Devitrifikasi Gelas (Studi Mikroskopik)

Journal of Non-Crystalline Solids 323 (2003) 84–90

www.elsevier.com/locate/jnoncrysol

Devitrification of natural rhyolitic obsidian glasses:petrographic and microstructural study (SEM+EDS)of recent (Lipari island) and ancient (Sarrabus,

SE Sardinia) samples

Domingo Gimeno *

Dept. Geoqu�ıımica, Petrologia i Prospecci�oo Geol�oogica, Fac. de Geologia, Universitat de Barcelona, 08028 Barcelona, Spain

Abstract

Microstructural evolution of devitrification of obsidian glasses is a process not fully understood, especially with

reference to preferred nucleation sites and anisotropic development of spherulites. Evidence is commonly hidden in

advanced devitrification textures. Two sets of naturally devitrified obsidian rocks, calcoalkaline rhyolitic in composi-

tion, have been used to test how this process develops. A petrographic and SEM+EDS comparative study of incipient

devitrified recent obsidian (from Lipari island) and advanced devitrified ancient rhyolitic rocks (from Sarrabus region,

Sardinia island, Italy) allows following successive stages in the generation of K-feldspar spherulites in natural glasses.

Spherulites show a preference for epitaxial nucleation over previous minute idiomorphic crystals and a number of

processes including rearrangement of initial mineral fibers and interstitial voids lead to the formation of blade-like

crystals and denser spherulites. In some cases bubbles with an inferred origin associated to secondary boiling of magmas

also favour spherulitic nucleation.

� 2003 Elsevier B.V. All rights reserved.

1. Introduction

Natural glasses are unusual in the geologicrecord. Most such glass is produced on the earth

surface during volcanic processes [1]. Volcanism

is widespread on the earth surface, but is strictly

related to particular geodynamic settings and

shows local tectonic control [1]. This fact is re-

lated to the required conditions for magma pro-

* Tel.: +34-93 402 1404; fax: +34-93 402 1340.

E-mail address: [email protected] (D. Gimeno).

0022-3093/03/$ - see front matter � 2003 Elsevier B.V. All rights res

doi:10.1016/S0022-3093(03)00294-1

duction and transport from crust or upper mantle

to the surface (positive thermal anomalies, re-

gional decompression, local availability of water)[1]. Most current volcanism is developing on mid-

ocean ridges under submarine conditions and is

mainly basaltic in character; on the other hand,

rhyolitic volcanism is prevalent in convergent

margin plates and back arc environments [1,2].

This situation has been maintained with minor

variations at least since the Paleozoic times over

600 Ma; anyway rhyolitic volcanism is well rep-resented over the entire geologic scale of time

[1,2].

erved.

mail to: [email protected]

D. Gimeno / Journal of Non-Crystalline Solids 323 (2003) 84–90 85

Effusive rhyolitic volcanoes are known in anumber of subaerial environments. They consist

of single to clustered mushroom-shaped high ratio

dome structures, kilometer in size, that in their

initial phases of eruption can erupt massive to

poorly vesiculated rhyolitic obsidians [3]. Analo-

gous rhyolitic volcanoes erupted in a subma-

rine environment have been recognized in the

geological record [4–6] and, more recently, in re-cent sea floor [7,8]. Thus scarcity of rhyolitic glass

in the geologic record is only due to its inst-

ability.

Late devitrification of obsidian occurs when

hydrous fluids, alkalies and secondary heating act

on the glass [9]. Independently of this fact,

obsidian glass undergoes a series of textural

modifications related to high temperature crys-tallization (mainly feldspars and silica phases),

the most prominent being the generation of a

spherulitic texture. This fact has been studied

under the microscope [9–12] but a more detailed

microstructural study is required to understand

the patterns of devitrification and the relation-

ships with other textures. The current study to

describe all the stages of this process of devitri-fication started with the characterization of an-

cient (early Paleozoic) devitrified obsidian rocks

in Sarrabus region (SE Sardinia, Italy) and was

complemented with analogous study of recent

obsidians from Lipari showing an incipient stage

of spherulitic growth.

Explosive silicic volcanism largely exceeds in

volume the effusive one. This difference results inthe case of explosive volcanism in the formation

of large volume of fragmentary vesiculated silicic

glass that is deposited mainly as planar beds on

the earth surface after an excursion through the

atmosphere [1,13]. Rocks made in this way are

labeled pyroclastic and glass fragments can be

totally or partially welded after their emplace-

ment. Pyroclastic fragmentary volcanic rocks areusually devitrified to zeolitic and clay mineral

assemblages; more rarely can undergo high tem-

perature spherulitic devitrification [14] and

therefore some of the evidence provided in this

paper can be applied in their study. We use the

terminology developed in [9] as recently revised in

[14].

2. Volcanic rocks studied

2.1. Sarrabus white porphyries (‘porfidi bianchi del

Sarrabus’)

The Paleozoic sequence in Sarrabus region (SE

Sardinia, Italy) is a thick (up to 1000 m) silici-

clastic and volcano-sedimentary succession de-posited within a shallow marine basin, ranging

from Upper Ordovician to Carboniferous in age

[4,5]. This succession contains a large quantity of

volcanic and volcano-sedimentary rocks with a

predominance of calcoalkaline products (mainly

silicic) distributed in several volcanic episodes

throughout Paleozoic time [4,5]. In a broad sense,

we can classify the �white porphyries� [15] asaphyric to poorly porphyritic rhyolites and

rhyodacites of several volcanic lithofacies (lava

flows, ignimbrites, minor sills, marginal border

lithofacies of compositionally zoned domes and

cryptodomes), while �grey porphyries� (�Porfidigrigi�) mainly correspond to holocrystalline mainbodies of domes of kilometric dimensions and

dacitic (and locally andesitic) character. Most sil-icic magmas were placed on (or near to) the sea

bottom as non-vesiculated domes, dikes and lava

flows; therefore explosive character was greatly

reduced [4,5,15].

All lava flow lithofacies of �white porphyries�exhibit conspicuous devitrification in the spheru-

litic stage [9]. This devitrification was locally de-

scribed [11] near the old silver mine of Tuviois butremained forgotten over a century [4,16]. As a

general rule we can remark that more viscous

erupted submarine dome and lava flow facies (as

inferred by aspect ratio of flows and domes) show

a less developed spherulitic devitrification pattern

as well as poor evidence of magmatic flow, while

low viscosity lava flows of metric thickness are well

banded and show prominent magmatic folds, aswell as marginal borders of intrusive sectors of

domes are sharply spherulitic [4,5,16]. The studied

samples in this work are calc-alkali rhyolites with

Na2O+K2O near to 9% (K slightly exceeds Na).

They come from the Punta Serpedd�ıı, BruncuMauru Lecca, Bruncu Murdegu and Rocca Arri-

celli localities in Western-Central Sarrabus region

and can be considered representative of the entire

86 D. Gimeno / Journal of Non-Crystalline Solids 323 (2003) 84–90

region with an approximate range of age of 450–360 Ma [4].

2.2. Lipari obsidian

Lipari is the most prominent volcanic island in

the Aeolian (or Lipari) volcanic archipelago placed

north of Sicily island, Italy. Rocce Rosse is a

classical site [17] of massive to highly vesiculatedrecent obsidian flows in N of the Lipari island that

shows a very incipient process of spherulitic devi-

trification. The sampling site is crossed by the

eastern road of the island. Rocce Rosse corre-

sponds to a lavic emission from Monte Pilato

volcano, belonging to Cycle X of evolution of

Lipari island and corresponding to an 580 A.D.

age [18]. It is calc-alkalic rhyolitic in compositionwith alkali values near to the 9% (and where like in

Sarrabus samples K slightly exceeds Na contents)

[4,18]. These rocks are glassy aphyric in texture,

massive to highly vesiculated with flattened

(compared to initial spherical ones) [1,13] and de-

formed vesicles and glassy cell walls, flow banded

with prominent flow folds, and possibly also af-

fected by diapiric folds related with gravitativedesequilibrium [19,20]. They originated from early

eruption of coarsely vesiculate obsidian followed

by successive overlapping by massive obsidian, as

suggested by aerial photographs and field features.

Flow banding in black obsidian is locally increased

by sparse spherulitic to continuous white axiolitic

spherulite disposition, spherulites being in general

lesser than 1 mm in size.

3. Experimental

Thin sections of natural glasses and devitrified

obsidians were studied under the petrographic

microscope. Whole rock composition was deter-

mined by X-ray fluorescence (XRF), by means of asequential X-ray spectrophotometer (Philips PW

1400) calibrated with a set of international stan-

dards using fused pearls (lithium tetraborate pearls

at a dilution 1/20). Na2O was determined by

atomic absorption spectrometry (AAS). Loss on

ignition (LOI) was performed in an oxidizing

furnace. The mineralogical composition of the

spherulites was investigated by means of X-raypowder diffraction (Siemens D500). Mineral

characterization of spherulites (microtextural and

semiquantitative chemistry) was carried out by

means of scanning electron microscopy (SEM,

JEOL J3M-840) served with an energy dispersive

spectrometer (EDS) system (LINK Microanalysis)

with variable operating conditions (10–15 kV,

18–33 mm of window conditions). The treatmentincludes dehydration to remove environmental

moisture, mounting on a metal stub with Ag so-

lution, and metal coating with vaporized Au. A

combination of thin polished samples and irregu-

larly crushed sample surfaces of the same samples

was studied. The homogeneity of chemical com-

position of the glasses as well as of the devitri-

fied crystal matrix was controlled by wavelengthdispersive spectrometer (WDS) electron microp-

robe analysis (Cameca Camebax SX-50) at Ser-

veis Cient�ııfico-T�eecnics, University of Barcelona(SCT-UB) calibrated with different natural and

synthetic silicates and oxides of certified compo-

sition. To avoid the effect of alkali migration in

glass irradiated by electrons [21] a strategy of

electron microprobe analysis including defocusingof beam, first testing for Na and a more reduced

time of acquisition in alkali elements to avoid

reaching an incubation time was developed [22].

Results in glass are consistent with those obtained

from whole rock XRF analysis. A set of certified

geological standards provided by Geological Sur-

vey of Japan has been used as internal standards in

XRF analyses; the analytical precision is within±1% for SiO2, TiO2, Al2O3, Fe2O3, CaO, K2O and

MnO, and ±4% for MgO, Na2O and P2O5.

4. Results

Lipari obsidians under the microscope show a

glassy to hypocrystalline character. Most crystalsare acicular to fiber-like microliths that show hy-

alopilitic disposition parallel to magmatic flow

banding. Composition of microliths is not deter-

minable under the microscope in most cases. Since

standard thickness of the section is calculated for

holocrystalline granular rocks where a section of a

crystal occupies all the section apparent crystal

Fig. 3. Thin section of a totally devitrified glassy porphyric

rock from Sarrabus region showing spherulite growth on feld-

spar (F) and quartz (Q) phenocrysts, as well as on the matrix.


content is increased in thin section. By contrast inobsidian we see by transparency a volume of glass

where there are only some small crystals immersed

in. Under the microscope the zenithal view of the

obsidian substitutes the volume that we explore by

a orthogonal projection on a plane (the thin slide

surface) and therefore suggests that sparse crystals

in the glass are much more closer than really they

are. Devitrification is manifested in some poorlyresolved dots ordered following magmatic flux

bands. SEM images (Figs. 1 and 2) show that these

dots are incipient spherulites growing on quartz

idiomorphic crystals; most spherulites are consti-

tuted of irregularly shaped amalgamated fibers,

with a presence of original porosity in between.

EDS spectra show that most of the fibers are K-

Fig. 1. SEM image of an incipient K-feldspar growing on a

quartz crystal, Rocce Rosse, Lipari.

Fig. 2. Close-up of Fig. 1 showing the spherulite surface con-

stituted by packed fibers leaving abundant interstitial cavities.

feldspar, with some intercalated silica phase. XRD

has not provided more details on the composition.

Sarrabus thin sections are now holocrystalline,

with a prevalent spherulitic and micropoikilitic

texture overprinted to a porphyritic hypo- to

mesocrystalline glassy texture (Fig. 3). Phenocrystsare idiomorphic feldspars (K-felspar, Na–Ca

plagioclase) and idiomorphic skeletal quartz.

Spherulites selectively overgrow phenocrysts and

eventually include microliths and dendritic to

skeletal crystalline forms (quartz, feldspar, iron

ore). Matrix ranges from poorly spherulitic with a

micropoikilitic [9] (quartz, feldspar) texture that

includes spherulites, to totally spherulitic. Based

Fig. 4. SEM image of sample from Fig. 3 showing spherulite

growth on feldspar crystal (left).


on petrographic evidence we can infer that somematrix spherulite grew on hollow gas vesicles, now

filled with a mosaic of microcrystalline quartz.

SEM images show an evolved spherulitic texture

mainly composed of thin blades of K-feldspar

(determined by EDS) and in a lesser degree silica

phases (Fig. 4); some residual porosity between

blades also exists (Fig. 5). The final evolution of

this process shows evidence of replacement of theK-feldspar blades for massive laths of the same

composition with associated elimination of po-

rosity (Fig. 6).

Fig. 5. Magnification of Fig. 5. See text for explanation.

Fig. 6. SEM image of spherulite growing on a quartz pheno-

cryst. Spherulite is constituted by compact packed tabular laths

of K-feldspar (Sarrabus region, Sardinia).

5. Discussion

A large number of authors have described de-

vitrification processes in natural rhyolitic systems

[1,10,11,13,14,23,24], but most of the works

available refer to an advanced devitrification pro-

cess. Lofgren [9] produced devitrification of ob-

sidian glass and studied this process andcharacterized it in successive stages: glassy (hy-

drated, with some spherulite), spherulitic and mi-

cropoikilitic. Lofgren [9] also provided most of

available evidence on these processes but his study

did not include SEM studies of devitrified samples.

Several authors have recently used SEM studies

to obtain information on crystallization and de-

vitrification processes in natural rhyolitic glasses[23,24]. Transmission electron microscopy has

been proposed as a good experimental approach to

study it [25]. Most of these reports are about recent

volcanism and therefore do not provide much in-

formation of advanced spherulitic stages of devi-

trification or evolution of devitrification with time.

In Sarrabus samples, aphyric devitrified obsid-

ians largely coexist with moderately porphyricquartz- and feldspar-bearing (devitrified) glassy

rhyolites [4]. Phenocrysts of quartz commonly

show a skeletal character. The so-called embayed

quartz phenocrysts have a large tradition in pet-

rographic literature as being due to corrosion by

magma; nevertheless experimental work [26,27]

has demonstrated that true skeletal quartz phe-

nocrysts are produced by supercooling of rhyoliticmagma, while more rapid supercooling forms of

dendritic silica phases. In Sarrabus samples this

supercooling is consistent with submarine em-

placement of lava flows and associated shallow

intrusion in water saturated unconsolidated sedi-

mentary rocks.

Sometimes it is difficult to distinguish early

crystallization (i.e. under the solidus) in a super-cooled liquid from devitrification processes. De-

spite this difficulty experimental work [9] and

geological evidence [14,28] show that K-feld-

spar spherulites in rhyolitic glasses form during

high temperature crystallization processes. A high

water content greatly increases crystallization [28].

In our cases, macroscopic spherulitic and axi-

olitic textures mimic the magmatic flow banding.


Taking into account that in most cases there isno evidence of differential chemical composition

between bands, this fact might be related to nu-

cleation. SEM images (Figs. 1–6) show that phe-

nocrysts, microphenocrysts and, in general, all

kinds of previous crystalline materials act as

nucleation sites for spherulites. Mineralogical and

chemical composition of crystalline substrate

and spherulitic fibers have no direct relationshipsand therefore spherulites grow epitaxially. The

comparison between Lipari and Sarrabus distri-

bution and abundance of spherulites is illustrative,

since advanced spherulitic devitrification prevents

resolution of processes. Lipari spherulites develop

over idiomorphic quartz microphenocrysts that are

not evident under the petrographic microscope.

Nevertheless, some of the matrix spherulites inSarrabus samples seem to nucleate on gas vesicles;

vesicles that are also arranged following flow

banding. This process can be related to the vis-

cosity of the magma, while [20] the process also

increases the role of crystallization during gas lib-

eration (secondary boiling) [14,24].

6. Conclusions

SEM+EDS study shows, in all studied cases,

that K-feldspar and silica spherulitic fibers selec-

tively overgrow previous crystalline magmatic

phases. Sarrabus samples are Paleozoic in age and

correspond to a submarine volcanism that under-

went supercooling. While the more viscous flowsand lavas show poor development of spherulites

and show microcrystalline devitrification, the inner

sectors of these lavas (and therefore affected by

supercooling to a lesser degree) show incomplete

development of spherulites and in some cases

hollow spherulites (microlithophysae) indicating

secondary boiling [4]. Low viscosity rhyolitic flows

and associated dikes show an advanced spheruliticstage of devitrification as well as marginal sectors

of intrusive domes at contact with host rock [4].

The studied devitrified samples of Lipari and

Sarrabus can be considered successive stages of the

same process. Therefore, this process is described

here entirely for the first time. SEM studies, es-

pecially SEM conducted on irregularly crushed

spherulitic samples (associated with petrographicstudy of thin samples), provide evidence of the

degree of developing of the process and can be

useful in the characterization of ancient obsidian

rocks. These spherulitic textures are particularly

sensitive to deformation and metamorphism and

therefore their widespread finding in cases as in the

case of Sarrabus provides evidence of rare devel-

opment of regional deformation and associatedregional metamorphism.

In the case of archeological pieces e.g. arrow-

heads made in obsidian can be placed on metal

stub and coated with a thin film of Au or graphite,

easily removable, in order to conduct a non-

destructive study obtaining a semiquantitative

chemical analysis as well as eventually good in-

formation of macroscopically non-evident devitri-fication features.

Acknowledgements

This study is a part of a larger one and was fi-

nanced by several institutions: CIRIT of Autono-

mous Government of Catalonia, Spain; Ministry ofEducation of Spain; Ministry of Foreign Affairs,

Italy; and Regione Autonoma della Sardegna,

Italy. It was carried out in the Istituto di Giacim-

enti Minerari (now DIGITA, Univ. Cagliari, Italy)

and Dept. Geoqu�ıımica, Petrolog�ııa i Prospecci�ooGeol�oogica (Universitat de Barcelona, UB). Ana-lytical study was developed at SCT-UB. The au-

thor specially thanks all people who providedvaluable help and council, both in Italy and Spain.

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Documents

Devitrifikasi Gelas (Studi Mikroskopik)