishing cloth using water as a lubricant. The samples were coated with a thin layer
of evaporated carbon for conduction and examined in a JEOL 840A analytical SEM at
25kV accelerating voltage. Preliminary analyses of the rosasite mineral samples were
carried out on the FEI Quanta SEM using an EDAX microanalyser, and microanalysis
of the clusters of fine crystals was carried out using a full standards quantitative
procedure on the JEOL 840 SEM using a Moran Scientific microanalysis system.
Oxygen was not measured directly but was calculated using assumed stoichiometries
to the other elements analysed.
Raman microprobe spectroscopy
The crystals of hydroxycarbonates were placed and orientated on the stage of
an Olympus BHSM microscope, equipped with 10x and 50x objectives and part of a
Renishaw 1000 Raman microscope system, which also includes a monochromator, a
filter system and a Charge Coupled Device (CCD). Raman spectra were excited by a
HeNe laser (633 nm) at a resolution of 2 cm-1 in the range between 100 and 4000
cm-1. Repeated acquisition using the highest magnification was accumulated to
improve the signal to noise ratio. Spectra were calibrated using the 520.5 cm-1 line of
a silicon wafer. In order to ensure that the correct spectra are obtained, the incident
excitation radiation was scrambled. Previous studies by the authors provide more
details of the experimental technique. Spectra at liquid nitrogen temperature were
obtained using a Linkam thermal stage (Scientific Instruments Ltd, Waterfield,
Surrey, England). Details of the technique have been published by the authors 32-35.
Infrared spectra were obtained using a Nicolet Nexus 870 FTIR spectrometer
with a smart endurance single bounce diamond ATR cell. Spectra over the 4000−525
cm-1 range were obtained by the co-addition of 64 scans with a resolution of 4 cm-1
and a mirror velocity of 0.6329 cm/s. Spectral manipulation such as baseline
adjustment, smoothing and normalisation was performed using the GRAMS®
software package (Galactic Industries Corporation, Salem, NH, USA).
Results and discussion
The minerals of the rosasite group have not been studied by single crystal X-
ray diffraction. Invariably because most of the crystals are composed of thin fibres.
Extensive powder X-ray diffraction suggests the minerals are triclinic, although
Anthony et el. states the mineral is monoclinic 36. This statement is apparently based
upon the assumption the structures are similar to that of malachite as shown in Figure
1. The structure shows that two of the three oxygens bond to separate copper atoms
but one carbonate oxygen bonds to two copper atoms (see Figure 1). The OH unit
serves to bridge two copper atoms. The model of the structure of malachite shows that
the carbonate anion is of C2v symmetry if the two cations (eg Cu) bonding to the
carbonate anion are identical. If however the two cations are different (eg Cu and Ni
or Cu and Zn) then the symmetry of the carbonate anion would be Cs. The mineral
glaucosphaerite is equally enigmatic and may be monoclinic. Williams reported the
structure of the rosasite group of minerals to be triclinic 1. It is not neccessarily true
that rosasite has the same structure as malachite 1. In the structure of rosasite some of
the copper atoms in the model of malachite will be replaced by zinc atoms.
Table 1 reports the chemical composition of the cations in the mineral samples
used in this work. If the atom ratio of Cu/Zn is 1:1 then every second position in the
model will be taken up by a Zn atom. The rosasite sample used in this work analysed
to a Cu/Zn ratio of close to 1:1. Rosasite does not necessarily maintain a ratio of 1; 1.
For example the rosasite from Rosas mine, Narcao, Cagliari, Sardegna (Sardinia),
Italy analyses as CuO 53.7 and ZnO 18.3 %. This gives a formula of the Narcao
rosasite as (Cu1.5Zn0.5)2(CO)3(OH)2. The mineral glaucosphaerite from the Carr Boyd
Nickel mine analyses as CuO 41.6 % and NiO 25.2 %. This gives the formula of
glaucosphaerite as (Cu1.1Ni0.7Mg0.06)2(CO)3(OH)2. According to Anthony et al. a
glaucosphaerite sample from Kasompi, Congo gave a formula of
(Cu1.23Zn0.71)2(CO)3(OH)2. 36 It is noted that the Cu/Zn ratio is not 1:1 and the total
cations is not 2.0, but 1.94. A similar formula exists for the mineral kolwezite from
Zaire namely (Cu1.33Co0.67)2(CO)3(OH)2. The fact that the two cation ratio is not 1:1
may have implications for the structure of the mineral and therefore for the vibrational
spectroscopy of the mineral. The mcguinnessite from California gives a cation ratio of
Cu/Mg as 1:1. Other measurements record a ratio of Mg/Cu as 1.5/0.5. The formula
for nullaginite based upon the chemical analysis is (Ni1.93Mg0.05Cr0.01)2(CO)3(OH)2.
This mineral is totally composed of Ni and may be regarded as a single cation
hydroxy carbonate. The mineral pokrovskite could be expected to clear or