Spectrochimica Acta Part A 61 (2005) 2273–2280

FT–Raman spectroscopic study of calcium-rich andmagnesium-rich carbonate minerals

Howell G.M. Edwardsa,∗, Susana E. Jorge Villara,b, Jan Jehlickac, Tasnim Munshia

a Department of Chemical and Forensic Sciences, University of Bradford, Bradford BD7 1DP, UKb Area de Geodinamica Interna, Facultad de Humanidades y Educacion, Universidad de Burgos, Calle Villadiego, s/n, 09001 Burgos, Spain

c Charles University, Institute of Geochemistry, Mineralogy and Mineral Resources, Albertov 6, 12843 Prague 2, Czech Republic

Received 10 June 2004; accepted 4 February 2005

Abstract

Calcium and magnesium carbonates are important minerals found in sedimentary environments. Although sandstones are the most commonrock colonized by endolith organisms, the production of calcium and magnesium carbonates is important in survival strategies of organismsand as a source for the removal of oxalate ions. Extremophile organisms in some situations may convert or destroy carbonates of calciumand magnesium, which gives important information about the conditions under which these organisms can survive. The identification ont udy of thesem f differentc ill be usefulf©

K

1

seie

ittom

cco

s andse, inct,an in-

par-forox-y-for

e

ivalbet-stru-

rtian

ions,and

1d

he surface of Mars of ‘White Rock’ formations, in Juventae Chasma or Sabaea Terra, as possibly carbonate rocks makes the stinerals a prerequisite of remote Martian exploration. Here, we show the protocol for the identification by Raman spectroscopy o

alcium and magnesium carbonates and we present a database of relevance in the search for life, extinct or extant, on Mars; this wor the assessment of data obtained from remote, miniaturized Raman spectrometers now proposed for Mars exploration.

2005 Elsevier B.V. All rights reserved.

eywords:Raman spectroscopy; Calcium carbonate; Magnesium carbonate; Mars exploration

. Introduction

The carbonates of calcium and magnesium are importantedimentary minerals that are currently attracting much inter-st because of their possible association with extinct organ-

sms in the geological record for extraterrestrial planetaryxploration[1–3].

The observation of the biological colonisation of rocksn extreme terrestrial environments in ‘limits of life’ situa-ions, where intense UV-irradiation and desiccation have con-ributed to the necessity for special strategies for the survivalf life forms, has identified the requirement for biogeologicalodification being effected[4–6].Only certain types of rock substrata are amenable to this

olonisation process, which in hot or cold deserts results inhasmolithic or cryptoendolithic structures; the productionf the biochemicals necessary for life in these circumstances

∗ Corresponding author.E-mail address:h.g.m.edwards@bradford.ac.uk (H.G.M. Edwards).

must involve UV-radiation protectants such as carotenecryogenic water-replacement molecules such as trehaloaddition to chlorophyll[7]. Even when life becomes extinthe presence of relict biomolecules such as scytonemin cdicate evidence of previous cyanobacterial colonisation[8,9].

Although endoliths are normally found in sandstone,ticularly in the Antarctic Dry Valleys, the requirementcarbonate ions of group II metals for complexation asalates with oxalic acid produced by hyphal or fungal mcobionts also results in novel biogeological signaturesspectroscopic analysis[10,11]. The ability to recognise thpresence of carbonates, particularly of Ca2+ and Mg2+ at themolecular level gives some important clues to the survstrategies adopted by extremophiles which will afford ater understanding of spectral responses from Raman inmentation that is now being considered for remote Maexploration.

Previous studies have shown that in extreme situatthe conversion or destruction of carbonates of calciummagnesium by extremophilic terrestrial organisms[5,12]

386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2005.02.026

2274 H.G.M. Edwards et al. / Spectrochimica Acta Part A 61 (2005) 2273–2280

gives important information about the habitats in which thesecolonies can survive. Coupled with recent analytical sweepsof the Martian surface by the Mars Global Surveyor, in whichseveral promising locations such as the ‘White Rock’ forma-tions in Juventae Chasma or Sabaea Terra have been iden-tified [13], the spectroscopic identification of carbonates ofmagnesium and calcium is a prime requirement for an analyt-ical database relevant to the behaviour of minerals in a spacescenario. For example, related experiments with cyanobacte-rial colonies in the hydromagnesite deposits at Salda Golu,Turkey [14], the conversion of calcite to aragonite by chas-moliths in an Antarctic felspar substrate, and the productionof calcite from dolomite in the first artificial meteorite ex-periment all depend on the accurate and unambiguous iden-tification of metal carbonates of calcium and magnesium byRaman spectroscopic techniques.

Here, we describe a series of experiments from which ithas been possible to define a unique database for the recogni-tion of calcium and magnesium carbonate-containing miner-als, which is of direct relevance to the application of Ramanspectroscopy to the search for extinct or relict life on Mars.

2. Experimental

2.1. Natural specimen geochemistry

relyf iumi om-b edw dis-t ervea be-h a-m sys-t -b ally

from 1085.7 cm−1 for CaCO3 (aragonite) to 1072 cm−1 forSrCO3 (strontianite). Indeed, significant quantities of calciumions (12.5 mol.%) can increase the stability of strontianitecrystals[15,16], and natural strontianites have been found tocontain up to 30 mol.% calcium[17]. The reverse situationis not reflected in strontium uptake by aragonite, where in amarine environment, for example, for calcium:strontium ra-tios of 1200:1 only 1 mol.% uptake of Sr is observed in thearagonite lattice.

Bottcher et al.[18–23]have reported the FT–IR spectra ofrhombohedral MgCO3 and CaCO3 containing bivalent metal-lic cations such as Fe2+, Mn2+ and Co2+ and have pointed outthe importance of these studies for geothermometry and pa-leoclimate reconstruction. Although there have been severalprevious Raman studies of natural simple carbonates[24,25],the investigation of natural alkaline-earth double carbonatesof Scheetz and White[26] has more relevance to the presentstudy. Here, we have selected the naturally occurring car-bonates of calcium and magnesium, which in the latter casealso includes sensu lato the hydroxycarbonate minerals, toestablish a spectroscopic geological database for applicationto biogeological colonisation studies; the latter is of signifi-cance for the detection of extant or extinct tracers of life inextremophilic habitats terrestrially and, possibly, extraterres-trially.

iuma solids cal-c

ed asn areg

2

6 in-f dule

TN ent wo

M

C ry, igneA ry me

peratD ry rockH h in Mg

ineraliment s

e-

M ry rockgnesiu

A igneouroduct

N gtonite

H ry rockium-ric

In nature, the carbonates of group II metals are raound as chemically pure minerals; for example, calcons in calcite or aragonite (the rhombohedral and orthorhic crystal forms of CaCO3, respectively) can be substitutith other divalent cations to give crystal structures of

orted symmetry. In special cases, it is possible to obsuthentic solid solutions, which are evident from a linearaviour of theν1(CO3)2− stretching wavenumber in the Ran spectrum with compositional change; hence, in the

em CaxSr1−xCO3, wherex= 0→ 1, the Raman wavenumer of (CO3)2− symmetric stretching changes monotonic

able 1atural calcium-rich and magnesium rich minerals studied in the pres

ineral Chemical composition Source

alcite CaCO3 Sedimentaragonite CaCO3 Sedimenta

and low temolomite CaMgCO3 Sedimentauntite Mg3Ca(CO3)4 In caves ric

uct of Mg mrecent sed

agnesite MgCO3 Sedimentaation of ma

rtinite Mg2CO3(OH)2·3H2O Ultrabasicalteration pals

esquehonite Mg (HCO3)(OH)2(H2O) With barrinbasalt

ydromagnesite Mg5(CO3)4(OH)2·4H2O Sedimentaof magnes

Another important point to realise here is that calcnd magnesium carbonates do not form a series ofolutions, although there is a natural mineral of 50:50ium:magnesium composition in dolomite, CaMg(CO3)2.

Hence, the specimens studied here are best describatural, calcium-rich and magnesium-rich minerals andiven inTable 1.

.2. FT–Raman spectroscopy

Raman spectra were recorded using a Bruker IFS6rared instrument with a dedicated FRA 106 Raman mo

rk

Crystallography

ous and metamorphic rocks Trigonal 32/mtamorphic (high pressuresures relatives) rock

Orthorhombic 2/m2/m2/m

Trigonal/3-rock. As weathering prod-s. As diagenetic mineral inequences

Rombohedral (trigonal)trigonal trapzohedral (32) R32

and as products of alter-m-rich minerals

Trigonal 32/m

s and metamorphic rocks ass of magnesium-rich miner-

Monoclinic 2/m

on the surface of olivine Monoclinic-Prismatic2/m

and as alteration productsh minerals

Monoclinic 2/m

H.G.M. Edwards et al. / Spectrochimica Acta Part A 61 (2005) 2273–2280 2275

attachment and Nd3+/YAG laser excitation at 1064 nm. Thenominal laser power for excitation at this wavelength for theminerals studied in this work was 100 mW, using a spec-tral resolution of 4 cm−1 and typically 1000 spectral scansto improve signal-to-noise ratios. It was realised that muchdefinitive spectral discrimination for the specimens can beachieved in the low-wavenumber region, so the wavenum-ber region 50–1200 cm−1 was judged to be critical in theseexperiments, which included crystal lattice modes and car-bonate bending vibrations as well as the familiarν1(CO3)2−wavenumbers quoted in the literature for some carbonates.It will become apparent that the single observation of thestrongest band in the Raman spectrum of carbonates, theν1stretching vibration, is not unambiguous for the identificationof the mineral; this is critically relevant for the discriminationbetween naturally occurring minerals containing calcium andmagnesium using Raman databases.

2.3. Spectroscopic theory

An isolated (CO32−) ion withD3h symmetry will have thefollowing vibrational activity

Γ = A′1 + A′′

2 + 2E′

of which the (A′ + 2E′) are Raman active and represent theν nd-i -e st yt

i vibra-t torsr f thisb Theh siumc pre-d ingp d2h n in-c ding,e

f( rw cbh bandae a-t ion( ans isef ase.T n the

lattice bands in the Raman spectrum is an interesting devel-opment which has merited little study so far[23,32]; Bischoffet al.[32] have synthesized calcite with 15% Mg2+ and havereported the disappearance of all the lattice modes except thatat 286 cm−1.

2.4. Samples

Eight samples of calcium and magnesium minerals, twocarbonate rocks and one beach shell have been analysed. Cal-cite, aragonite, magnesite, and dolomite are from our scien-tific collection of minerals; artinite, huntite and nesquehonitecome from the Geological Sciences collection, Charles Uni-versity, Prague, Czech Republic; a hydromagnesite samplecomes from Salda Golu lake, Turkey. Travertine and coquinarocks belong to our scientific rock collection.

Calcite and aragonite are polymorphs of calcium car-bonate (CaCO3). Calcite is more stable at higher pressuresand temperatures than aragonite and is more common.Magnesite (MgCO3) is isomorphous with calcite andhas an origin that is both sedimentary and metamorphic.Hydromagnesite, Mg5(OH)2(CO3)4·4H2O, and artinite,Mg2CO3(OH)2·3H2O, are hydrated varieties of magnesiteand they are frequently found in association which eachother. Nesquehonite is another hydrated magnesium car-bonate variety, Mg(HCO)(OH)·2(H O), related to basalticr n-t reash ngp erali ition,o n inT

pro-c se ofC cip-i edi-m velyo t.

beens erec

3

3

diedh gionw

3

a ne-s 1098

1

1, ν3 andν4 modes, one symmetric stretching and two beng modes. Through crystal effects thisD3h symmetry is lowred to site symmetries ofCs orD2h; for the former, all mode

hen become Raman-active (A′ +A′′) and for the latter onlhe (A1g +B1g +B2g +B3g) modes are Raman-active.

The symmetric stretching mode,ν1(A′1 inD3h symmetry)

s the strongest band observed in the Raman spectrum;ional coupling between neighbouring carbonate oscillaeduces this symmetry and can result in an asymmetry oond, which is normally then seen in infrared absorption.igherν1 Raman stretching wavenumbers for the magnearbonates compared with their calcium analogues areicted[27] on the basis of the significantly higher polarisower of Mg2+ relative to Ca2+, with P-values of 1.8 an.1 eV (λ)−1/2, respectively. A diminution of theν1 value forighly polarising cations in carbonates is explained by arease in covalent character of the metal–oxygen bon.g. PbCO3 has aν1 value of 1053 cm−1.

The antisymmetric stretchingν3(E′) Raman band oCO3)2− is of lower intensity thanν1 but occurs at a higheavenumber, usually near 1400 cm−1. The antisymmetriending vibration,ν4(E′), occurs near 700 cm−1 and thereas been some confusion in the literature with regard tossignments for the calcium carbonates[25,28–30]. Latticexternal modes[30] consist of ionic translations and librions (restricted rotations) in the low wavenumber reg100–300 cm−1) and the strongest of these in the Rampectra of carbonates[30,31] have been calculated to arrom librations of the carbonate ions in the molecular phhe effect of cationic substitutions in pure carbonates o

3 2ocks. Dolomite, MgCa(CO3)2, is a common sedimeary mineral and usually appears with calcite wheuntite, Mg3Ca(CO3)4, can form either as weatheriroduct of magnesium minerals or as diagenetic min

n sedimentary sequences. The chemical composrigin and structures of these minerals can be seeable 1.

Travertine is a carbonate rock formed by biochemicalesses in continental carbonated springs. The decreaO2 content of the carbonated water results in the pre

tation of this form of calcium carbonate. Coquina is a sentary rock formed by the accumulation almost exclusif shells in continental lakes or in a marine environmen

Samples of nesquehonite, artinite and huntite havetudied by X-ray diffraction to confirm that the samples worrectly catalogued.

. Results

.1. Raman wavenumbers

Table 2shows the Raman bands of the minerals stuere. Spectroscopic details from each wavenumber reill now be considered.

.1.1. The 1200–1000 cm−1 regionThe main band for calcium carbonate (stretching,ν1) is

t 1087 cm−1; for dolomite and huntite, calcium and magium carbonates, the wavenumber position changes to

2276 H.G.M. Edwards et al. / Spectrochimica Acta Part A 61 (2005) 2273–2280

and 1123 cm−1, respectively; for magnesite this band appearsat 1094 cm−1. In the artinite mineral, with two Mg2+ ions andthree water molecules, the band occurs at 1094 cm−1 and at1100 cm−1 for nesquehonite, another magnesium carbonatewith water of crystallization. In hydromagnesite, containing

five Mg2+ ions and four water molecules, the band appearsat 1119 cm−1 (Fig. 1).

Theν1 stretching band can hence be used to differentiatebetween the groups of either calcium or magnesium carbon-ates, but it is not valid for the differentiation between the cal-

Table 2Raman spectral signatures of calcium and magnesium carbonates studied in the present work

Calcite Aragonite Dolomite Huntite Magnesite Artinite Nesquehonite Hydromagnesite

2934vw

2906vw 2905vw 2906vw 2905vw 2906vw 2905vw 2905vw 2904vw

2835vw 2835vw 2813vw

2707vw 2690vw

1903vw 1904vw

1891vw 1890vvw 1871vw

1749vw

1759vw 1761vw

1708vvw

1574vw

1521vw

1515vvw

1487vw

1462vw 1459vw 1460vw

1451vw

1443vw 1444vw

1436vw 1437vw

1428vvw

1351vw

1094v

1

7

2

1

v

1123vs

1098vs

086vs 1086vs

882vvw 878w

854vw

784bw

742vw 738m–

725w 723w

13w

704w 705vw

717sh 713sh

693vw

682vw

386vw

364vw

339w 330m

316m

83m 283sh,w 300m–s

273vw 272m–s

261vw 253m–s

249vw

231vw

208m 213w

191vw

181vw

177m–s

56m 154m 156sh 155m

120m

118m

s = very strong; s = strong; m = medium; w = weak; vw = very weak; b = broad

1119vs

s 1094vs 1100vs

935vw

792vw

772vvw 757w

w

727w

704w 703vvw 706w

690sh

669vw

653sh,vw

472w

380vvw

344vw

329w

326vvw 311vw

277vvw 273vvw

242w 247m

228m 232m

211vvw

199m 202m

187m 184m

173m

158m 167m

147m

119m

; sh = shoulder.

H.G.M. Edwards et al. / Spectrochimica Acta Part A 61 (2005) 2273–2280 2277

Fig. 1. Raman spectra of the calcium and magnesium carbonates studied inthe present work in the 1200–1000 cm−1 wavenumber region.

cium carbonates or the magnesium carbonates within eachgroup.

3.1.2. The 1000–500 cm−1 regionThe bands in this region have a medium to weak relative

intensity for all samples studied.Theν2 bands are at 713 cm−1 in the calcite spectrum and

at 704 cm−1 in the aragonite spectrum, with a very weakshoulder at 717 cm−1 (Fig. 2).

In the magnesite spectrum, one medium intensity bandat 738 cm−1 can be seen. The dolomite spectrum in this re-gion shows a band at 725 cm−1 with a very weak shoulder at713 cm−1. The most significant signature in this region for thedolomite mineral is the presence of a broad band at 784 cm−1

which is characteristic of this mineral and which is absent inthe other calcium and magnesium carbonate mineral spectra.A weak band with two peaks at 693 and 682 cm−1 appearshere, too. In the spectrum of huntite, two main bands are vis-ible with weak intensity at 878 and 724 cm−1 and a furthertwo weaker bands at 705 and 742 cm−1 (Fig. 3).

In the artinite spectrum there is a band at 704 cm−1 with ashoulder at 690 cm−1 and another two weaker bands at 935and 792 cm−1. Three bands are present in the hydromagnesite

F cmw

Fig. 3. Raman spectra of huntite and hydromagnesite in the 1200–100 cm−1

wavenumber region.

spectrum in this region at 757, 727 and 706 cm−1 and anotherweak doublet at 669 and 653 cm−1. A broad band at 703 cm−1

and another weaker one at 772 cm−1 are characteristic fornesquehonite in this region.

Because of the relatively weak intensity of the bands in thisregion, the differentiation between the calcium carbonatesand among calcium–magnesium carbonate mixtures whenthe components appear in small quantities or when the spec-trum is not of a good quality is compromised and discrimina-tion must therefore be corroborated with other characteristicbands. For example, in the aragonite and artinite spectra, thebands in the 1000–500 cm−1 region are in the same positionbut theν1 mode must be used to differentiate between the twominerals. The same situation can occur with the dolomite andhydromagnesite minerals.

3.1.3. The 500–100 cm−1 regionIn the lattice vibration region only two bands appear in the

calcite spectrum, at 283 and 156 cm−1. The aragonite spec-trum presents bands of medium intensity at 208 and 154 cm−1

and other weak intensity bands at 283 (shoulder), 273, 261,249, 191 and 181 cm−1.

Medium–strong bands at 300 cm−1 (with a very weakshoulder at 283 cm−1) and at 177 cm−1 (with a very weakshoulder at 156 cm−1) appear in the dolomite spectrum witha −1 mw 272,2 231a

cma

giona and1a rh

veralb , 184a 99,1 ite,

ig. 2. Raman spectra of calcite and aragonite in the 1200–100−1

avenumber region.

nother weak band at 339 cm. Huntite presents a spectruith several bands with medium–strong intensity (316,53 and 118 cm−1) and three weaker bands at 386, 364,nd 155 cm−1.

The magnesite spectrum has one main band at 330−1

nd a weaker band at 213 cm−1.The artinite spectrum presents the main band in this re

t 173 cm−1 and a broad doublet with two maxima at 15847 cm−1. Two medium–weak bands at 472 and 242 cm−1

nd very weak bands at 380, 326, 277 and 211 cm−1 appeaere (Fig. 4).

The hydromagnesite spectrum in this region has seands of medium–weak intensity at 329, 247, 232, 202nd 147 cm−1. A broad band with several peaks at 228, 187, 167 and 119 cm−1 is characteristic of nesquehon

2278 H.G.M. Edwards et al. / Spectrochimica Acta Part A 61 (2005) 2273–2280

Fig. 4. Dolomite, magnesite, artinite and nesquehonite Raman spectra in the1200–100 cm−1 wavenumber region.

which, furthermore, presents weak bands at 344, 311 and273 cm−1.

3.1.4. The 2000–1200 cm−1 regionThree very weak bands are visible in calcite (1903, 1749

and 1436 cm−1), aragonite (1904, 1574 and 1462 cm−1)and dolomite (1891, 1759 and 1443 cm−1) spectra. A bandat 1444 cm−1 appears in the magnesite spectrum whereashuntite gives very weak bands at 1761 and 1459 cm−1. Theartinite spectrum has a large spectral background but weakbands at 1460, 1437 and 1351 cm−1 appear. Bands at 1871,1521, 1487 and 1451 cm−1 appear in the hydromagnesitespectrum. The nesquehonite spectrum shows very weak in-tensity bands at 1890, 1708, 1515 and 1428 cm−1.

3.1.5. The 3000–2500 cm−1 regionThe hydromagnesite spectrum shows a band at 2904 cm−1

which appears at 2905 cm−1 for aragonite, nesquehonite andhuntite, at 2906 cm−1 in the calcite, dolomite and magne-site spectra. The artinite spectrum has a doublet at 2934and 2905 cm−1. Aragonite and calcite spectra have a veryweak band at 2835 cm−1, and a band at 2707 cm−1 also ap-pears in the calcite spectrum. Bands at 2813 and 2690 cm−1

are present in the dolomite spectrum but no other bandsappear in the hydromagnesite magnesite and huntite spec-t

3o

min-e ativec . Weh anda itivityt esenti

n ofa anda lcite

Fig. 5. Raman spectra of aragonite 100%; aragonite 98%–calcite 2%; arag-onite 95%–calcite 5%; aragonite 93%–calcite 7%; aragonite 90%–calcite10% in the 500–50 cm−1 wavenumber region.

and 2% aragonite, and 98–2%, 95–5%, 93–7%, 90–10% and50–50% aragonite-calcite).

As the strong band in the lattice mode for aragonite(208 cm−1) occurs with clear spectroscopic ‘window’ be-tween the 283 and 156 cm−1 Raman signatures of calcite,a mixture of 2% of aragonite is enough to assess this mineralis present.

It is a different situation when calcite is the mineral whichappears in a proportion of 2% in admixture with aragonite.The 283 cm−1 band of calcite coincides with a weak shoulderof aragonite in the region of 273–249 cm−1; as a result a pro-portion of 2% of calcite in 98% aragonite is indistinguishablefrom pure aragonite but as the proportion of calcite increasesfurther the feature becomes identifiable as calcite at about 7%or so (Fig. 5).

A mixture of 50% calcite–50% aragonite gives a spec-trum with similar relative intensities of the bands at 283 and208 cm−1 for calcite and aragonite, respectively (Fig. 6); thismeans that it should be possible to estimate the proportions ofcalcite and aragonite in a specimen from the relative intensityratio of these bands in the Raman spectrum.

F in the1

rum.

.2. Evaluation of band intensities of different mixturesf calcite and aragonite

Calcite and aragonite are the most common calciumrals and they have been found together in extraputolonies in stressed environmental terrestrial conditionsave prepared mixtures in different proportions of calciteragonite to determine the Raman spectroscopic sens

owards discrimination between calcite and aragonite prn the same sample.

To assess the spectral sensitivity for the detectiomounts of one mineral mixed with another, calciteragonite were mixed in different proportions (98% ca

ig. 6. Raman spectrum of a mixture of aragonite 50%–calcite 50%200–100 cm−1 wavenumber region.

H.G.M. Edwards et al. / Spectrochimica Acta Part A 61 (2005) 2273–2280 2279

Fig. 7. Comparative Raman spectra of coquina rock, aragonite and calcitespectra in the 1200–100 cm−1 wavenumber region.

3.3. Rock and shell spectra

Rock compounds of calcium carbonates and a shell havebeen analyzed and in each case it has been possible to dif-ferentiate between the carbonate and mixture of carbonatespresent.

The travertine spectrum shows bands at 1086, 713, 282 and155 cm−1 characteristic of calcite. No other bands appear inthe spectrum, so we can conclude that the only mineral in thissample is calcite.

The coquina spectrum (Fig. 7) gives a very strong bandat 1086 cm−1, which corroborates the presence of a calciumcarbonate only. The existence of a band at 282 cm−1 confirmsthat calcite is present in the rock and the band at 207 cm−1

indicates the presence of aragonite. A band at 705 cm−1 anda shoulder at 712 cm−1 are characteristic of aragonite andcalcite, respectively. Hence, the coquina rock contains bothcalcite and aragonite, which are clearly distinguishable usingRaman spectroscopy. According to our studies, the propor-tion between these minerals present in this specimen is 50%aragonite–50% calcite.

The analysis of the spectrum of a sample from theSalda Golu Lake (Fig. 8) shows a very strong band at1119 cm−1 characteristic of hydromagnesite and a weak band

F sam-p mw

Fig. 9. Comparative Raman spectra of a beach shell with aragonite andcalcite in the 1200–100 cm−1 wavenumber region.

at 1086 cm−1, which is assigned to one of the calcium car-bonates. The relative intensity of the strong hydromagnesitebands masks the weaker bands of the calcium carbonates inthe 1000–500 cm−1 region. Because of interference from thecharacteristic bands of hydromagnesite, the band at 208 cm−1

from aragonite is not visible but from the absence of a signalat 282 cm−1, we can therefore dismiss the presence of calciteand assert that the calcium carbonate present in this sampleis therefore aragonite.

The shell spectrum (Fig. 9) shows the strongest bands at1086, 704, 207 and 154 cm−1 from aragonite. No other cal-cium or magnesium carbonate signatures appear in the shellspecimen.

4. Conclusions

Specimens from the calcium carbonate group and calciumand magnesium carbonate group are not easily differentiatedwith the sole observation of theν1 stretching band wavenum-ber only.

The low wavenumber lattice modes can be used to rec-ognize the calcium carbonate present in the sample; the208 cm−1 band for aragonite and 283 cm−1 band for calciteare enough to assert, unquestionably, the composition of thec andi undsi , theν ne oro

h hy-dm s-s eent

cal-c banda e

ig. 8. Comparative Raman spectra of a natural hydromagnesitele from Salda Golu Lake (Turkey) with calcite in the 1200–100 c−1

avenumber region.

alcium carbonate mineral. When the ratio of Raman bntensities between calcium carbonate and other compon the sample permits the obtaining of a good spectrum2 mode may be used to corroborate the presence of other carbonate.

The carbonates with magnesium have aν1 vibration inigher positions than calcium carbonates. Huntite andromagnesite are characterized by very closeν1 vibrationodes (1123 and 1119 cm−1, respectively) and it is nece

ary to resort to the lattice mode region to distinguish betwhem.

The lattice mode bands are very distinctive for theium and magnesium carbonate group; a medium strongppears at 242 cm−1 in the artinite spectrum, while in th

2280 H.G.M. Edwards et al. / Spectrochimica Acta Part A 61 (2005) 2273–2280

Table 3Corroborative patterns of distinctive Raman bands for calcium and magne-sium carbonates of our present work

Calcite 1086 713 283 156Aragonite 1086 704 208 154Dolomite 1098 725 300 177Huntite 1123 316 272 253Magnesite 1094 704 330 213Artinite 1094 704 472 173 147Nesquehonite 1100 228 187 167 119Hydromagnesite 1119 232 202 184

dolomite spectrum it is at 300 cm−1 and occurs at 330 cm−1

for magnesite. Huntite has three characteristic bands at 316,272 and 253 cm−1 (Table 3).

It has been demonstrated here that it is possible to dif-ferentiate unambiguously mixtures of different carbonates inrocks and minerals from the study of their Raman spectra.Since this analysis can be carried out remotely, microscopi-cally and macroscopically, it is clear that the geological char-acterization of calcium and magnesium carbonate minerals ispossible using Raman spectroscopic techniques. Since sev-eral of these compounds provide suitable host matrices forbacterial colonization in extreme habitats and others are pro-duced by the biogeological modification of the host geologythrough extremophilic activity, the detection of these miner-als could provide a useful adjunct to the search for evidenceof life in the terrestrial geological record using Raman spec-troscopy. The location of suitable microniches in extrater-restrial planetary exploration is now considered to be a vitalstage in lander and rover and sedimentary deposits are keyspectral markers in the selection of future landing sites onMars.

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