Retrieving magma composition from TIR spectra: implications for terrestrial planets investigations

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Re ie ing magma composi ion
om iR spec a: implica ions o
e es ial plane s in es iga ions
Alessand o pisello1*, ancesco p. Ve e e1,2, Ma eo Bisol a i1, Alessand o Ma u illi3,
Daniele Mo ga i1, c is ina pauselli1, Gianluca iezzi
4,7, Michele Lus ino
5,6 &
Diego pe ugini1
Emissi i y and e lec ance spec a ha e been in es iga ed on wo se ies o silica e glasses, ha ing
composi ions belonging o alkaline and subalkaline se ies, co e ing he mos common e es ial
igneous ocks. Glasses we e syn hesized s a ing om na u al end-membe s ou c opping a Vulcano
Island (Aeolian Islands, I aly) and on Snake Ri e Plain (USA). Resul s show ha he shi o he spec a,
by aking Ch is iansen ea u e (CF) as a e e ence poin , is co ela ed wi h SiO2 con en , he Sc M ac o
and/o he deg ee o polyme iza ion s a e ia he nBo/ and empe a u e. he mo e e ol ed is he
composi ion, he mo e polyme ized he s uc u e, he sho e he wa eleng h a which CF is obse able.
CF shi is also dependen on empe a u e. The shape o he spec a disc imina es alkaline cha ac e ,
and i is ela ed o he e olu ion o Qn s uc u al uni s. Vulcano alkaline se ies show la ge amoun o Q4
and Q3 species e en o ma ic samples compa ed o he subalkaline Snake Ri e Plain se ies. Ou esul s
p o ide new and obus insigh s o he geochemical cha ac e iza ion o olcanic ocks by emo e
sensing, wi h he ou look o in e o igin o magmas bo h on Ea h as well as on e es ial plane s o
ocky bodies, om emissi i y and e lec ance spec a.
A majo challenge in plane a y in es iga ions is he emo e de e mina ion o composi ions o ocky plane s’ su -
ace, o he eason ha his is a key ea u e o assess he geological e olu ion o ha body1. As consequence o
he di icul y o di ec ly sample ocky plane s and sa elli es (excep ion a e Moon and, possibly, Ma s), sa elli e
da a ep esen he only sou ce o in o ma ion. An impo an pa o plane a y s udies is based on using spec os-
copy o cha ac e ize analogue ma e ials, in o de o pe o m compa isons wi h spec oscopic da a ob ained wi h
elescopic and/o in si u me hods. On his conce n, spec al esponse o geologically ele an samples has been
in es iga ed in a ious anges: isible (VIS), nea in a ed (NIR)2–4, mid in a ed (mid-IR) and he mal in a-
ed (TIR)5,6, o o o he ib a ional spec oscopy echniques7,8. A p esen , he in e p e a ion o spec oscopic
da a mainly in ol es models calib a ed using single mine als9,10 o ocks11,12. These models a e hus use ul3,13 bu
incomple e in cases o amo phous ma e ials, such as hypohyaline o obsidianaceous olcanic ocks. Consequen ly,
exis ing models canno be au oma ically ex apola ed o o mula e high- esolu ion hypo heses, especially when
olcanic ac i i ies a e equen and widesp ead. In ac , e usi e and explosi e olcanism ex ensi ely occu ed on
ocky plane s and sa elli es (Me cu y, Moon, Ma s, Io, Ves a), co e ing huge a eas o hei su aces and shaping
peculia mo phology14,15. The e o e, he s udy o he chemical composi ion o ex a e es ial olcanism is o
g ea impo ance o in e he geological his o y o Sola Sys em ocky bodies, only i non-c ys alline ac ions a e
co ec ly accoun ed.
Spec al esea ch on silica e glasses was indeed pe o med in pas imes, wi h a ious aims. Glasses we e spec-
oscopically in es iga ed in he mid-IT and TIR o gene al ma e ial cha ac e iza ion16–21, bu also o pu poses
ela ed o plane a y in es iga ions: glasses we e used o cha ac e ize basal ic composi ions on Ma s and on he
1Dep . o Physics and Geology, Uni e si y o Pe ugia, Piazza Uni e si à 1, 06123, Pe ugia, I aly. 2Ins i u e o
Mine alogy, Leibniz Uni e si ä Hanno e , Callins . 3, D-30167, Hanno e , Ge many. 3Ins i u e o Plane a y
Resea ch, DLR, Ru he o ds asse 2, 12489, Be lin-Adle sho , Ge many. 4Dipa imen o di Ingegne ia e Geologia,
Uni e si à degli s udi Chie i, Via dei Ves ini – Campus Uni e si a io, 66100, Chie i, I aly. 5Dipa imen o di Scienze
della Te a, Uni e si à Roma La Sapienza, Piazzale Aldo Mo o, 5, 00185, Roma, I aly. 6Is i u o di Geologia Ambien ale
e Geoingegne ia (IGAG-CNR) c/o Dipa imen o di Scienze della Te a, Uni e si à Roma La Sapienza, Piazzale Aldo
Mo o, 5, 00185, Roma, I aly. 7INGV o Rome, Via di Vigna Mu a a, 605, 00143, Rome, I aly. *email: alessand o.
[email p o ec ed]
open
Con en cou esy o Sp inge Na u e, e ms o use apply. Righ s ese ed
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Moon22,23, o cha ac e ize al e a ion o eph a on plane a y su aces24,25, o in es iga e possible silica coa ings on
plane a y e ains26–28, o o ocus on spec al ea u e o possible impac glass29. Howe e , no s udy exis s ocusing
on glass as a p oxy o magma ic bodies on plane a y su aces, especially o wha conce ns a ia ion o spec al
esponse o silica e glasses wi h e ol ing composi ions ( om ma ic o silicic) o be ween di e en magma ic
se ies ( om subalkaline o alkaline).
In his s udy, we p opose an in e p e a ion o spec a ega ding wo se s o glass composi ions, ep esen ing
di e en magma ic se ies, showing a iabili y in SiO2 con en (which de ines composi ional e olu ion) and in
alkali con en (which on Ea h can be associa ed o speci ic geodynamic se ings). We ocus on he TIR egion, in
pa icula o wha conce ns he wa eleng h ange om 7 o 14 µm, wi h he aim o iden i y possible sys ema ic
a ia ions ha migh be used o in dep h in e p e a ion o spec oscopic da a.
In 2018 he BepiColombo mission was launched o Me cu y, ca ying he Me cu y Radiome e and The mal
In a ed Spec ome e (he eon, MERTIS), whose goal is o map he en i e plane wi h ~500 m spa ial esolu ion
and ~90 nm spec al esolu ion o a wa eleng h ange om 7 o 14 µm30. Me cu y su ace is hough o be widely
co e ed by p oduc s om e usi e olcanism in i s no he n hemisphe e, known as No he n Smoo h Plains
(NSP)31 o No he n Volcanic P o ince (NVP)32, bu addi ional olcanism- ela ed ea u es a e p esen on o he
egions33,34. Thus, Me cu y could ep esen a challenging example o he in e p e a ion o TIR spec oscopic da a
om olcanic e ains, ela ed o bo h e usi e and explosi e olcanism.
Resul s
Two se s o glasses whose composi ion esembles he ones o wo magma ic se ies we e syn hesized and spec ally
cha ac e ized in he TIR ange. Me hodologies a e ex ensi ely desc ibed in he Me hods sec ion a he end o his
pape . The majo oxide chemical composi ion o expe imen al samples is epo ed in Table1. I is obse able how
e e y pa ame e is linea ly a ying om an end-membe o he o he . Thus, we can s a e ha mixing o glasses
was success ul, esul ing in he expec ed egula a ia ion o chemical composi ions as shown in Fig.1. Calcula ed
i on specia ion is epo ed on Table2.
Spec oscopic cha ac e iza ion: e lec ance and emissi i y. Figu es2 and 3 show he acqui ed emis-
si i y spec a o he Vulcano and he Snake Ri e Plain glasses, espec i ely, o di e en acquisi ion empe a u e.
This s udy will ocus on he spec al ange be ween 7 and 14 µm wa eleng h. In Fig.4, e lec ance spec a a e
epo ed, acqui ed a 20 °C ( oom empe a u e, he eon R.T.) and 500 °C. Spec a a e shown as 1- e lec ance (1-R),
o a be e compa ison wi h emissi i y.
All he spec a p esen a peak o maximum emissi i y (minimum e lec ance) a ~8 µm, whe e emissi i y
is close o he uni y and e lec ance is close o ze o. This spec al ea u e is ela ed o Ch is iansen e ec , which
appea s a he wa eleng h a which he ma e ial e ac i e index app oaches he e ac i e index o he medium,
esul ing in a backsca e ing minimum35. We a e able o e e a his emissi i y maximum caused by Ch is iansen
e ec as he Ch is iansen Fea u e (he eon CF, also p esen in li e a u e as Ch is iansen maximum)3,36. O he
Sample (no
o analyses) SiO2TiO2Al2O3FeO MnO MgO CaO Na2O K2O P2O5Cl To al
SW . % 53.33 0.65 15.29 7.63 0.16 4.15 7.76 5.08 3.04 0.50 97.61
(10) S. D. 0.34 0.02 0.12 0.17 0.03 0.08 0.14 0.11 0.06 0.04
S7 W . % 59.66 0.48 14.55 5.94 0.13 2.85 5.66 4.63 3.77 0.33 0.17 98.18
(8) S. D. 1.03 0.05 0.30 0.08 0.02 0.05 0.05 0.10 0.06 0.03 0.02
S5 W . % 64.08 0.38 14.03 4.86 0.12 2.11 4.38 4.32 4.15 0.29 0.16 98.90
(8) S. D. 0.46 0.02 0.22 0.10 0.03 0.05 0.10 0.06 0.05 0.03 0.03
S3 W . % 67.96 0.28 13.40 3.73 0.08 1.37 3.05 3.96 4.49 0.17 0.16 98.63
(8) S. D. 0.65 0.02 0.30 0.10 0.03 0.04 0.08 0.04 0.07 0.04 0.02
RS W . % 73.96 0.11 12.79 1.98 0.05 0.21 1.07 3.63 5.12 0.04 0.22 99.18
(8) S. D. 0.70 0.02 0.14 0.05 0.02 0.02 0.05 0.08 0.10 0.03 0.03
BW . % 48.03 1.68 15.01 10.98 0.17 7.30 10.55 2.39 0.32 0.35 0.02 96.79
(4) S. D. 0.24 0.03 0.29 0.08 0.04 0.12 0.07 0.09 0.02 0.03 0.01
B8 W . % 53.03 1.48 14.63 9.67 0.16 5.93 8.74 2.52 1.20 0.29 0.01 97.66
(7) S. D. 0.34 0.04 0.29 0.20 0.03 0.09 0.08 0.08 0.03 0.02 0.01
B6 W . % 57.07 1.30 14.09 8.36 0.12 4.50 7.00 2.69 2.09 0.25 0.01 97.47
(7) S. D. 0.45 0.04 0.25 0.16 0.03 0.07 0.05 0.05 0.05 0.04 0.01
B4 W . % 62.64 1.06 13.53 6.89 0.10 3.17 5.20 2.74 2.98 0.21 0.01 98.52
(7) S. D. 0.41 0.04 0.22 0.10 0.04 0.07 0.10 0.09 0.05 0.02 0.01
B2 W . % 67.47 0.82 13.03 5.42 0.07 1.70 3.44 2.79 3.83 0.20 0.01 98.79
(7) S. D. 0.66 0.04 0.20 0.11 0.01 0.06 0.06 0.06 0.05 0.03 0.01
RB W . % 72.28 0.63 12.53 3.87 0.03 0.25 1.61 2.77 4.74 0.16 0.00 98.86
(7) S. D. 0.65 0.05 0.24 0.15 0.02 0.01 0.03 0.06 0.06 0.03 0.00
Table 1. Chemical composi ions o syn hesized glasses epo ed as an a e age o mul iple measu emen s o
each sample oge he wi h S anda d de ia ion (S.D.). In b acke s he numbe o analyses o each sample.
Con en cou esy o Sp inge Na u e, e ms o use apply. Righ s ese ed
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wo ks ocusing on emissi i y and o e lec ance o silica es ha e indi idua ed CF as a use ul spec al ea u e37–39,
and i is pa icula ly e icien when glasses a e aken in o accoun , because o he sca ci y o spec al ea u es in
amo phous ma e ials10,40,41.
The ange be ween he CF and ~14 µm wa eleng h is he a ea whe e main es s ahlen bands, linked o ib a-
ions o bonds ega ding 4-coo dina ed oxides (such as Si and Al), appea 16,19,20. Beyond CF, almos all ou spec-
a, wi h he excep ion o sample B, p esen an e iden ea u e be ween 11.5 and 12 µm as an emissi i y minimum/
e lec ance maximum. This is a diagnos ic ea u e, he T anspa ency Fea u e, (he eon, TF)6 which is cha ac e is ic
o ex emely ine samples42. Appea ance o TF is linked o he su ace/ olume a io o a pa icle, which de ines
he a io o he amoun o sca e ed/abso bed ligh 43. This ea u e is obse ed o be s onge o elsic mine als44
and we obse e TF o be s onge o mo e e ol ed composi ions e en o ou da ase o glasses. E en i TF may
o e p in ou spec al in o ma ion, i is o g ea in e es as such a ea u e is possible o be obse ed in ex a e -
es ial samples, and i migh be a use ul ea u e o de ec ine-g ained e ains and ela i e e olu ion o chemical
composi ion.
Thus, wo a e he main spec al cha ac e is ics on which we will ocus: he shi in wa eleng h o spec a
by aking CF as a e e ence poin , and he change in he ea u es ha de e mine he shape o he spec a in he
7–14 µm egion.
Shi o he spec a. Fo wha conce ns e lec ance, a g adual shi o he en i e spec um om longe o sho e
wa eleng hs is obse ed wi h inc easing SiO2 con en (Fig.4). By using CF as a e e ence poin , his shi can be
quan i a i ely de e mined. CF shows shi s o ~0.5 µm h oughou he en i e se ies. The shi appea s o be sligh ly
sho e o he Vulcano se ies, which indeed has a sligh ly mo e es ic ed SiO2 ange (Table1 and Fig.1).
Fo emissi i y spec a, CF composi ion-dependen shi consis s o ~0.5 µm o each se ies. Mo eo e , a
empe a u e-dependen shi is obse ed, i appea s o be weake han composi ional-d i en shi .
0
2
4
6
8
10
12
14
16
35 40 45 50 55 60 65 70 75
80
Na
2
O+K
2
O (w .%)
SiO
2
(w %)
B8
B6
B4
B2
RB
RS
S3
S5
S7
S
B
Figu e 1. To al alkali-silica diag am (TAS)61 o end-membe s and in e media e composi ions used in his
s udy. Composi ions a e no malized on anhyd ous basis. S and RS e e o he Vulcano Island shoshoni e and
hyoli e end-membe s, while B and RB e e o he Snake Ri e Plain basal and hyoli e end-membe s. De ails
a e epo ed in o he main ex .
NBO/T FeO/Fe o FeO (w .%) Fe2O3 (w .%) SCFM
S 0.36 0.35 2.74 5.08 0.79
S7 0.24 0.34 2.06 3.99 0.85
S5 0.17 0.33 1.64 3.28 0.89
S3 0.11 0.33 1.23 2.55 0.92
RS 0.02 0.32 0.63 1.37 0.97
B 0.50 0.36 4.07 7.27 0.69
B8 0.39 0.36 3.54 6.36 0.75
B6 0.29 0.35 3.03 5.55 0.80
B4 0.20 0.35 2.41 4.58 0.85
B2 0.11 0.33 1.83 3.66 0.91
RB 0.02 0.30 1.17 2.74 0.93
Table 2. Va ious pa ame e s calcula ed o each sample. NBO/T is calcula ed wi h Eq.1, Fe specia ion is
calcula ed aking in o conside a ion ai ugaci y59,SCFM is calcula ed wi h Eq.2.
Con en cou esy o Sp inge Na u e, e ms o use apply. Righ s ese ed
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Shape o he spec a: om CF o 14 µm. By aking in o conside a ion e lec ance spec a, Snake Ri e Plain se ies
shows weake in ensi y o e lec ance and smoo he shape o he spec a (Fig.4b). B sample spec um shows
a unique p o ube ance a he han a peak, oughly cen e ed a ~10.2 µm. A subs an ial spec al shape change
is obse ed as chemical composi ion a ies owa ds mo e e ol ed, SiO2- iche , composi ions. B8 composi ion
shows wo peaks maxima a ~9.8 and ~11.8 µm (TF), wi h a smalle shoulde a ~8.5 µm. As we mo e again
owa ds SiO2- iche composi ions hese peak ea u es become clea e and mo e dominan , and a shi owa ds
sho e wa eleng hs is obse ed. The hyoli ic end-membe (RB) shows a la ge peak cen e ed a ~9.3 µm wi h wo
small shoulde s, espec i ely a ~8.3 µm and TF a ~11.7 µm.
Fo wha conce ns e lec ance o Vulcano se ies (Fig.4a), wo peaks a ~9.7 µm and TF a ~11.9 µm and a small
shoulde a ~8.7 µm a e obse ed o he ma ic end-membe S. As we mo e owa ds mo e e ol ed SiO2- iche
composi ions, hese ea u es shi owa ds sho e wa eleng hs and become g adually mo e e iden and dis in-
guishable, un il clea ly showing h ee peaks a ~8.2 µm, ~9.3 µm and TF a ~11.5 µm o he mo e silicic hyoli ic
end-membe .
Fo bo h se ies o e lec ance spec a, he inc ease in empe a u e o ~500 °C (Fig.4c,d) has no a s ong
in luence on he shape o he spec a o Vulcano se ies, whe eas o Snake Ri e Plain se ies an iden i-
cal inc ease in empe a u e esul s in a s onge con as o he spec a. In pa icula , h ee peaks a e much
mo e clea ly dis inguishable o he wo end-membe s B and RB. Quali a i ely, we obse e a mo e e iden
composi ionally-dependen change in he shape o he e lec ance spec a o he Snake Ri e Plain ela i e o
Vulcano samples. These la e show, ins ead, a g adual and sligh ansi ion.
Fo wha conce ns he se s o emissi i y spec a, hei shape o e laps when simila composi ions and di e en
empe a u e a e aken in o conside a ion (Figs2 and 3). Howe e , a gene al abso p ion inc ease is obse ed wi h
no g ea e ec on he shape o he spec a: his can be app ecia ed in pa icula when aking in o accoun local
emissi i y minimum a ~9–9,5 µm, ha deepens wi h inc easing empe a u e. Excep ions a e B2 and B8 compo-
si ions, a 450 and 600 °C (Fig.3). The gene al empe a u e-dependen inc ease in abso p ion does no seem o be
linea ly co ela ed wi h he inc ease o empe a u e: emissi i y spec a ob ained a 300 and 450 °C a e e y simila ,
wi h a bold s ep when inc easing up o 600 °C. By changing he chemical composi ion a cons an empe a u e,
he obse a ions ha can be made o he e olu ion o emissi i y spec a a e e y simila o he ones ha we e
made o he e olu ion o e lec ance spec a. Peaks and ea u es de ec ed wi h e lec ance a e s ill p esen in he
emissi i y spec a, wi h iden ical ends, bu TF is slowly disappea ing wi h inc easing empe a u e. Mo eo e ,
he e a e mo e ea u es de ec able o he emissi i y spec a a he han e lec ance spec a. As an example, close
o 8.3–8.5 µm, wo ea u es (ins ead o only one obse able in he e lec ance spec a) can be no iced: (i) a peak
ab
cd
810121416
0.76
0.80
0.84
0.88
0.92
0.96
1.00
Wa eleng h (µm)
S
S7
S5
S3
RS
600 °C
810121416
0.76
0.80
0.84
0.88
0.92
0.96
1.00 300 °C
S
S7
S5
S3
RS
810121416
0.76
0.80
0.84
0.88
0.92
0.96
1.00 150 °C
y i issimE
S
S7
S5
S3
RS
810121416
0.76
0.80
0.84
0.88
0.92
0.96
1.00 450 °C
y i issimE
Wa eleng h (µm)
S
S7
S5
S3
RS
Figu e 2. Emissi i y spec a collec ed o he Vulcano (S-RS) se ies in he 7–16 µm wa eleng h ange. Each sub-
igu e ep esen s he emissi i y o he en i e se o glasses, collec ed a (a) 150 °C, (b) 300 °C, (c) 450 °C, and (d)
600 °C. Spec a a e s acked in o de o be e highligh di e ences, wi h a e ical o se o −0.02, s a ing om
he ma ic endmembe (da kes , a he op) down o he mos e ol ed composi ion (ligh es , a he bo om).
Con en cou esy o Sp inge Na u e, e ms o use apply. Righ s ese ed
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a sligh ly sho e wa eleng hs, and (ii) a bulge a sligh ly longe wa eleng hs (Figs2–4). A mino peak can be
no iced a 10.5 µm o all emissi i y spec a. I does no appea o show ele an a ia ions as a unc ion o em-
pe a u e and/o chemical composi ion.
Discussion
Elec omagne ic wa es in e ac wi h solid ma e causing molecules o ib a e by changing he bond leng hs
and/o he angle be ween di e en bonds. This cons i u es he basis o ib a ional spec oscopy and i s applica-
ions10,16, esul ing in he ac ha wa eleng hs o abso p ion/emission a e de e mined by di e en bond leng hs.
The whole in a ed egion (IR) is he ange whe e phenomena ela i e o he abso p ion/emission o ene gy
due o ib a ion o bonds la gely occu . The gene al model used o desc ibe ib a ional beha io o bonds is he
anha monic quan um oscilla o 45. This model es ablishes ha bonds display a disc e e numbe o ib a ions when
hi by elec omagne ic wa e. The e o e, he e is a disc e e numbe o wa eleng hs ha a e abso bed/emi ed due
o bond ib a ion. C ys als a e long- ange o de ed ma e ials easily in es igable h ough di ac ion me hods,
he e o e hey ha e u nished exhaus i e in e p e ed da abases. Con e sely, amo phous/glassy ma e ials a e no
in es igable by di ac ion me hods, and de ailed s udies esul ing in sys ema ic e e ence da a a e consequen ially
sca ce9,30,35.
Al hough glasses lack long- ange o de , sho o medium ange o de a e p esen as a s uc u al ea u e in he
o e all chao ic geome ical a angemen o he elemen s, allowing us o use ib a ional spec oscopy on glasses46.
In e p e a ion o he IR spec a om glasses has been p oposed in li e a u e40,41 mainly conce ning syn he ic
simple composi ions ela ed o ea h as well as e es ial plane s17–29.
In a silica e glass, SiO2 e ahed a a e he dominan s uc u al ea u e. These e ahed a a e linked by bond-
ing oxygens wi h each o he o wi h o he molecula clus e s wi h di e en geome ical a angemen s. These
sho - ange ea u es de ine he mos impo an physical p ope ies o he sys em, such as iscosi y and, con-
sequen ly, chemical di usi i y47–49. Mo eo e , o he elemen s (such as Al, Fe, Ti, e c.) may coo dina e wi h 4
oxygens, unde speci ic condi ions. These elemen s a e known as ne wo k o me ca ions (he ea e , T), whe eas
o he ca ions, which a e no 4-coo dina ed wi h oxygen, a e called ne wo k modi ie s50. The beha io o alkali
elemen s (Na and K) in ela ionship wi h Al and Fe2+/Fe o de e mines impo an s uc u al ea u es o glasses:
alkalis may ac as ne wo k balance s o Al and Fe, so ha he e ahed ical s uc u e is no in e up ed and T-O
bonds become longe 15,16,51. Since alkalis ha e such an impo an in luence on he s uc u e, we ha e chosen o
in es iga e se ies wi h subs an ial di e ence in alkali con en (Table1).
ab
cd
810121416
0.76
0.80
0.84
0.88
0.92
0.96
1.00 600 °C
Wa eleng h (µm)
B
B8
B6
B4
B2
RB
810121416
0.76
0.80
0.84
0.88
0.92
0.96
1.00 300 °C
B
B8
B6
B4
B2
RB
810121416
0.76
0.80
0.84
0.88
0.92
0.96
1.00 150 °C
y i issimE
B
B8
B6
B4
B2
RB
810121416
0.76
0.80
0.84
0.88
0.92
0.96
1.00 450 °C
y i issimE
Wa eleng h (µm)
B
B8
B6
B4
B2
RB
Figu e 3. Emissi i y spec a collec ed o he Snake Ri e Plain (B-RB) se ies in he 7–16 µm wa eleng h
ange. Each sub- igu e ep esen s he emissi i y o he en i e se o glasses, collec ed a (a) 150 °C, (b) 300 °C, (c)
450 °C, and (d) 600 °C. Spec a a e s acked in o de o be e highligh di e ences, wi h a e ical o se o −0.02,
s a ing om he ma ic endmembe (da kes , a he op) down o he mos e ol ed composi ion (ligh es , a he
bo om).
Con en cou esy o Sp inge Na u e, e ms o use apply. Righ s ese ed

6
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In o de o de e mine polyme iza ion deg ee o he s udied composi ions, we use he NBO/T pa ame e
de ined as:
=
−NBO
T
OT
T(1)
whe e NBO s ands o non-b idging oxygen ( he numbe o oxygen a oms no linking wo ne wo k o me s)52, O
ep esen s o al oxygen moles and T ep esen s o al moles o all ca ions ac ing as ne wo k o me s. As o Al and
Fe3+, only moles exceeding ha o alkaline (Na, K) and ea h alkaline ca ions (Mg, Ca) a e conside ed ne wo k
o me s and he e o e included in he calcula ion o T50. Thus, his pa ame e akes in o accoun no only he ole
o he classic ne wo k builde s such as Si, bu also o he ca ions which a e in luenced by alkalis and ea h alkalis.
Ano he use ul pa ame e o be used o quan i ying he s uc u al p ope ies o glasses is he so-called SCFM
pa ame e ( aking i s name om he oxides in ol ed in he de ini ion: SiO2, CaO, FeO, MgO)10,and de ined in
mass p opo ion as:
=+++
SCFM
SiO
SiOCaO FeOMgO
(2)
2
2
This pa ame e akes in o accoun SiO2, which is he main ne wo k o me , and he mos impo an di a-
len ca ions, which a e conside ed as he main ne wo k modi ie s. Since i s de ini ion, SCFM pa ame e has
been widely used o build up in e p e a i e models o plane a y science11,36,42. Howe e , his is a me e empi ical
pa ame e linking chemical composi ion and spec al esponse, and has no s uc u al meaning o amo phous
ma e ial such as glasses. Calcula ed NBO/T and SCFM a e epo ed on Table2.
The shi owa ds sho e wa eleng hs mo ing om he less e ol ed (ma ic) o he mo e e ol ed ( elsic) com-
posi ions is one o he mos s iking ea u es obse ed in ou se o spec a (Figs2–4). This phenomenon is well
known16, and has been in e p e ed as a po en ial ma ke o he e olu ion o he e ahed al ne wo k wi hin he
s uc u e, o he so-called “deg ee o polyme iza ion”. F om compu e simula ions and expe imen al s udies18,46
we know ha , as e ahed a link o each o he , he a e age T-O bond dis ance becomes sho e . Thus, we compa e
he shi in ou spec a wi h: (1) SiO2 con en , because i is he main ne wo k o me , (2) SCFM, which is a widely
used pa ame e co ela ing SiO2 o di alen ca ions, which a e he main ne wo k modi ie s, and (3) NBO/T
pa ame e , which is a pa ame e quan i ying he p esence o ne wo k o ming/ne wo k modi ie s elemen s in he
s uc u e. Figu es5 and 6 show he ela ionship be ween shi o CF ( o e lec ance and emissi i y, espec i ely)
in ela ionship wi h SiO2 con en , SCFM and NBO/T o each o he expe imen al sample.
810121416
0.90
0.95
1.00
d
Wa eleng h (µm)
B
B8
B6
B4
B2
RB
500°C
810121416
0.90
0.95
1.00
b
B
B8
B6
B4
B2
RB
Room empe a u e
810121416
0.90
0.95
1.00
c
500°C
ecna cel eR-1
Wa eleng h (µm)
S
S7
S5
S3
RS
810121416
0.90
0.95
1.00
ecna cel eR-1
S
S7
S5
S3
RS
Room empe a u e
a
Figu e 4. Re lec ance spec a collec ed o (a,c) Vulcano (g een spec a), and (b,d) Snake Ri e Plane (o ange
spec a) se ies. Da a a e collec ed in 7–16 µm wa eleng h ange a (a,b) oom empe a u e (R.T.) and (c,d)
500 °C. Spec a a e s acked in o de o be e highligh di e ences, wi h a e ical o se o −0.01, s a ing om
he ma ic endmembe (da kes , a he op) down o he mos e ol ed composi ion (ligh es , a he bo om).
Con en cou esy o Sp inge Na u e, e ms o use apply. Righ s ese ed
7
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I we ocus on e lec ance shi s epo ed in Fig.5, he same linea ela ionship is obse ed o all he se ies.
As he SiO2 inc eases, SCFM inc eases, NBO/T dec eases, and CF shi s owa ds sho e wa eleng hs. I only SRP
se ies is aken in o accoun , we obse e ha he inc ease in empe a u e shi s CF o sho e wa eleng hs, wi hou
modi ying he slope o he gene al linea i y. This is ue o all he conside ed expe imen al SRP da a. On he
con a y, he Vulcano se ies p esen a sligh ly di e en slope o he wo se ies acqui ed a oom empe a u e and
a 500 °C: he wo ends in e sec a ca. SiO2 = 65 w %, SCFM = 0.89 and NBO/T = 0.18 (Fig.5b,d, ).
Fo SRP se ies, whose chemical composi ion s ongly a ies along he se ies in e ms o SiO2 and alkali con-
en , he inc ease in empe a u e seems o in ol e equally all composi ions in he shi o CF. Fo Vulcano se ies,
whose chemical composi ion does no s ongly a y in alkali con en , (Table1 and Fig.1) empe a u e is di e -
en ly in luencing he shi in CF o ma ic and elsic composi ions. Indeed, conside ing he SiO2- iches glasses o
he Vulcano se ies, an inc ease in empe a u e p oduces a shi o he spec a owa ds longe wa eleng hs.
a
c
e
b
d
50 55 60 65 70 75
7.6
7.7
7.8
7.9
8.0
8.1
8.2
)mµ(h gnele awFC
SiO2 w %
0.00.1 0.20.3 0.40.5
7.6
7.7
7.8
7.9
8.0
8.1
8.2
)mµ(h gnele awFC
NBO/T
0.65 0.70 0.75 0.80 0.85 0.90 0.95
1.00
7.6
7.7
7.8
7.9
8.0
8.1
8.2
)mµ(h gnele awFC
SCFM
50 55 60 65 70 75
7.6
7.7
7.8
7.9
8.0
8.1
8.2
)mµ(h gnele awFC
SiO2 w %
0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
7.6
7.7
7.8
7.9
8.0
8.1
8.2
)mµ(h gnele awFC
SCFM
0.00.1 0.20.3 0.40.5
7.6
7.7
7.8
7.9
8.0
8.1
8.2
)mµ(h gnele awFC
NBO/T
Figu e 5. Pa icula o he Snake Ri e Plain (SRP) and Vulcano se ies Ch is iansen Fea u e CF s. (a,b) SiO2,
(b,c) SCFM and (c,d) NBO/T ela ed o e lec ance spec a. All plo s well co ela ed wi h a linea ela ionship
whose R2 and slope a e p o ided in Table3. Please e e o main ex o de ails and discussions.
Con en cou esy o Sp inge Na u e, e ms o use apply. Righ s ese ed
8
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I we ocus on emissi i y shi s epo ed in Fig.6, he shi ends a e iden ical o hose obse ed o e lec-
ance, bu gene ally shi ed o sligh ly sho e wa eleng hs. An inc ease in empe a u e is always shi ing he
spec a owa ds sho e wa eleng hs. This e ec is sligh ly la ge o in e media e composi ions and o he 450–
600 °C s ep, esul ing in a cu a u e o he se ies o highe empe a u es. Table3 epo s co ela ion coe icien s
R2 and linea slopes o he da a shown in Fig.6. Fo emissi i y, R2 sligh ly dec eases as empe a u e inc eases,
oge he wi h a ligh dec ease in he slope o he ends. Di e en ly o e lec ance, he change in he slope o he
se ies is no deeply di e en o he wo se ies.
The shape o he spec a be ween he CF and ~14 µm ela es on he ib a ion o T-O bonds, in pa icula
Si-O and Al- O16. Re lec ance spec a p esen ed in Fig.5 clea ly show he di e ence be ween he wo analyzed
se ies, especially o oom empe a u e (R.T.) acquisi ions. Fo e ahed al Si, he la ge he amoun o BO, he
sho e he a e age Si-O dis ance, and he sho e he abso p ion/emission wa eleng h. The e o e, i appea s
easonable ha he mos e ol ed chemical composi ions will show ea u es ela i e o sho e wa eleng hs. The e
a e e iews16 epo ing he app oxima e wa eleng hs whe e, despi e he obse ed shi , ib a ions o T-O bonds
occu in ela ionship wi h he ype o s uc u al uni Qn52. Fo Q4 wo ib a ional egions a e epo ed a ound
0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
7.3
7.4
7.5
7.6
7.7
7.8
7.9
8.0
8.1
8.2
SRP150°C
SRP300°C
SRP450°C
SRP600°C
CF wa eleng h (m)
SCFM
0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.
00
7.3
7.4
7.5
7.6
7.7
7.8
7.9
8.0
8.1
8.2
Vulcano150°C
Vulcano300°C
Vulcano450°C
Vulcano600°C
SCFM
45 50 55 60 65 70 75
7.3
7.4
7.5
7.6
7.7
7.8
7.9
8.0
8.1
8.2
SRP150°C
SRP300°C
SRP450°C
SRP600°C
CF wa eleng h (m)
SiO
2
w %
45 50 55 60 65 70 75
7.3
7.4
7.5
7.6
7.7
7.8
7.9
8.0
8.1
8.2
Vulcano150°C
Vulcano300°C
Vulcano450°C
Vulcano600°C
SiO
2
w %
0.00.1 0.20.3 0.40.5
7.3
7.4
7.5
7.6
7.7
7.8
7.9
8.0
8.1
8.2
SRP150°C
SRP300°C
SRP450°C
SRP600°C
CF wa eleng h ( m)
NBO/T
0.00.1 0.20.3 0.40.5
7.3
7.4
7.5
7.6
7.7
7.8
7.9
8.0
8.1
8.2
Vulcano150°C
Vulcano300°C
Vulcano450°C
Vulcano600°C
NBO/T
ab
cd
e
Figu e 6. Pa icula o he Snake Ri e Plain (SRP) and Vulcano se ies Ch is iansen Fea u e CF s. (a,b) SiO2,
(b,c) SCFM and (c,d) NBO/T ela ed o emissi i y spec a. All plo s well co ela ed wi h a linea ela ionship
whose R2 and slope a e p o ided in Table3. Please e e o main ex o de ails and discussions.
Con en cou esy o Sp inge Na u e, e ms o use apply. Righ s ese ed
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8–8.3 µm and 9–10 µm, Q3 shows ib a ion a 9–10 µm, Q2 a 10–11 µm, Q1 and Q0 a 11–12 µm. The ac ha in all
he e lec ance spec a he shoulde a ound 8.4 µm is inc easing wi h SiO2 con en allows us o assign his ea u e
o Si-bea ing Q4 species. The p og essi e inc ease o he peak a ~9.5 µm can also be in e p e ed as a consequence
o he inc ease o Q4 and Q3 species. Mo eo e , he conca e shape isible be ween 10 µm and TF can be ela ed o
a dec ease o Qn. Iden ical obse a ions can be made o emissi i y spec a a ~8.3–8.5 µm. As epo ed abo e, he
ea u e showing sho e wa eleng h is inc easing as he chemical composi ion becomes iche in SiO2.
The de elopmen o spec al ea u es when mo ing om ma ic o elsic composi ions, is clea ly mo e ma ked
o he Snake Ri e Plain han o he Vulcano se ies, whose spec a a e much mo e simila o each o he . I
we look a he TAS diag am in Fig.1 i is clea ha he SiO2 ange is oughly o e lapping o he wo se ies. By
obse ing chemical composi ion a ia ions (Table1) we obse e how Al2O3 and he o al Fe a y cohe en ly
be ween he wo se ies. The e o e, he di e ence be ween he wo se s o spec a may no be explained by aking
in o conside a ion a ia ions o Fe, Al o Si oxides only. In ac , de ailed analyses o he chemical composi ion o
he wo se ies indica e ha he only subs an ial composi ional di e ence is ela ed o he alkali con en . Indeed,
he Vulcano se ies shows a oughly cons an alkali con en be ween 8 and 9 w %, whe eas Snake Ri e Plain sam-
ples show an inc ease in alkalis wi h SiO2. In pa icula , o he Vulcano se ies, he dec ease in Na2O om 5.08
o 3.63 w % is compensa ed by an inc ease in K2O om 3.04 o 5.12 w %, whe eas o Snake Ri e Plain se ies
he Na2O con en is sligh ly inc easing om 2.39 o 2.77 w % and K2O shows a s ong inc ease om 0.32 up o
4.74 w %. Alkali-O bonds do no show ib a ion in he in es iga ed ange, bu , as men ioned abo e, hey play an
impo an ole in de e mining coo dina ion o ca ions de e mining hei ole as ne wo k o me o modi ie , and
he e o e in luencing Qn specia ion. The e o e, a undamen al di e ence be ween he wo se ies conce ning how
much a e seconda y ne wo k o me s like Fe and Al con ibu ing o polyme iza ion eme ges. This migh explain
why spec al shapes om he wo se ies appea di e en in spi e o hei SiO2 anges looking simila .
The inc ease in empe a u e o expe imen al samples shows he ypical e ec s o dec ease in con as and band
shi ing al eady obse ed in li e a u e52. P e ious s udies ha e epo ed how he e iciency o his e ec depends
on ea u es such as he mal conduc i i y and g anulome y30,53.
By ocusing on e lec ance, empe a u e-d i en shi owa ds sho e wa eleng hs appea s o all chemical
composi ion o he Snake Ri e Plain se ies wi h he excep ion o B2. On he o he hand, o Vulcano se ies, he
elsic composi ions show ha an inc ease in empe a u e is shi ing he spec a owa ds longe wa eleng hs. A
simila phenomenon is obse able o SRP se ies o B2 bu no o RB (Figs5 and 6). I is no clea whe he his
should be aken in o accoun as an a i ac o i i migh be co ela ed o some pa icula low ene gy chemical
eac ion in ol ing cha ge-balancing ca ions ela ed o alkaline con en .
As o emissi i y spec a, he mal inc ease is de ini ely mo e e icien in de e mining a CF shi owa ds
sho e wa eleng hs. In Fig.7, he empe a u e-d i en CF shi can be app ecia ed o all he emissi i y spec a.
I is clea ha , e en i no negligible, empe a u e-d i en shi is decidedly weake han composi ion-d i en shi .
Figu es5–7 also show how empe a u e-d i en CF shi is la ge o he in e media e composi ions, especially
in he ange 450–600 °C. This gene a es a sligh non-linea i y o he ends (Fig.5), pa icula ly e iden o he
Vulcano se ies. This non-linea i y can be co ela ed o expe imen al ou ine bias: e en i we a e able o see a
wha empe a u e we a e acqui ing he spec a, we do no know he exac empe a u e o he emi ing body i sel .
Mo eo e , he ex en o empe a u e-d i en spec al shi is sligh ly la ge o glasses wi h highe alkali con en .
F om he spec a obse able in Figs4–6, we can calcula e linea in e pola ion equa ions desc ibing he CF
shi wi h he h ee pa ame e s aken in o accoun . In Table3, esul s om linea in e pola ion o sca e ed
SRP SiO2Vulcano SiO2SRP SCFM Vulcano SCFM SRP NBO/T Vulcano NBO/T
Re lec ance R.T. Eq. CF = −0.0201x + 9.14 CF = −0.0211x + 9.2245 CF = −1.7463x + 9.3489 CF = −2.23x + 9.8309 CF = 1.0016x + 7.6578 CF = 1.2493x + 7.6378
R² 0.9813 0.9984 0.9855 0.9986 0.9904 0.9977
Re lec ance
500 °C
Eq. CF = −0.0194x + 9.077 CF = −0.0177x + 8.997 CF = −1.6863x + 9.2791 CF = −1.8693x + 9.5065 CF = 0.9675x + 7.6461 CF = 1.0473x + 7.668
R² 0.9853 0.9972 0.9903 0.9994 0.9957 0.9986
Re lec ance (bo h
empe a u e)
Eq. CF = −0.0198x + 9.1085 CF = −0.0194x + 9.1107 CF = −1.7163x + 9.314 CF = −2.0496x + 9.6687 CF = 0.9846x + 7.652 CF = 1.1483x + 7.6529
R² 0.979 0.9897 0.9836 0.9908 0.9887 0.99
Emissi i y 150 °C Eq. CF = −0.0215x + 9.0037 CF = −0.0202x + 8.9344 CF = −1.861x + 9.2247 CF = −2.1442x + 9.5232 CF = 1.065x + 7.4232 CF = 1.2052x + 7.4138
R² 0.996 0.9712 0.9979 0.9799 0.9982 0.9855
Emissi i y 300 °C Eq. CF = −0.0219x + 8.9976 CF = −0.0192x + 8.8206 CF = −1.8969x + 9.2243 CF = −2.035x + 9.3774 CF = 1.087x + 7.3876 CF = 1.1419x + 7.3757
R² 0.9867 0.9837 0.9899 0.9892 0.9929 0.9915
Emissi i y 450 °C Eq. CF = −0.021x + 8.9255 CF = −0.0191x + 8.7545 CF = −1.8174x + 9.1415 CF = −2.031x + 9.3125 CF = 1.0394x + 7.3823 CF = 1.1408x + 7.3145
R² 0.9867 0.9365 0.9887 0.9454 0.9877 0.9494
Emissi i y 600 °C Eq. CF = −0.0199x + 8.7702 CF = −0.0182x + 8.6533 CF = −1.7289x + 8.981 CF = −1.9442x + 9.1944 CF = 0.9954x + 7.3059 CF = 1.0967x + 7.281
R² 0.9466 0.9112 0.957 0.9307 0.9689 0.9427
Table 3. Equa ions ob ained om linea in e pola ion o sca e ed da ase s oge he wi h hei ela i e
coe icien s o de e mina ion R. He e, CF is shown as dependen on x, which is espec i ely SiO2 w %, SCFM
o NBO/T. Fo emissi i y spec a, in e pola ion was pe o med only by aking in o accoun each acquisi ion
empe a u e sepa a ely. Fo e lec ance, in e pola ion was pe o med by aking in o accoun wo di e en
acquisi ion empe a u es bo h as wo di e en da ase s ( i s wo ows in he able) and as a unique da ase
( hi d ow).
Con en cou esy o Sp inge Na u e, e ms o use apply. Righ s ese ed