https://doi.org/10.1007/s00710-020-00709-9

ORIGINAL PAPER

Mid-infrared spectroscopy of alkali feldspar samples for spaceapplication

Maximilian P. Reitze1 · Iris Weber1 ·Herbert Kroll2 · Andreas Morlok1 ·Harald Hiesinger1 · Jörn Helbert3

Received: 18 November 2019 / Accepted: 18 May 2020© The Author(s) 2020

AbstractFeldspars are major components of terrestrial planetary surfaces. For future space application and the setup of acomprehensive reference database, Na- and K-rich alkali feldspars, NaAlSi3O8 – KAlSi3O8, have been investigatedby infrared reflectance spectroscopy. We related the feldspar spectra to the chemical composition and state of Al,Siorder/disorder. The infrared measurements were analyzed with respect to band shifts and peak shapes using theautocorrelation function. Natural samples served as starting materials. Some samples were treated by the alkali exchangemethod to produce pure end-members, which were then heated to generate various states of Al,Si disorder. X-ray diffraction(XRD) methods served to determine the Al,Si distribution. Our autocorrelation allowed to differentiate between thecompositional and the order/disorder influences seen in the spectra in the wavelength range between 7 μm up to 14 μm(1429 cm−1 to 714 cm−1). Space missions often analyze the surfaces of planetary bodies using remote sensing. Therefore,our results are essential to characterize and distinguish alkali feldspars on the surfaces of terrestrial planetary bodies likeMercury.

Keywords Alkali feldspars · Infrared spectroscopy · X-ray diffraction · Al,Si distribution

Introduction

Feldspars are rock-forming minerals, on Earth and onother terrestrial planetary bodies. In the Earth’s crust, theyaccount for more than 50 vol% (Okrusch and Matthes2014). Lunar highlands, which cover approximately 83%of the lunar surface (Jolliff et al. 2006), are composedmostly of anorthosites and gabbros with a large amount ofanorthite (Wood et al. 1970). Furthermore, Mercury’s crustis feldspatic with a labradorite-like composition predicatedon the basis of Earth-based infrared emission spectroscopyby Sprague et al. (1997), who measured a Christiansen

Editorial handling: A. Beran

� Maximilian P. Reitzemaximilian.p-reitze@uni-muenster.de

1 Institut für Planetologie, Westfälische Wilhelms-Universität,Wilhelm-Klemm-Str. 10, 48149, Münster, Germany

2 Institut für Mineralogie, Westfälische Wilhelms-Universität,Corrensstr. 24, 48149, Münster, Germany

3 Deutsches Zentrum für Luft- und Raumfahrt, Rutherfordstr. 2,12489, Berlin, Germany

feature (CF) at 8 μm (1250 cm−1) near the equator atapproximately 120◦ longitude. After spectral unmixing ofEarth-based observations, Mercury’s surface is thought tocontain potassium-rich feldspar (Sprague et al. 2009).

Direct sampling of Solar System bodies other than Earthrequires landing on the surface and returning samples,which is extremely expensive and currently not feasiblefor most objects. Thus, remote sensing data of planets andmoons are necessary to provide the missing information.

Between 2011 and 2015, the MESSENGER (MErcurySurface, Space ENvironment, GEochemistry and Ranging)mission investigated the hermean surface (see Solomonet al. 2018, for a detailed description). The gamma-rayspectrometer (GRS) onboard the MESSENGER missionmeasured high Mg/Si and low Al/Si and Ca/Si contents,which excludes an anorthite-rich feldspatic crust like thelunar highlands (Nittler et al. 2011). GRS detected relativelyhigh abundances of sodium, which suggest that albite is amajor mineral on the surface of Mercury (Evans et al. 2012).

In October 2018, the ESA/JAXA BepiColombo space-craft was launched to investigate the innermost planet ofour Solar System via remote sensing. Among several otherinstruments, MERTIS (MErcury Radiometer and ThermalInfrared Spectrometer) will investigate Mercury’s surface

/ Published online: 30 July 2020

Mineralogy and Petrology (2020) 114:453–463

http://crossmark.crossref.org/dialog/?doi=10.1007/s00710-020-00709-9&domain=pdfhttp://orcid.org/0000-0002-7092-3404mailto: maximilian.p-reitze@uni-muenster.de

in the mid-infrared (mid-IR) wavelength region (Hiesingeret al. 2010; H. Hiesinger and co-workers, in preparation).The goals of MERTIS are to study the surface mineralogyand composition, to map the mineralogy, and to measure thesurface temperatures as well as the thermal inertia (Hiesingeret al. 2010; H. Hiesinger and co-workers, in preparation).To achieve these goals, MERTIS operates in the wavelengthregion from 7 μm to 14 μm (Spectrometer Channels/TIS1429 cm−1 to 714 cm−1) and from 7 μm to 40 μm (Radio-meter Channels/TIR 1429 cm−1 to 250 cm−1) (Hiesingeret al. 2010; H. Hiesinger and co-workers, in preparation).

Reflectance and absorption are not directly convertible(e.g., Kirkland et al. 2002; Thomson and Salisbury 1993).Therefore to apply spectral deconvolution algorithms weneed reflectance spectra, which are comparable to remotelyacquired data for the remote sensing applications.

For the interpretation of data returned from spacemissions, laboratory work is essential. Known factors whichcontrol reflectance and emission mid-IR data are, forexample, the chemical composition, grain size distribution,bulk sample density, viewing geometry, pressure, andtemperature (e.g., Pieters and Englert 1993). In this context,analyses of different grain sizes are necessary, because thesurfaces of atmosphereless planetary bodies, like Mercury,are highly affected by larger and smaller impacts, whichcause the formation of a poorly-sorted regolith.

To setup our spectral database that is valuable for remotesensing of various Solar System bodies, it is necessary toinvestigate well characterized samples of the alkali feldsparsystem. This includes both chemical composition and degreeof Al,Si order in addition to other known factors controllingthe shape of mid-IR spectra (Weber et al. 2018).

The study of feldspars includes their formation con-ditions, occurrences, chemical, and physical behaviors.Alkali feldspars are solid solutions of the end-membersK[AlSi3O8] (Or) and Na[AlSi3O8] (Ab). They mostly con-tain small amounts of Ca[Al2Si2O8] (An). In addition totheir chemical compositions, they are characterized by thespecific distributions of Al and Si onto the tetrahedral sitesof their framework structures (“structural state”). The mostcommon rock-forming alkali feldspars, ranging in compo-sition from Or100Ab0 to Or30Ab70, occur in two differentmodifications. Monoclinic sanidine (K,Na)[AlSi3O8] is thestable modification at high temperatures. The Al and Siatoms (1 Al plus 3 Si per formula unit) are distributed in ahighly disordered, but not completely random manner ontotwo T1 and two T2 tetrahedral sites. The larger Al atomslightly prefers the T1 over the T2 site (Scambos et al. 1987),denoted by t1 > t2 with t = Al/(Al + Si) of the respec-tive site (Kroll 1973). Provided a rate-enhancing influencebeing present, such as a fluid phase, mineral-fluid replace-ment processes lead at T ≤ 450 ◦C - 480 ◦C to the triclinic

modification of microcline that under the polarizing micro-scope shows a characteristic tartan twinning reflecting theloss of monoclinic symmetry elements (Parsons 1978). Thephase transition causes the T1 and T2 sites to split into T10,T1m and T20, T2m sites, respectively. Aluminum concen-trates in the T10 site so that at full order t10 = 1, t1m =t20 = t2m = 0. At less favorable conditions, due to thehigh kinetic Al,Si exchange barrier, the transition results in akinetically stranded, metastable pseudomorph called ortho-clase. On a submicroscopic scale, orthoclase consists ofintimately intergrown triclinic domains. All textural stagesbetween orthoclase and microcline may be present in oneand the same morphological unit. Na-rich alkali feldsparsare as well symmetrically monoclinic at high temperature(monalbite). Changes to triclinic symmetry are inducedduring cooling through two different, though coupled mech-anisms: A displacive transition at 980 ◦C, followed by acontinuous Al,Si ordering process (Salje et al. 1985). Onslow cooling, Al,Si ordering proceeds at T < 980 ◦C inthe triclinic state with Al gradually enriched in the T10 sitewhile leaving the other three sites so that in the end t10 = 1,t1m = t20 = t2m = 0 like in microcline. By contrast, whenrapidly cooled, monalbite collapses to triclinic metastableanalbite because the Na ion is too small to prevent structuralcollapse in contrast to the larger K ion in sanidine. The termshigh albite and low albite denote different states of order. Athigh temperatures, the solid solution series between monal-bite and sanidine is continuous. A wide miscibility gap opensat lower temperatures, leading to the formation of intimatelyintergrown phase segregation textures of which perthites(Na-feldspar in a matrix of K-feldspar) are most common(Okrusch and Matthes 2014, and referenes therein).

Characteristics of alkali feldspars concerning the struc-ture, formation conditions, exsolution processes, and behav-ior at high temperatures are well known (e.g., Ribbe 1983).Previous infrared spectroscopy work on the feldspar systemshows that changes in absorption spectra occur accordingto chemical composition and degree of Al,Si order (e.g.,Laves and Hafner 1956; Hafner and Laves 1957; Martin1970; Zhang et al. 1996, 1997). Hecker et al. (2010) gavea review of different infrared techniques and their use-fulness for remote sensing application. Iiishi et al. (1971)correlated single bands in K-feldspar absorption spectrabetween 8.7 μm and 23.3 μm (1150 cm−1 and 430 cm−1)with specific stretching, bending, translation, and expan-sion/contraction vibrations of Si-O, Si-Si, and Si-O-Sibonds, which are controlled by the state of Al,Si order. How-ever, most of the bands described by Iiishi et al. (1971)cannot be observed as single peaks in powder reflectancespectra.

The objective of our study is to measure mid-IR powderreflectance spectra of well characterized alkali feldspar

454 M.P. Reitze et al.

samples to build a database for remote sensing applications.This database is necessary especially for MERTIS.

Samples and analyses

Samples

To differentiate and quantify controlling factors of thepowder mid-IR reflectance spectra, the analyzed samplesmust be well characterized. Natural alkali feldspars wereused as raw materials, which were first thoroughlycharacterized and then treated in different ways described inthe subsection “Sample Treatment”. All samples presentedin this study, both natural and treated, have an internaldatabase identification number (ID, Table 1), which refersto our database which is accessible at www.uni-muenster.de/Planetology (Weber et al. 2018). Our starting materialincluded sanidine from Volkesfeld, Eifel, Germany (ID73), microcline from Prilep, North Macedonia (ID 151),and albite from Cazadero, California (ID 125). ID 73sanidine, (Na15K85)[AlSi3O8], is of gem-stone quality,being transparent. Its Al,Si order is low, 2t1 = 0.61 (Krolland Knitter 1991). ID 151 microcline, (Na5K95)[AlSi3O8],is a light greenish powder sample fraction (100 μm to125 μm) prepared from the original perthitic microcline(amazonite) from Prilep, which contains some exsolvedalbite. The Al content of the T10 site is 0.99 (Strob 1983). ID125 albite has a white color and represents an endmemberof the alkali feldspar solid solution in terms of Ab content(Ab99.9) and Al,Si order (t10 = 1.00) (Wenk and Kroll1984). These natural samples were treated in different waysas described in section sample treatment.

Sample characterization

Natural as well as commercially available minerals are oftenimpure. We checked for impurities using light microscopyof thin sections, which also allowed us to distinguishbetween different compositions and states of order.

To determine the degree of Al,Si order, XRD patternswere obtained with a Philips X’Pert powder diffractometer,equipped with a primary Ge monochromator and with aBruker D8 ADVANCE. For the XRD measurements, somesample material was crushed in an agate mortar to receivea “X-ray fine” grain size (approx. 1 μm to 10 μm),controlled by light microscopy. The powder was mixedwith Si powder as internal standard to correct the zeroshift of the X-ray diffractometers. Rietveld analysis ofthe XRD patterns were performed with the FULLPROFsoftware (Rodriguez-Carvajal 2005) to refine the latticeparameters. Atomic coordinates and lattice parametersfor albite were taken from Wenk and Kroll (1984), for

Table1

Overviewof

theused

samples,theirtreatm

ent,lattice

parameters,Ab-Orcontent,anddegree

oforder

IDSamplename

Origin/p

recurser

IDTreatment

ab

cα

βγ

VAb/Or

∑t 1

73Sanidine

Volkesfeld

–8.5454(8)

13.0191(8)

7.1827(9)

–115.993(7)

–718.27(9)

15/85

0.62(1)

125

Albite

Cazadero

–8.132(2)

12.783(2)

7.159(2)

94.29(2)

116.59(2)

87.69(2)

663.6(2)

100/0

1.01(2)

129

Sanidine

73850

◦ C/11d

8.5480(7)

13.0256(6)

7.1799(4)

–115.981(6)

–718.63(8)

15/85

0.58(1)

149

Or100

73KCl/8

50◦ C

/5h

8.6038(8)

13.0198(9)

7.1860(4)

–116.016(7)

–723.41(9)

0/100

0.62(1)

150

Or100

129

KCl/8

50◦ C

/5h

8.6057(7)

13.0284(6)

7.1839(4)

–116.003(6)

–723.91(8)

0/100

0.59(1)

151

PrilepMicrocline

Prilep

–8.580(2)

12.962(2)

7.2211(7)

90.65(2)

115.974(8)

87.69(2)

721.4(2)

5/95

0.99(1)

152

MicroclineOr100

151

KCl/8

50◦ C

/5h

8.593(2)

12.963(2)

7.2223(6)

90.62(2)

115.969(1)

87.70(2)

722.6(2)

1/99

0.99(2)

163

Albite

125

1060

◦ C/7

barH2O/8

d8.146(3)

12.799(4)

7.144(2)

94.22(3)

116.57(2)

88.07(3)

664.4(3)

100/0

0.90(3)

164

Albite

125

1060

◦ C/7

barH2O/1

5d8.144(4)

12.816(8)

7.149(2)

94.21(5)

116.61(3)

87.93(5)

665.3(6)

100/0

0.86(7)

165

Albite

125

1060

◦ C/>

7bar

H2O/1

8d8.142(4)

12.846(6)

7.126(3)

93.92(5)

116.47(4)

89.33(5)

665.5(5)

100/0

0.79(4)

166

Albite

125

1060

◦ C/>

7bar

H2O/4

7d8.137(3)

12.826(6)

7.130(2)

94.02(4)

116.47(2)

89.08(4)

664.4(4)

100/0

0.77(5)

167

Albite

125

1060

◦ C/7barH2O/9

2d8.158(2)

12.874(3)

7.111(1)

93.51(2)

116.45(2)

90.22(2)

667.0(3)

100/0

0.55(5)

Num

bersin

bracketsindicateuncertaintyof

thelastdigit

455Mid-infrared spectroscopy of alkali feldspar samples for space application

www.uni-muenster.de/Planetologywww.uni-muenster.de/Planetology

microcline from Strob (1983), and for sanidine from Krollet al. (1986). Based on the obtained lattice parameters, Al,Sisite occupancies were derived from equations given by Krolland Ribbe (1987) (Table 1).

Sample treatment

For our investigation we needed alkali feldspars withendmember compositions, which possess a maximumspread of ordering states. Natural feldspars do not fulfillthese requirements. Therefore, we performed annealingexperiments to vary the state of order as well as alkaliexchange experiments to change compositions. Volkesfeldsanidine, ID 73, has been heated at 850 ◦C for 11 d (days),thereby slightly decreasing its state of order. Much largerchanges were obtained for the Cazadero low albite. Aportion of ID 125 was filled in platinum tubes (ca. 1g pertube) and sealed in evacuated silica glass tubes togetherwith gibbsite (=hydrargillite, Al(OH)3). The samples wereheated at 1060 ◦C to produce increasing states of disorder(see Tab. 1 for details). H2O released by gibbsite acts asa flux that promotes disordering. The amount of gibbsitewas chosen so that the H2O pressure was about 7 bar.The samples were recovered after 8 d (ID 163), 15 d (ID164), 18 d (ID 165), 47 d (ID 166) and finally after 92 d(ID 167). While Cazadero low albite is a pure Na-feldsparendmember, Volkesfeld sanidine and Prilep microclinecontain some Ab component. They were thus treated by thealkali exchange method to produce K-feldspar endmembers(Kroll et al. 1986). Charges of ID 73, ID 129 (63 μm to125 μm) and ID 151 (100 μm to 125 μm) were heated at850 ◦C for 5 h in an excess of molten KCl in a covered Ptcrucible in order to exchange Na ions by K ions. The weightratio sample/KCl was set at 1:4.5 (ID 73 → ID 129), 1:70(ID 129 → ID 150), and 1:5 (ID 151 → ID 152). Both smalland large weight ratios led to the desired results. After theexperiments, the samples were washed with distilled water,filtered and then dried at 50 ◦C for several hours. The Orcontents of the exchange products were determined fromthe position of the composition-sensitive (2̄01) reflection inX-ray powder diagrams (Kroll et al. 1986, their Table 9)resulting in Or99 and Or100, respectively (Table 1).

Infrared and analysis procedure

For the IR measurements, all samples were crushed in asteel mortar and/or agate mortar and sieved to grain sizes< 25 μm, 25 μm to 63 μm, 63 μm to 125 μm, 125 μmto 250 μm, and grains greater than 250 μm. Mid-infraredmeasurements were performed with a Bruker 70v FTIRspectrometer with a spectral resolution of 4 cm−1 using

a grazing angle unit A513. We used a liquid nitrogencooled MCT (mercury cadmium telluride) detector, and allmeasurements took place at pressures of 2 hPa at roomtemperature (ca. 23 ◦C). For the background spectrumwe used a commercially available diffuse gold standard(INFRAGOLDTM). The powder samples were placed inan aluminum sample holder, 1cm in diameter and 1.5mm in depth, and flattened with a spatula. Each grainsize fraction was measured at two different geometries,with an incident angle (i) of 20◦ and emergent angle (e)of 30◦ (i20◦ e30◦) and with an incident and emergentangle of 30◦ (i30◦ e30◦). For each sample and backgroundmeasurement, 512 single interferogram scans in doublesided foreward-backward scan mode were summed up, andthe spectrumwas calculated through Fourier transformation.In the wavelength region we measured, silicates showcharacteristic spectral features: (1) The Christiansen Feature(CF), which is a reflectance minimum at which the sampleis mostly transparent for the infrared radiation because thereis a rapid change of the real part of the refractive index inthe sample (Conel 1969). This feature is diagnostic for themineralogy (Conel 1969). (2) The Reststrahlen Bands (RB),as a result of dominating surface scattering of photons.RBs arise from the Si-O and Al-O stretching (e.g., Pietersand Englert 1993; Iiishi et al. 1971). (3) The TransparencyFeature (TF) results from increased volume scattering of theinfrared radiation and is only visible in samples with smallgrain sizes (< 25 μm) (Pieters and Englert 1993).

Because peak fitting is complex (Malcherek et al. 1999),especially in powder reflectance spectra with broad andoverlapping peaks, we decided to use the autocorrelationmethod, which gives us a measure of the peak widths. Thismethod was used successfully to determine the degree oforder in cordierite and Al,Ge ordering in Ba[Al2Ge2O8]feldspar (e.g., Malcherek et al. 1995; Salje et al. 2000).Following this, we analyzed the feldspar infrared spectra notonly in terms of band positions but also with autocorrelationof specific band regions. For this we had to slightly modifythe method given by Salje et al. (2000) and calculated thefollowing correlations. From the autocorrelation functionwe used the FWHM (Full Width at Half Maximum) of thecomplete autocorreleation function for two specific bandregions from 8 μm to 9 μm and from 9 μm to 10 μm,because of the remarkable changes in the spectra in thesebands without baseline correction. Gaussian fits to thecentral part of the autocorrelation function from 7.27 μmto 12.11 μm were calculated with a constant offset aroundzero considering our spectral resolution (Offset values forfitting: -26 cm−1 and 26 cm−1. From this fit to the centralpart of the autocorrelation function we used the width ω,which corresponds with k2 in Salje et al. (2000).

456 M.P. Reitze et al.

Fig. 1 a Spectra of all sampleswith Ab100 with different degreeof order and grain sizes between63 μm and 125 μm (i20◦, e30◦).The gray region indicates themeasurement range of MERTIS.The black lines indicate afeature shifting with

∑t1 and

Or content. The dashed linesindicate the borders of theregion for autocorrelationanalysis and the associated fitanalyzes to determine the degreeof order. The dashed-dottedlines indicates the bands forautocorrelation analyzes for thedetermination of the Or content.b The same as in a but forsamples with Or100 to Or85

Results

X-ray diffraction

Heating of the Volkesfeld sanidine (ID 73) for eleven daysat 850 ◦C led to further disorder resulting in a final valueof

∑t1 = 0.58 (ID 129). Heating of samples ID 73 and

ID 129 in molten KCl lead to an Or content of 100% in thesamples ID 149 and ID 150. The degree of order remainedunchanged. After ion exchange of the Prilep Microcline ID151 in molten KCl, the produced sample (ID 152) had anOr content of 99% with no observable Ab component in

the XRD diagram. The calculated degree of order was alsounchanged within the range of uncertainty (Table 1).

The degree of order in the low albite sample ID 163that had been heated for 8 days at 1060 ◦C decreased from∑

t1 = 1 to 0.90; after 15 d it was down to 0.86 (ID164). In the experiment that lasted 18 d (ID 165), the silicaglass tube was bulging and was possibly broken, most likelybecause of a too high water vapor pressure. The crystals,however, were not molten. XRD analysis showed two albitepopulations, 32% of the original low albite had not yetstarted to disorder, 68% were high albite with

∑t1 =

0.69. An average value of∑

t1 = 0.79 was assigned to

Fig. 2 a Spectra of all sampleswith Ab100 with different degreeof order and grain sizes smaller25 μm (i20◦, e30◦). The grayregion indicates themeasurement range of MERTIS.The black lines indicate afeature shifting with

∑t1 and

Or content. The dashed linesindicate the borders of theregion for autocorrelationanalysis and the associated fitanalyzes to determine the degreeof order. The dashed-dottedlines indicates the bands forautocorrelation analyzes for thedetermination of the Or content.b The same as in a but forsamples with Or100 to Or85

457Mid-infrared spectroscopy of alkali feldspar samples for space application

the sample. The silica glass tube of sample ID 166 alsostrongly bulged but did not break and for this sample, thedegree of order was determined from XRD refinment to∑

t1 = 0.77. For sample ID 167, after opening the tube asmall hole near the weld seam was noticed. At this positionthe tube wall was probably thinner because of weldingand therefore a hole could form due to the water vaporpressure. The calculation of the degree of order gave ahighly disordered

∑t1 = 0.55. All samples, their precursor

material, treatment, refined XRD data, Ab-Or-content, aswell as degree of Al,Si order are listed in Table 1 (comparesection results).

Infrared

Our main goal was to investigate the spectral rangebetween 7 μm and 14 μm (marked by a gray backgroundin Figs. 1 and 2 /≈ 1429 cm−1 to 714 cm−1),which corresponds to the measurement range of theMERTIS instrument. The overall reflectance of our samplesdecreases with decreasing particle size because surfacescattering is decreased compared to volume scattering. Theautocorrelation function was used for the analysis of thisspectral range (ranges of autocorrelation calculations: 8 μmto 9 μm (1250 cm−1 to 1111 cm−1), 9 μm to 10 μm(1111 cm−1 to 1000 cm−1), and 7.27 μm to 12.11 μm(825 cm−1 to 1376 cm−1).

Figure 1 shows the spectra of samples with a grain sizeof 63 μm to 125 μm, measured at i20◦ and e30◦ for sampleswith compositions Ab100 (Fig. 1a) and Or100 to Or85 (Fig. 1b). The spectra change with the degree of Al,Si order. RBpeaks, which are related to Si-O and Al-O stretching (Iiishi

et al. 1971), broaden and disappear with increasing disorder.In the ordered species (ID 151, 152, 125), four clear bandsand one to two shoulders can be distinguished between8 μm (1250 cm−1) and 10.5 μm (952 cm−1) in the region ofthe RBs. In contrast, the spectra of disordered species showonly two or three broad peaks and two or three shoulders.Spectral differences resulting from the varying Or contentin the disordered samples (ID 73, 129, 149, 150) are onlyminor. The absolute intensities of the RB peaks change withthe degree of order as well. There are no observable RBshifts resulting from Al,Si distribution changes. The meanwavelength of the CFs is ca. 7.83 μm (1277 cm−1) for theOr-rich samples (ID 73, 129, 149, 150, 151, 152) comparedto 7.72 μm (1295 cm−1) for the Ab100 samples (ID 125,163, 164, 165, 166, 167). As an effect of small grain sizes,spectral contrast and overall reflectance are reduced (Fig. 2).Thus, the peaks in the spectral region of the RB in sampleswith a high Al,Si order are weak but still distinguishable.In the spectra of the more disordered species (ID 73, 129,149, 150, 165, 166, 167), the small grain sizes also led toa decrease of the RB bands, so that the spectra are difficultto distinguish. In addition, the TF is clearly visible in thespectra as a large broad peak between 11 μm and 12 μm(909 cm−1 and 833 cm−1).

Discussion

General remarks

In the following, we discuss the infrared powder reflectancespectra results in terms of peak shift and the autocorrelation

Fig. 3 a Band position of thefeature marked in Fig. 1 forgrain sizes between 63 μm and125 μm. Dashed-dotted lineOr100 fit; dotted line Ab100 fit. bSame as a for figure but forgrain sizes smaller 25 μm

458 M.P. Reitze et al.

analyzes for the three different grain size fractions. In addi-tion, we debate implications that arise for remote sensing.

IR absorption spectra show decreasing band intensities,increasing linewidths, and shifts of bands with decreasingAl,Si order (e.g., Zhang et al. 1997). Our powder reflectancespectra exhibit the same changes. Zhang et al. (1997)correlated peak positions and linewidths of absorptionspectra above 16.6 μm (below 600 cm−1) and in the farinfrared around 33 μm (300 cm−1) with Or content anddegree of order of alkali feldspars. In contrast, our studyfocused on the mid-IR, especially the wavelength regionfrom 7 μm to 14 μm (1429 μm to 714 cm−1), which is themeasurement range of the MERTIS experiment (Hiesingeret al. 2010; H. Hiesinger and co-workers, in preparation).

Following the classification of Iiishi et al. (1971), theIR bands between 8.6 μm (1162 cm−1) and 17.2 μm(580 cm−1) should only depend on stretching and bendingof the bonds between Si or Al and O-ions. In contrast, wefound in our analysis, that this wavelength region is alsoaffected by the Or-content. This is probably associated withthe change of the bond length in the alkali feldsparcrystal structure due to the Or-content (Ribbe 1983).

Feature shift analysis

In Fig. 3, the position of the maximum of the spectralfeature, denoted as 15.48 μm (646 cm−1) feature thereafter,that is bounded by two black lines in Figs. 1 and 2 isplotted versus the degree of order,

∑t1. The marked feature

shifts from 15.26 μm (655 cm−1) in pure low albite to15.40 μm (649 cm−1) in disordered high albite. Theseresults are in agreement with the peak shifts in absorptionspectra reported by Zhang et al. (1996) for low albite but

for the high albite sample it is shifted around � = 0.12 μmtowards shorter wavelengths (higher wavenumbers; � =5 cm−1). In microcline with Or100 the feature is at 15.41μm(649 cm−1) and shifts to 15.69 μm (637 cm−1) in highlydisordered sanidine with Or100 (Fig. 3). Zhang et al. (1996)reported an absorption feature in microcline also at around15.43 μm (648 cm−1) and for highly disordered sanidinewith Or100 at around 15.67 μm (638 cm−1), which isin agreement with our reflectance data. These discussedfeatures in the spectral range around 15.48 μm (646 cm−1)should be originating from bending of O − Si(Al) − Obonds (Zhang et al. 1996). The intensity of this bonddepends on the degree of order (Iiishi et al. 1971). Ourresults confirm these findings as seen in Fig. 3a. In addition,the Or-Ab content also affects shape and position of theanalyzed feature in reflectance spectra. Therefore, we fittedthe wavelength of the 15.48 μm (646 cm−1) feature tothe degree of Al,Si order of the samples with Or100 andAb100, respectively. The 15.48 μm (646 cm−1) feature inthe spectra of samples with grain sizes smaller than 25 μm(Fig. 3b) is not as well resolved as in the spectra for thelarger grain sizes (Fig. 3a). In the microcline-sanidine series(ID 73, 129, 149, 150, 151, 152) the 15.28 μm (646 cm−1)feature is only visible as a shoulder. In these cases, fittingof the spectral region of interest was done with a functionof the form R(λ) = a · (b − λ)3 + d · λ + c (R reflectance,λ wavelength in μm). The variation of the band positionwith

∑t1 is about the same for both grain size fractions

(Fig. 3). The slopes and intercepts with the y-axis of thestraight lines are, however, affected by the grain size, whichhighlights the need of powder reflectance spectra for remotesensing (Table 2). The 15.28 μm (646 cm−1) feature shiftsslightly (mean deviation �125−25 ≈ 0.06 μm) to shorter

Table 2 Functions of the different fits calculated with the software gnuplot v.5.2

Correlation Figure Function

Band position Or100 samples –∑

t1 63 μm to 125 μm 3 a −1.42 · λ + 22.91Band position Ab100 samples –

∑t1 63 μm to 125 μm 3 a −2.02 · λ + 31.90

Band position Or100 samples –∑

t1 < 25 μm 3 b −1.11 · λ + 18.00Band position Ab100 samples –

∑t1 < 25 μm 3 b −1.34 · λ + 21.38

Ratio of FWHM of autocorrelation functions (�) – Or-content 63 μm to 125 μm 5 a −321 · � + 403Ratio of FWHM of autocorrelation functions (�) – Or-content 25 μm to 63 μm 5 b −360 · � + 454Ratio of FWHM of autocorrelation functions (�) – Or-content < 25 μm 5 c −428 · � + 568ωGauss –

∑t1 all samples 63 μm to 125 μm 6 a −0.0005 · (26 − ω)2 + 1.02

ωGauss –∑

t1 Ab100 25 μm to 63 μm 6 b −0.032 · ω + 1.9ωGauss –

∑t1 Ab100 25 μm to 63 μm 6 b −0.016 · ω + 1.5

ωGauss –∑

t1 Or100 < 25 μm 6 c −0.12 · ω + 4.3ωGauss –

∑t1 Ab100 < 25 μm 6 c −0.06 · ω + 2.8

λ in μm, � is dimensionless, ω in cm−1

459Mid-infrared spectroscopy of alkali feldspar samples for space application

Fig. 4 Typical autocorrelationfunction (Auto.corr. is the valueof the autocorrelation function)with marked FWHM and fit tothe central peak between -26cm−1 and 26 cm −1 (inset withω of the Gaussian fit): ID 125albite, spectral range 7.27 μm to12.11 μm

wavelengths in the sample fraction with smaller grain sizes.This deviation is 0.01 μm larger than our estimated upperbound of the uncertainty of the feature wavelength of0.05 μm. In particular, two microcline samples (ID 151,152) and the albite sample (ID 125) show larger deviations(�125−25,MICRO,ALB ≈ 0.12μm) from the large to the smallgrain sizes. For all other samples, the mean deviation is atthe upper bound of uncertainty; therefore, the grain size-dependent shift of the feature could be neglected. Based onour study we conclude, that with the feature shift analysis ofthe 15.28 μm (646 cm−1) feature, the degree of order of analkali feldspar can be estimated if the chemical compositionis known.

Autocorrelation analysis

The calculations of the autocorrelation analyses wererelated to the degree of order and the Or content. Figure 4shows a typical autocorrelation function. We tested severaldifferent ways of combining autocorrelation analyses(linear, polynomial, and ratio correlations) and degree oforder as well as Or content. Ratios of different spectralregions are a common way in multi- or hyperspectral dataanalysis to identify distinct chemical compositions (e.g.,Vincent and Thomson 1972; Kirland et al. 2002). In Fig. 5,the ratio FWHM of the autocorrelation function of thespectral range from 8 μm to 9 μm (1250 μm to 1111 cm−1)

Fig. 5 Correlation between Orcontent and �, which is definedas the ratio of the FWHM of theautocorrelation function of thespectral range from 8 μm to9 μm to that of the spectralrange from 9 μm to 10 μm (seedashed-dotted lines in Figs. 2and 3) a Grain sizes of 63 μm to125 μm; b Grain sizes from25 μm to 63 μm, and c Grainsizes smaller than 25 μm. Theyellow data point is sample ID12 with Or62. Although itsupports the linear trendbetween the endmembers it wasnot used for the fit because ofcontamination with quartz

460 M.P. Reitze et al.

to that of the spectral range from 9 μm to 10 μm (1111 μmto 1000 cm−1) (denoted as � thereafter) is plotted againstthe Or content for all analyzed samples and grain sizefractions. Samples with smaller Or contents have a larger �value than Or-rich samples.

One measured sample with an intermediate chemicalcomposition between Or and Ab (Or62) suggest a linearrelationship of � value and Or content (yellow data pointin Fig. 5). However, because of some accessory mineralscontained in the sample, it was not suitable for exactspectral analysis. To prevent uncertainties resulting of thiscontamination we fitted our data with a linear function,without using of the data of the sample with intermediatecomposition. The slope of the fitted straight line and itsintercept with the y-axis increase with decreasing grainsizes, but the overall behavior is the same for all fractions(Table 2). The respective fit functions are Or (mol%) =−321 ·� +403 for samples with grain sizes between 63 μmand 125 μm, Or (mol%) = −360 ·�+454 for samples withgrain sizes between 25 μm and 36 μm, and Or (mol%) =−428 · � + 568 for the smallest grain size fraction.

In Fig. 6, the width ω = FWHM/√ln 4 of a Gaussianfit (note: this FWHM is from the Gaussian fit and thereforesomething different as the FWHM from above) to thecentral peak from -26 cm−1 to 26 cm−1 on the x-coordinateof the autocorrelation function of the spectral band regionfrom 7.27 μm to 12.11 μm (1376 μm to 826 cm−1) isplotted versus the degree of order. The black line in Fig. 6ais a quadratic fit to all the samples with a grain size rangingfrom 63 μm to 125 μm. The fit function to the dependenceof the width ω of the Gaussian fit curve on

∑t1 is given by∑

t1 = −0.0005 · (26 − ω)2 + 1.02 (Fig. 6a). In Fig. 6band c, the results for the grain size fractions from 25 μm to63 μm and smaller than 25 μm are plotted. The analyses

of the smaller grain size fractions led to a split in thecorrelation of ω and and degree of order for Or-rich samplesand Ab-rich samples (

∑t1Or100 = −0.032 · ω + 1.9 and∑

t1Ab100 = −0.016·ω+1.5 for grain sizes from 25 μm to63 μm and

∑t1Or100 = −0.12 · ω + 4.3 and ∑ t1Ab100 =

−0.06 · ω + 2.8 for the smallest grain size fraction). Thisimplies that the grain size distribution has a large effect onthe form of the autocorrelation function in the regions ofthe RB (Table 2). This is probably an effect of the reducedspectral contrast in combination with decreasing reflectancein spectra of samples with smaller grain sizes compared tothose with larger grain sizes. The combination of the effectsleads to a reduced FWHM of the peaks which is visiblethrough the calculation of the autocorrelation function.

Iiishi et al. (1971) stated that the mid-IR spectral regionof alkali feldspars is unaffected by alkali ions and onlyaffected by the degree of order. In contrast, with theautocorrelation analysis we can distinguish not only thedegree of order within infrared reflectance spectra between7 μm and 14 μm but also the Or content. Nevertheless,the reflectance spectral behavior is more complex, becausethe analyzed grain size fractions have an influence on theRBs, which in turn have an influence on the autocorrelationfunction. Therefore, knowing the grain sizes of the samplesis necessary for accurate determinations of Or content anddegree of order from the spectra. Best results were obtainedfrom samples with a grain size between 63 μm to 125 μm.

Conclusions

We investigated reflectance mid-IR powder spectra ofendmember alkali feldspars with respect to their Ab-Orcontent and their Al,Si distribution which were derived from

Fig. 6 Correlation between ω ofGaussian fit to the central part ofthe autocorrelation function ofthe wavelength region from7.27 μm to 12.105 μm (seedashed lines in Figs. 2 and 3)and the degree of order (

∑t1). a

Grain size range from 63 μm to125 μm. The black line is aquadratic fit to all samples. bGrain size range from 25 μm to63 μm. Dashed-dotted lineOr100 fit; Dashed line Ab100 fit,and c Same as b but with grainsize smaller than 25 μm

461Mid-infrared spectroscopy of alkali feldspar samples for space application

XRD analysis. The results of this study showed that theAl,Si degree of order as well as the Or content can bedetermined by reflectance spectroscopy. Thus, these resultsare important and applicable for future space missionscarrying TIR spectrometers.

Mid-infrared spectra of alkali feldspars are stronglyaffected by the distribution of Al and Si on the non-equivalent tetrahedral sites of the frame-work structure andare less affected by the amount of K and Na. Previouswork showed that the positions of specific absorption bandsare related to the Al,Si distribution (e.g., Martin 1970).On the basis of our study, we demonstrate that a specificfeature found in all of our investigated spectra around15.48 μm (646 cm−1), which is in the same spectralregion as investigated by Martin (1970), is a measureof the state of order. This feature shifts from 15.26 μm(655 cm−1) in pure low albite to 15.40 μm (649 cm−1) indisordered high albite and from 15.41 μm (649 cm−1) inmicrocline to 15.69 μm (637 cm−1) in highly disorderedsanidine (Fig. 3). As different means to investigate thedegree of order, Malcherek et al. (1995) introducedthe autocorrelation analysis of absorption spectra. Incontrast to the feature shift analysis discussed above, theautocorrelation analysis, which provides insights into thechemical composition as well as the degree of order, can beapplied to the measurement range of MERTIS. We used theautocorrelation analysis on reflectance spectra for differentspectral regions. The ratio of FWHM of the autocorrelationfunction of 8 μm to 9 μm (1250 μm to 1111 cm−1) tothe autocorrelation function of 9 μm to 10 μm (1111 μmto 1000 cm−1), called �, is linearly dependent on theOr content (Fig. 5). Iiishi et al. (1971) predicted that thealkali ions do not affect the region of the RB in absorptionspectra of alkali feldspar. With our approach it is possibleto determine the Or content of (pure) alkali feldsparsfrom their reflectance spectra. Salje et al. (2000) suggestedautocorrelation of the absorption spectra from 12.11 μmto 7.27 μm (825 cm−1 to 1376 cm−1) for cordierite andfitting of the central peak of the autocorrelation functionwith a Gaussian curve to investigate the degree of order. Weused the same approach to analyze our powder reflectancespectra because MERTIS measures in the same spectralrange and we are also observing spectral changes in thesewavelength region. For samples with grain sizes between63 μm and 125 μm the correlation of

∑t1 and the width

ω of a Gaussian fit to the central part of the autocorrelationfunction is independent of the Or-content of the samplesand thus allows to identify the degree of order withoutprevious determination of the chemical composition. Forsamples with a grain size below 63 μm, however, therelation between ω and

∑t1 becomes dependent on the Or

content.

Depending on the spectral deconvolution of data returnedfrom MERTIS, it will be possible not only to determinethe degree of order of an alkali feldspar, but also itschemical composition. The data gained in this studyas well as the calculated correlations are the basis forfurther investigations of the feldspar system, especiallyfor remote sensing of other planetary surfaces. Furtherwork on the alkali feldspar system will include sampleswith intermediate chemical compositions plus mechanicalmixtures with the same bulk composition.

Acknowledgements We are grateful to Stephan Klemme and Chris-tian Rengli for their help and permission to use their laboratory,to Ludger Buxtrup for technical support with the furnaces, to PeterSchmid-Beuermann for help with XRD at Institut für Mineralogie,and to Uta Rodehorst for advice and the possibility of using the lab-oratory at Münster Electrochemical Energy Technology. Constructivecomments by Charles Geiger, an anonymous reviewer and handlingeditor Anton Beran are gratefully acknowledged. This work is partlysupported by Deutsches Zentrum für Luft- und Raumfahrt funding 50QW 1701 in the framework of the Bepi-Colombo mission.

Funding information Open Access funding provided by ProjektDEAL.

Compliance with Ethical Standards

Conflict of interests The authors declare that they have no conflict ofinterest.

Open Access This article is licensed under a Creative CommonsAttribution 4.0 International License, which permits use, sharing,adaptation, distribution and reproduction in any medium or format, aslong as you give appropriate credit to the original author(s) and thesource, provide a link to the Creative Commons licence, and indicateif changes were made. The images or other third party material in thisarticle are included in the article’s Creative Commons licence, unlessindicated otherwise in a credit line to the material. If material is notincluded in the article’s Creative Commons licence and your intendeduse is not permitted by statutory regulation or exceeds the permitteduse, you will need to obtain permission directly from the copyrightholder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.

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463Mid-infrared spectroscopy of alkali feldspar samples for space application

https://doi.org/10.1524/zkri.1957.109.1-6.204https://doi.org/10.1524/zkri.1957.109.1-6.204https://doi.org/10.1524/zkri.1956.108.1-2.52

Mid-infrared spectroscopy of alkali feldspar samples for space applicationAbstractIntroductionSamples and analysesSamplesSample characterizationSample treatmentInfrared and analysis procedure

ResultsX-ray diffractionInfrared

DiscussionGeneral remarksFeature shift analysisAutocorrelation analysis

ConclusionsCompliance with Ethical StandardsReferences