Weathering of volcanic rocks from Mt. Vesuvius associated with the lichen Stereocaulon vesuvianum

9 Pages·1991·1.2 MB·English

by  Jean-François Ponge

l'.:doh1olng1a 35. 2()') - 217 ( 199 I 1
Ciu'lil' Ftschi:r Vi:rla!,! Ji:na
l1i-111u10 d1 Ch11111ca Agrarn1. Untvi:r~ila dcglt S111d1 dt Napoli. Port1c1. l1:il1a
Weathering of volcanic rocks from Mt. Vesuvius associated
with the lichen Stereocau/011 ves11via1111111
PAOLA ADAl-.H> and P. V101 A"-TI
W11h 10 figure'
1/ \cci:r11cJ: 90-0-1-0 I )
I. Introduction
The effecti\t.:ness or lichen!> as agents ror rock \\eathenng anti :-.oil formation ha:- long
hcen recogn17cd (FRY. 1927: .IAC"KSO:-. & KELLER. 1970: ASCASO ('/al .. 1976: JONES l'/ 11/ ..
1980: GAL V '' C'I 11/. . 1981: JONES C'/ 11/ .• J 981 . WILSON e/ 11/ .• 1981 : WILSON & JONI'.S. 1984:
.fo,i:s & W11.so:-.. 1985). Lichens arc among the first organisms to coloni1c a freshly 9poscd
rock surface. Such lichen-encrusted rocks provide an ideal environment ror stud1ng the
Oll)geochcmical weathering or mincrab !>Ince. unlike the !>ltUalion in soil. the 1111luc11cc nr
11nl\ two orga111~m' the alga and the fungus on any one mineral. ha\C tl> hl· invcst1gatctl.
S)Il~S & ISKA'-'DAK (1973) suggest that it 1s convcnienI to evaluate the pcdogc11i.:11c
or
sig111fica nee lichens in terms or the biogcophy~ical and biogcm:hemical proCCS!>CS i 11\ olved
1n the weathering of rocks. The llHht important mechanisms implicated in the physical
\leathering o!" the '-Uhstrntum arc·
rh1z1ne pi:netration.
thallus exransion and contraction.
The main chemu.:al processes b\ which lichen., arc .ihlc tn dccomrose mmeral'> .trc:
till' production of col
the excretion of oxalic acid:
the prodUL'tion or \\<llCr--;olublc b1ochcmical COlllpOUnd~ CidJeJ lichen dCiUs.
In the pre!>ent parer the changes mduccd by the growth or the frutico~I.' lichen
)terc'1JC<111!1111 re\11ria1111111 on lcucitic rock' of \!It Vesuvius lwve been studied.
2. lVlatcrials and methods
Sten'11n11tl1111 1c11ir1c11111111 (Pl·KS.J '' \\ldi:>pri:aJ <Ill kuctlc-hea11ng rocks 1>1' Mt. \'csll\lll,. 12 km
"1u1h-ca>l or N.q1k, in thi: Campania Region.
S:1111pk' or 11ch.:11. ltl"i1cn1/cd volca111c rock .ind 'uh,tralc .il thi: 1n1crfocc hi:I\\Ccn 1hc 1ni:k and
thi: lti:hen 1hallu' "i:rc c11lli:c1c<.l. Oplu:al .1nJ-; I· \1 0Ji,i:na1io11' \\Cre made upon.
fr<1gmi:n1' or l1chi:n-i:ncru>lcJ rock.
1111ni:ral grain' remaining ai"lcr h~drogcn pcrnx1di: 1rca1111cn1 or 'crapcd wi:alhcrcd material hcnealh
1hc· hi:hen:
the rock ~urrace 1mmcd1a1ely hclO\\ the· lichen
for 'cann111g electron microscopy the· 'ampli:' \\i;rc mounted \\llh colloidal carbon 1>n alum1111u111
'tuJi,. ''er.: LOalcd \\ith carbon or \\llh gold and ''.:re cxam1n.:J in a Cambridge Sti:ri:o>can 250
\111..2 instrument c4u1ppcd with an energy d1spcrs1\e X-ray analyser. In order lo 'ludy closely the
na!lm: .ind rnmpo,ition nf the wca1h.:rcd material al the intcrraci: bciwc.:n the rock :111d thl' lichen
th;ilJu,. the \Ill. da~ and lino.: cla~ (0 < 0.5 um) fractions were scparat.:d. aft.:r 11~0~ 1rca1111i:n1. from
Ihc ,i;rnpcd ~uh,trale hcne<tlh the lichen
14
These frac11ons were examined by X-ray diffraction, transmission electron microscopy and l.R.
spectroscopy.
A Philips 2k W X-ray <.l11Trac1om..:tcr with iron filtered Co Ka radia11on was used to record X-ray
po\\lder patterns. For the T.E.M. observations a Siemens Elmiscope 102 instrument was used.
lnfrared spectra \\/ere recorded with a Perkin Elmer 567 Spectrometer using 13 mm-diameter K Br
pressed disks containing 0.8 mg of sample.
Dry deposition collected by \\lashing the fresh thallus samples was analyzed for organic and inorganic
anions hy ion chromatography using a Dionex SP/2000i instrument (analytical column AS4A: guard
columnAG4A.lih..:r~uppressor).Thcclucnluse<.lwas0.75x 10-J M NaHC0 in2.2x 10 3 M Na~CO,
3
al a flO\\ rate or~ cm3 min I with a pump pressure or 7000 k Pa.
3. Results
3.1. Optical observations
The substrate rock is a vesuvite. The dominant minerals are·leucite, occurring as large
pseudo-isometric crystals and exhibiting extremely weak birefr;(~gence with low first-order
grey interference colours and low relief, and augite, readily identified by its optical properties.
Both leucite and augite crystals are surrounded by the dark-grey, homogeneous, microcry
stalline matrix. which consists largely of calcium-rich plagioclase feldspar.
The interface between the rock and the lichen is not well defined and there is clear
evidence of disruption of the rock surface. The "roots" of the lichen had penetrated deeply
into the rock
3.2. S.E.M. observations
The rock-lichen 111terface is characterized by extensive disintegration of the rock surface
with separation of numerous rock fragments (figure I a).
Many of them are located immediately beneath the lichen thallus, and others are
incorporated into the thallus (figure I b). The X-ray spectra from all of these mineral
fragments are dominated by aluminium, silicon, potassium and calcium but appreciable
amounts of iron, magnesium and sodium can also be detected (figure 2 a, b, c).These results
indicate that the mineral fragments are fragments of vesuvite substrate. Never1hless some
Fig. I. Scanning electron mi,crographs (SEM) or volcanic rock-S1ere9ca11/011 ve.mviam1m lichen interface
5howingex1ensivc disintegration of the rock surface (a): volcanic rock fragments in lichen thallus (b).
210 Pedob1ologia 35 ( 1991} 4
Si
Al K
a
Ca
Mg Al
Fe
b
Si
Al
Fe c
d
Ca
e
0 2 4 6 8 10
keV
l71g 2. X-rn) \pcctra of mineral fragments d1rcetl) beneath lichen and 111 the lichen thallus. Leuc1te
ta). aug11c (bJ: plag1oclasc feldspar (c); quart? (d): calcium oxalau: (?) (c).
fragmenb observed 111 the lichen thallus yielded X-ray spectra completely dominated by
silicon !figure 2d) and calcium (figure 2e).
The surfaces of mineral grains found al the rock-lichen interface or embedded in the
thallus are extensi\ely etched. Electron probe microanalysis enabled differentiation of the
separate minerals. these being mainly leucite and augite. Leucite is characterized by rounded
etch pits (figure 3a), and augite by two sets of cleavage planes at right angle to each other
(figure 3bl
Pcdob1ologia 35 ( 1991) 4 211
Fig. 3 Scann1ng cli.:<.::rnn micrograph~ tSFM) of wca1hcrcd mineral grain> al the ;od.-lic:hcn 111tcrfacc
,md Ill thc thallu; or St('r('O('(lll/011 ('('\/ll'/(/////111. l~lllllldcd etch pit~ in lcucite ta): ctehcd ckavagt' ph1111.:~
or
augite lh)
Q
3.34
L
UNORIENTED Au 3.26 L
2.9..8._ PI 3.43 )
3.21
PI
3.75 Q L
4.24 5.36
ORIENTED
45 40 35 30 25 20 15 10 5 29
i·1g. -l X-1a\ dilfrac.:1,imc1cr trace' (CoK -i. 1adia11011) of the ~lit fractinn fwm thc wcalhcrcd 111a1crial
hcnc;11h 1hc ·lichen. Unnrrcntcd (al and oricntcd mount lh). L = lcm:itc. /\u = augilc. PI = plag1<1clast'
feldspar. Q = quarl/. K = kaolrn11c. I = rll11c
212 Pcdnh1t1log1a J5 t 1991l .i
3.3. X-ray diffraction
Thi! unoricnted and oriented mounts of the silt fraction from the weathered material
beneath the lichen yielded X-ray dilTractometer traces which could be interpreted in terms
of a mixture of rock-forming minerals. according with the E.D.X.R.A. results presented
a hove (lil!ure 4a. b)
The ;cncct1ons which occur at 5.36. 3.43 and 3.26A [l A= I· I0-10 m = IOnm]
c.:nrre~pond to the main diagnostic rencctions of leucite, at 2.98. 2.94 and 2.52 A ln augite
X-ra;. rcnect1on:.. at 3.75. 3.2 l and J.18 A lo plagioclase feldspar lines.
'\. 'imall amount of quart7 also occurs. as indicated by the diagnostic peak~ at
4.24 and ·u4 A. The presence of this mineral in the thallus clearly indicah.:s that
u1ntJmi11at1on from acnlian ~ourccs had occurred. a~ quartz docs not occur in the sub-;tratc
rock
The LJ11ented mounts prepared b) scd1mt:nting the :.ill fraction on glass slides reveal
Jl~L) tht! presence L)f 'a nous cla) 111111erals (ligure 4 bl.
X-rn) d1ffrac11on traces of the clay fraction of the weathered material beneath the lichen
c.:onlirm the cla) m111erals presence (ligurc Sa). Ethylene glycol solvation and heating at
550 C demonstrate kaolinit..-: and illite presence (ligure 5b-e). A rather high background
al about 3.5 4.5 A :.uggests the presence of amorphous material. This 1s consistent with
X-ray d1ffract1on observations on the line clay fraction (0 < 0.5 µm) which indicate the
presence of appreciable amounts of amorphous material.
A
The rencctions at 6.20 and 4.45 in the X-ray diffractometer traces of the cla) fracllon
and the renect1ons at 5.95 and 2.97 A 1n the X-ray diffractometer trace of the powdered
thallus (ligure 6) suggest the presence of the monohydrate and dihydrate forms of calcium
oxalate.
Ethylene
Glycol
35 30 25 20 15 10 5 20
1-i!!. 5. '\-ra\ d1ITrac.:1om..:1cr 1ral:cs (CoJ... :x rad1a11onJ of 1hc c.:la~ fra..:11011 from 1hc wcallu:rcd ma1crial
h.:~1..::i1h 1hc i1d1c11. Orn:n1cd moun1 (a). c1hylcn.: glycol solva1ed (b) and healed al 550 C (c). L = lcuc11c.
\u at11!1h:. PI - plag11H:la~c feldspar. 0 = quart7. K = kaolini1c. I = illi1c. CaOx = cal\:111111 oxa-
1:11,
Pcdob1olog1a 35 (1991) 4 213
45 40 35 30 25 20 15 10 5 20
Fig. 6. X-ra) d1ITrac1ometer trace (CoK:x rndiallon) of powdered lichen thallus. K = kaolin11e. I = 1ll1tc.
0 = <.1uanz. Ca(h = calcium oxalate.
•
a
•
..---4
0.1µm
Fig. 7. Transmission electron micrograph (TEM) of fine clay fraction (0 < 0.5 µm) (a); electron
diffraction pattern from selected area of microaggregates (b).
Al
0 2 4 6 B 10
keV
Fig 8. X-ra) spectrum of fine clay frac11on (0 < 0.5 pm).
2 I 4 Pcd0b1ologia JS ( 1991) 4
1720
-~·------..1 1620
COOH
CaOx
500
K.I
1020
K,I
4000 3000 2000 1600 1200 BOO 400
WAVENUMBER (cm-1)
Fig. 9 lnfrared spectra or fine clay fractmn (0 < 0.5 µm).
3.4. T.E.M. observations
Tran:.mission electron mtcroscopy reveals that the line clay fraction (0 < 0.5 µm)
consists of microaggregates of very finegrained particles (figure 7a). These microaggregates
give a weak electron difTraction pattern with diffuse and poorly defined rings (figure 7b)
consistent with amorphous material and in agreement with X-ray diffraction results.
Microanalysis or these microaggregatcs showed that they contain considerable amount
of aluminium and silicon (figure 8)suggesting the presence or amorphous alumino-silicates.
3.5. I nfrared spectroscopy
lnfrared spectro:.copy confirms the presence or kaolirnte and il11tc. The rather wide band
at 3500 cm- 1 is assignable to the presence of poorly-ordered material (figure 9). Additional
bands at 2930. 2850. 1750. 1620 and 1320 cm - 1 can be auributed to organo-mincral
complexes and to small amounts or calcium oxalate. It is concluded, therefore, that the
finegrained material rrom the rock-lichen interface consists of a mixture of rock-rorming
minerals, or clay minerals and of poorly ordered alumino-silicate.
3.6. Jon chromatography
The concentration of oxalate detected in the dry deposition ranged between 4.64 and
7.50 ppm (figure I OJ
4. Discussion and conclusions
All the results prc:.ented provide information on the physical and chemical processes of
weathering associated with the growth of S1ereoca11/011 ues11via11w11 on leucitic Ml. Vesuvius
rocks. The optical observation on the interface between the rock and the lichen clearly
Pcdob1ologia 35 ( 1991) 4 215
2
S04-
NAME ppm
F-
F - 4.24
c1- 17.82
NOJ 0 90
HP042- 1.69
so/-
27.21
2
C204 - 6.29
u
ell C20/-
~
0 2 4 6 8 10 12
Fig. I 0. Dr} dcpo~ll1on. typical chromatogram.
~hows a ,·er) deep physical disaggregation or the substrate rock by the lichen covering.
Such intense disaggregation is a result of both the physico-chemical properties of the
substrate rock .ind the nature or the lichen thallus. The mineralogical composition or la vie
rock and the presence 1n ii or many vesicles and less coherent areas make the penetration
llf the pscudopodellO .ind 1ls ramilicallons easie1.
The t) p1cally strong adhesion of Stereocau/011 re.1·111w11111111 to the substrate added 10
the thallu::. c\pansion and contraction. promote disaggregation of the rock surface.
the detachment or mineral fragments and their incorporation into the thallus. Th<:
chcm1<:al - \H h1ogc:ochcmical decomposition of rock substrate proceeds al the sam<:
lime a~ the ph) su::al - or b1ogcoph~ sical - disintegration, as shown by the extensive
,urface c1eh111g in 1he grams incorporated into the lichen thallus and in the rock ~urfaccs
immediately hclo\\ the lichen. Thc only secondary products which seem to have formed
as a result of ~ui:h chemical weathering are poorly ordered alumino-silicate materials
and \ariou~ c:la~ minerals. These products occur intimately admixed al the rock-lichen
llllerfilCC.
In these c:1rcums1ancc\ most. if not all. or above changes could be accounted for the
reactions or lichen acids with thc mineral particles. The present study does nol provide
cnough evidence to consider calcium oxalate a product of lichen weathering of rock forming
m111crals. Calcium oxalate crystals were not observed by Scanning Electron Microscopy
(SEM) and. furthermore. the treatment of weathered material beneath the lichen with
hydrogen peroxide could produce oxalates. Nevertheless it is not possible lo exclude
completely the effect~ or oxalic acid as well as of other low-molecular-mass organic acids
rroduced by the mycobionl
5. References
,,, ,,, I. l . .I lu\I \A'· & c. OK 11 (.,\. 1976 The pc1lt1gcrm: .ll0t1nn or 1'110111•/1111'1111.\/l<'F\I/. Rlr1:11u1rp1111
s:e11grnpl11c 11111 and l.'111hilte·11rw p11.110/1110. L11. .: he1wlogist 8. 151 - 171.
l-K\. f: J .. 1927 The 111ccha111cal action 111' l'fl"taceou~ lil'hcr1' on suh:.trata of shale ,c111,t. gnci".
hmc,tone ,111d oh,1c.l1an /\1111.1b of 13ot.rn~ -H. -137- 460
(j.\I ''"'· J. . c. ROl>RI(,! 17. & c. AS<"A!>O. 1981 The pe<logc111c actlllll or lichens 111 111etan111rph1L'
rnt:k,. Pcd11h10log1a 21. (\() 7J
I\( KSO • T ,\ . & w I) KH 11 K. 1970. ,\ t:omparall\'C ~tllll) or the role or lrt:hClb .1nd the "'111org.rn1t:00
procc"c' 111 the chcmrl"al l\Cathcnnt! of rct:cnt I la\\a11an hl\a ll1l\1~. American Journal ol Scrcnt:c
269 +t(l- 4(\(i
Jo"~- D . \I J \.\'11 \<>'. & J M T.\tT. 1980 Weathcr111g of a ha,ah h) 1'1•r111.111n11 , ora//11111
L1t:hcnolog"t 12. 277 - :!89
& W J 'v1cl1AKl>Y. 1981 Lichen \\Catheringol'rnd.-fonn1ng 1111nc1ab: applrt:at1on ol'st:annrng
ckctrnn nrn:ro-.cop) and m1croprohc analysis Journal of Mrt:rnscop) 124. 95- 104
1985 ( hc1111cal actl\it\ oflrchcn~11n m111cral surfaces· A rc\le\\' lntcrnatronal Brodctcrioratrnn
21. 2. l/9 - I 04
ur
S\ I K~. J K. & I K hi.. '")AK. 197J J>o.:Jogenctrc ~1g111lit:am:c lrchc1h In \I All\I \l)Jf,\' & M
l. I I \I I (L·d,.). 225 248. Ne" YorL London. At:ademK· Pre~'
W11so'. M J . IJ Jo"'· & W.J. McllARln. 1981. The 11cathcringol°>crpcnt1n1tc h) l.<'rn1111rac11r11
L1chcnol,1g.1,t I J. 16 7 - 176
61. . 191\4 fhc Ot:t:urrcm:c and s1g111(il:allt:C of lllilllganC'L" oxalate 111 J'crltlHll'ltl c m11//111t1.
l'i:d11h1olog1rn 26 171-379
S) nopsis: Ong11wl Hie111i/11 1111pi·r
\l>A\10. p. & p \ 1111 ·\"I. 1991 \\'i:athcrrng of volcanrc rot:k, rrom ~1l. Vc,11\lll' .1,snL·1atcd \\Ith
s,,.,.""'
thi: lrchcn c111/1111 l'l'.\tll'ie11111111 Pc:doh1olog1a 35. :!09-::! 17
The wca1hcri11g phenomena n.:sulling rrnm the coloni1.atio11 or lcucitH.: rot:~s or Mi. Vcsuvru~ by
Sl<'rt'1111111!tu1 11•1111·1111111111 11'1 KS.) lit:hi:n ha\ c bci:n 'tudicd Lichen "i:a t hcnng n:,ull~ 111 thi: disrupt ion
of the roe~ ,ub~t rntc .1nd rn the ex tcn-.,rvc i:tchrng and degradation or the primary rnc~-formrng 111111crab
panicularl) kucrtc and augite to ~idd poorl) ordcrcd alu1111110-sif1catc matcrials and chi} 111111.:ral~. It
1s suggi:stcd that thi: \\cathcnng prot:cs' could hi: .1ccou111i:d for thi: reactions of lichrn acid., with the
mineral paniclcs Ne1crthclcss rt rs not po.,siblc to exclude complctd) the effect; of oxalu: acid ;i-\\t:ll
.1s of Other low-11wlct:11lar-ma" organil acids produced h) th<: mycob1011l.
Kc) ''ords: \\cathcring lc1Jl:1t1c rock. lichen. lichen acids. oxalic acid. mvcobion1
\ddrc!>' of the corn:,ponding authorc": Dr l'\OI A AIJ\\IO. ht1ll1to dr C.h1m1c.1 Agrarr.1 \l11't1tutc
'lf 1\grrcult11ral Chcm"ln l. U111,cr.,1ti1 dcgh Studr J1 :qwlr. l'onic1. 1-X0055. lwlra.
Pcdobiolog1a 35 r 1991) 4 217