ORIGINAL ARTICLE

Analysis of fibrous zeolites in the volcanic deposits of the ViterboProvince, Italy

Andrea Cattaneo • Andrea Rossotti •

Giorgio Pasquare • Anna Somigliana •

Domenico M. Cavallo

Received: 21 September 2009 / Accepted: 26 September 2010 / Published online: 9 October 2010

� Springer-Verlag 2010

Abstract Since Etrurian age, the Viterbo Province

(Central Italy) is famous for its ancient towns carved out of

local ignimbrite deposits which geologically represent the

sedimentation of pumice-rich, volcano-related pyroclastic

flows. Almost the entire study area is geologically charac-

terized by a thick succession of ignimbrites, tephra fallouts

and lava flow deposits locally subjected to zeolitization.

Among zeolites, fibrous erionite represents a well-known

health hazard and so this work aims at locating and quan-

tifying the presence of fibrous zeolites in volcanic deposits

of the Viterbo area, suggesting at the same time a standard

operational procedure useful in other areas showing the

same possible hazard. 41 samples collected in the Viterbo

area were analyzed by X-ray powder diffraction, optical and

electron microscopy. Fibrous zeolites were detected in five

samples and quantified in amounts ranging from 0.35 to

1.66% vol. They generally occur in tetragonal or ortho-

rhombic prismatic habit and showed chemical composition

mainly consistent with K-phillipsite. Fibrous zeolites

occurred with aspect ratios (mean = 6.3), comparable or

lower than those of erionite fibers reported in previous

studies and mean diameters[3 lm.

Keywords Fibrous zeolites � Viterbo Province � Volcanic

deposits � X-ray powder diffraction � Scanning electron

microscopy � Digital image processing

Introduction

Geological setting

Viterbo is geologically located on the northern sector of the

Roman Comagmatic Province (RCP). RCP is characterized

by a large amount of Quaternary potassic and ultrapotassic

volcanic deposits (Marra et al. 2004) covering the Meso-

Cenozoic sedimentary substrate and extending over the

greater portion of the Thyrrenian margin of Central Italy.

The RCP products were originated by strongly explosive

volcanic activity associated with lava flows. Such large

magmatic production is responsible for the creation of dif-

ferent volcanic districts distinguishable on the basis of their

respective ages, lithology, morphology and eruptive styles.

The Cimino Volcanic District is the oldest volcanic

district in the area of Viterbo. The K/Ar radiometric age of

this lava dome complex ranges between 1.35 and 0.95

million years (Ma) (Nicoletti 1969). More than 50 lava

domes are still recognizable, whose growth was associated

with violent explosive activity and with the emission of

huge pyroclastic flows.

A. Cattaneo (&)

Department of Occupational and Environmental Health,

Universita degli Studi di Milano, Via San Barnaba 8,

20122 Milan, Italy

e-mail: andrea.cattaneo@unimi.it

A. Cattaneo

Unit of Epidemiology,

Fondazione IRCCS Ca’ Granda–Ospedale Maggiore Policlinico,

Via San Barnaba 8, 20122 Milan, Italy

A. Rossotti � D. M. Cavallo

Department of Chemical and Environmental Sciences,

Universita degli Studi dell’Insubria,

Via Valleggio 11, 22100 Como, Italy

G. Pasquare

Department of Earth Sciences ‘‘Ardito Desio’’, Universita degli

Studi di Milano, Via Mangiagalli 34, 20133 Milan, Italy

A. Somigliana

Environmental Protection Agency of Lombardy Region (ARPA),

Via Juvara 22, 20129 Milan, Italy

123

Environ Earth Sci (2011) 63:861–871

DOI 10.1007/s12665-010-0756-3

The Vulsini Volcanic District (VVD) extends along the

Central and Southern Italy and represents the largest vol-

canic district considered in this work (Fig. 1). VVD vol-

canic activity probably started from 1.3 to 0.9 Ma

(Martinelli 1967; Nappi and Valentini 2005) with the

production of lava flows and scoria cones emitted by fis-

sure vents in the western sector of the district. In a second

stage a large conical central edifice was built. Its successive

collapse, followed by an explosive volcanic activity,

formed a large caldera now filled by the Bolsena Lake

(Varekamp 1980).

The third district, called Vico Volcanic District, is

located on the southern flank of the Cimino volcano

(Fig. 1). Its earliest products, dated between 0.82 and

0.6 Ma (Nappi and Valentini 2005; Nicoletti 1969), are

represented by latitic and trachitic tephra associated with

phonolitic and tephritic lava flows. On top of these deposits

the volcanic activity followed with the production of a

large quantity of lava flows responsible for the develop-

ment of a central volcanic edifice. Afterwards, a second

explosive cycle produced four large pyroclastic flows

which triggered the collapse of the main edifice and led to

the formation of the Vico caldera. After the formation of

the Vico Lake in the caldera basin, between 0.14 and

0.095 Ma, a series of explosive plinian events led to the

emplacement of thick pyroclastic flow deposits covering

the area (Locardi 1965; Martinelli 1967).

Zeolite minerals

Zeolites are secondary minerals (Na, K and Ca hydrated

Al-bearing tecto-silicates) formed exclusively under high

alkalinity (pH [ 7), low temperature (\300�C) and low

pressure conditions, due to diagenetic, deuteric or hydro-

thermal processes affecting volcanic rocks. Although eri-

onite is the most studied fibrous zeolite owing to its related

health outcomes, many other fibrous zeolites such as

offretite, mesolite, mordenite, natrolite, paranatrolite, tetr-

anatrolite, gonnardite, scolecite, mazzite and thomsonite

can be found in amygdales or in the groundmass of vol-

canic rocks or in soils. Erionite can be chemically

expressed as K2(Ca0,5Na)7[Al6Si27O72]�28H2O although K,

Na and Ca relative ratios can be highly variable.

Very little presence of erionite in Italian volcanic

deposits was reported so far (Passaglia et al. 1974; Passa-

glia and Tagliavini 1995), but the common and strong

zeolitization processes of Viterbo tuffs could suggest the

local presence of such hazardous mineral mixed with other

innocuous and commercially exploited zeolites.

Health effects arising from erionite exposure

Exposures to erionite fibers can cause adverse health effects

identical to those of asbestos, including mesothelioma

(Dogan et al. 2008), which is a rare cancer arising from the

Fig. 1 Geographical and

geological setting of the study

area. Labeled black dots refer to

the 41 field samples analyzed in

this work. Bold text refers to the

five key samples

862 Environ Earth Sci (2011) 63:861–871

123

mesothelial cells of the pleura, pericardium, peritoneum and

tunica vaginalis testis. Such pathology is of public health

concern, as it can develop also at low doses and is always

followed by fatal outcomes. The causal relation between

exposures to erionite and mesothelial tumors was firstly

reported in some Anatolian villages and was verified on the

basis of mineralogical studies and analyses of lung fiber

burden (Sebastien et al. 1981). The incidence of pleural and

peritoneal mesothelioma in Anatolian villages was much

higher than any reported (Baris et al. 1987), accounting for

200 and 700 cases per 100,000 people per year in the villages

of Karain and Sarıhıdır (Baris and Grandjean 2006). Such

strong health effects were attributable to environmental

exposures since infancy to fine zeolite fibers included in

volcanic rocks and soils, as well as to the genetic predispo-

sition, which could also explain the high incidence in certain

families (Carbone et al. 2007). Any relevant crystallographic

difference was actually found between Anatolian erionites

and those from other countries, even though a certain amount

of chemical differences may exist from place to place

(Dogan and Dogan 2008). Recently, one case of erionite-

associated mesothelioma was also reported in the USA

(Kliment et al. 2009).

Toxicity and carcinogenicity of fibrous zeolites

There is sufficient scientific evidence that fiber size is a

major determinant of fiber toxicity and carcinogenicity.

Firstly, it is crucial in determining lung deposition,

then, it influences the efficacy of clearance mechanisms

(Lippmann 1993). Fiber diameter is the major determinant

for the deposition of fibers in the lower respiratory tract,

while macrophage phagocytosis is more dependent on fiber

length. Fiber biopersistence is also an important factor for

fiber toxicity and carcinogenicity. It was hypothesized that

a fiber with critical dimensions will be carcinogenic if it is

sufficiently durable to remain chemically and physically

intact in lung tissue in close contact with the target cells

(Pott et al. 1994). Other fiber characteristics such as surface

area, chemistry, structure, net charge and microtopography

could also play some role in fiber-induced toxicity and

carcinogenicity (Hochella 1993).

Erionite is the only zeolite which was classified as a

human carcinogen (IARC 1987), although other fibrous

zeolites should not be considered intrinsically safe

(Stephenson et al. 1999). Despite of the lack of epidemiological

information on populations exposed to natural asbestiform

minerals other than asbestos and erionite, experimental

results suggest that all mineral fibers of similar size, habit,

and biopersistence may carry the same risks for humans

(WHO 1986).

Erionite is poorly cytotoxic but is nowadays considered

as the most human health-threatening mineral in nature due

to its strong carcinogenic power, estimated up to 300–800

times higher than that of chrysotile asbestos and 100–500

times higher than that of crocidolite (Carthew et al. 1992).

Erionite is assumed to be more carcinogenic than asbestos

due to its induction capacity of proliferating signals which

lead to a high proliferation rate of human mesothelial

cells (Bertino et al. 2007). Erionite can also generate

reactive oxygen species from macrophages (Long et al.

1997).

Aim of the study

The Viterbo Province, located in central Italy, has an

ancient tradition of buildings made out of bricks quarried

from local volcanic deposits. This kind of rocks can con-

tain some health-threatening components such as erionite

and other fibrous zeolites. In this work some of the several

volcanic deposits of the Viterbo Province were analyzed, in

order to identify and quantify the possible presence of

health-threatening minerals. Another aim of this work is to

stimulate future systematic studies on the possible presence

of such hazardous minerals in rocks and soils, suggesting at

the same time a methodology to be applied to hazard

identification processes in areas characterized by the pos-

sible natural presence of hazardous mineral phases.

Until now, the presence of hazardous zeolites was

mainly reported in Turkey (Dogan 2003). Some studies

about the spatial distribution of fibrous zeolites outcrops

were also conducted in the US (Chipera and Bish 1989;

Triplett et al. 2010), but this field of research is practically

neglected in Europe, where fibrous zeolite deposits have

been mainly studied for their possible technological

applications.

Materials and methods

Collection and preparation of samples

At a first stage, the selection of the main geological units to

be surveyed in the study area was carried out through an

accurate geological analysis of the existing literature, ver-

ified on-site during an extensive surveying campaign.

Locations with the largest vertical and horizontal outcrops

were preferred, as well as areas showing strong alteration

zones or vesicle mineralization. Thanks to the extensive

quarrying activity in this area, about 100 different volcanic

horizons were surveyed during our fieldwork, over an

investigated area of about 2,300 km2. Moreover, each

visited site was accurately GPS-surveyed and positioned in

a geographic information system. Such operation enabled

the comparison of the existing geological information with

the actual field data. In addition, sampling sites were

Environ Earth Sci (2011) 63:861–871 863

123

chosen in correspondence with active quarries or in the

immediate vicinity of freshly scraped pits in order to col-

lect samples in stratigraphic agreement with the geological

map of the area.

According to the few data available about zeolite-

bearing sites in the investigated area (Gottardi and Galli

1985; Passaglia and Tagliavini 1995), every facies of

major layer was carefully sampled taking into account its

relative stratigraphy. Other samples of known strati-

graphic position were collected, where possible, among

areas with stronger hydrothermal alteration (worst-case

scenario). More than 100 samples belonging to several

volcanic formations were collected, cleaned from residual

external material and preliminarily analyzed under a

binocular microscope to verify their quality. In accor-

dance to this preliminary screening, 41 samples were

considered suitable for a more detailed laboratory analysis

(Table 1). These samples were then prepared for optical

microscopy (OM) and X-ray powder diffraction (XRPD)

analysis.

Qualitative XRPD and OM analysis

The collected samples were firstly analyzed by XRPD, in

order to determine the mineral association. Data were

collected by a powder diffractometer equipped with a real-

time multi strip detector with a maximum active length of

2.118 mm, using CuKa radiation (40 kV; 40 mA) in a h/hBragg–Brentano geometry. Samples were ground in an

agate mortar and back-loaded on a circular steel 12 mm

sample holder. The sample spinning was obtained by a

rotating sample stage regulated at 2 s/round. Data were

collected in the angular range 5–70� 2h with an integrated

step scan of 0.017� 2h, divergence slit of 0.5�, antiscatter

slits of 0.5�, Soller slits of 0.04 rad and 10 mm masks. The

total time for the data acquisition was about 14 min. The

crystalline phase identification was conducted by a search–

match software and using the ICDD PDF2 database. The

detection limit of XRPD for erionite is about 1 wt%. The

same samples were then analyzed by the dispersion stain-

ing method (Mc Crone 1974) using OM, in order to iden-

tify the presence of fibrous zeolites having a concentration

below the XRPD lower detection limit. Optical observa-

tions were performed on samples treated with an immer-

sion liquid having a refraction index of n = 1.48 by a

125–5009 Reichert Polyvar microscope linked with a

digital camera. The dispersion staining color obtained by

the use of a central stop was useful to detect the presence of

zeolites, having refraction indexes ranging from 1.47 to

1.50.

Results from the combined analyses were used to select

samples with possible content of fibrous zeolites (key

samples) for further morphological and elemental analysis

with a scanning electron microscope (SEM) equipped with

an energy dispersive spectroscopy (EDS).

Quantitative OM analysis and qualitative SEM analyses

An in-depth analysis of key samples was carried out by

dispersion staining with OM using a digital image pro-

cessing (DIP) software. 30 digital images for each sample

were obtained with a 10 Megapixel resolution, where

15.2 pixels correspond to an average linear size of 1 lm.

Coalescing particles in the pictures were manually split

and, throughout an automatic count/size processing, the

total area occupied by the particles was calculated as a

function of their respective pixel intensity value against a

contrasting background. Particles were identified as zeo-

lites according to their dispersion staining color and then

pigeonholed into two categories (L/D C 3; L/D \ 3) on

the basis of their aspect ratio, where L/D C 3 corresponds

to the fibrous habit definition threshold for health protec-

tion purposes (WHO 1997). This procedure allowed the

quantification of the area percent occupied by fibrous

zeolites with respect to the total particle area. In addition,

assuming the fibers have cylindrical shape, it was possible

to estimate their total volume and their relative amount (%

vol.) in the bulk sample.

Key samples were then prepared for subsequent SEM

investigations. About 0.5 g of the ground sample was

suspended in 100 ml of bidistilled water and vigorously

shaken. 1 ml of the suspension was filtered on a 0.8 lm

porosity polycarbonate filter and a square 5 9 5 mm of the

filter was fixed on a 0.5 in. stub for SEM–EDS analysis

(Oxford INCA Energy 350, UK). The stub was metallized

with gold using a sputter coater. The analyses were carried

out using a Zeiss EVO40 SEM–EDS with LaB6 filament in

order to define the chemical composition of fibers with

identification purposes. INCA software package was used

to compute the semi-quantitative composition of particles

performing normalized stoichiometric calculation. Fibers

were then identified as zeolites on the basis of their

chemical composition and stoichiometric ratios, from the

relative amount of Si, Al, Mg, Fe, Na, Ca and K cations.

Results

Lithology

Almost the entire study area is geologically characterized

by extensive volcanogenic lithologies such as large

ignimbrite deposits and lava flows alternated with thick

fallout layers. Three main lithologies were distinguished:

pyroclastic flows, lava flows and pumice fallout deposits,

often in partial overlapping.

864 Environ Earth Sci (2011) 63:861–871

123

Pyroclastic flows in the Viterbo Province are repre-

sented by widely spread and strongly welded decametric

layers constituted by centimetric pumice layers showing

localized strong alteration. Several well-preserved dark

scoria blocks are embedded in the fine matrix composed by

yellow-orange ash and fine lapilli. Thick massive dark

lava flows (up to 20 m) largely outcrop in the area, often on

top of ignimbritic deposits. Lava (leucitic, phonolitic or

Table 1 Geographic coordinates and geological descriptions of the 41 selected samples

Sample name Coordinates (UTM, WGS 84) Deposit type Vent

Zone Easting Northing

VT 3 32T 719161 4721390 Plinian pumice fallout Latera

VT 5 32T 719286 4720841 Lacustrine deposit Latera

VT 7 32T 724714 4730087 Cbs-bearing plinian Pm and Ash fall Latera

VT 13a 32T 718064 4695067 Lacustrine deposit Bolsena

VT 14 32T 718064 4695067 Basaltic flow Latera

VT 14B 32T 718064 4695067 Basaltic flow Latera

VT 17 32T 724090 4701648 Pumice fallout ? pumice flow Bolsena

VT 19 32T 725308 4704753 Plinian ignimbrite (tuff) Bolsena

VT 22 32T 731324 4712382 Scoria and lapilli cone w/bas dykes Latera

VT 24 33T 260215 4722949 Lct-bearing trakite flow (‘‘basaltite’’) Bolsena

VT 25 33T 265660 4717624 Pumice fallout and flow Bolsena

VT 29 33T 268574 4707724 Reworked pumice and ash fall Bolsena

VT 32 33T 281459 4697722 Thick pumice and scoria fall w/lct bas Vico

VT 34a 33T 281459 4697722 Thick pumice and scoria fall w/lct bas Vico

VT 35a 33T 281459 4697722 Thick pumice and scoria fall w/lct bas Vico

VT 36 33T 262651 4723288 Pumice flow Bolsena

VT 39 33T 277106 4686931 Red pumice flow w/black scoria Vico

VT 40 33T 277106 4686931 Red pumice flow w/black scoria Vico

VT 42B 33T 285685 4690358 Pumice and ash flow (‘‘Tufo finale’’) Vico

VT 43a 33T 284257 4683972 Red pumice flow w/black scoria Vico

VT 44 33T 275091 4691504 Phonolitic trakitic ignimbrite Vico

VT 45 33T 274297 4694364 Leucitite (fish-eye) Vico

VT 46 33T 275012 4696328 Pumice flow Vico

VT 47 33T 275386 4696289 Pumice fallout and flow Vico

VT 53 33T 275386 4696289 Pumice fallout and flow Vico

VT 54 33T 274672 4696147 Red pumice flow w/black scoria Vico

VT 57 33T 269756 4699437 Latitic lava flow Cimino

VT 58 33T 265993 4700711 Gray, massive flow w/fiamme Cimino

VT 60 33T 268230 4703254 ‘‘Peperino’’ tuff Vico

VT 60B 33T 268625 4703402 ‘‘Peperino’’ tuff Vico

VT 60C 33T 268625 4703402 ‘‘Peperino’’ tuff Vico

VT 60D 33T 268625 4703402 ‘‘Peperino’’ tuff Vico

VT 63 33T 271236 4679493 Red pumice flow w/black scoria Vico

VT 64 33T 266945 4711346 Pumice fallout Bolsena

VT 65B 33T 269069 4714075 Red pumice flow w/black scoria Bolsena

VT 67B 33T 264147 4716698 Lapilli and ash fallout Bolsena

VT 70 32T 736231 4699987 Leuciti tephrite lava flow Bolsena

VT 74 32T 732408 4704709 Black pumice flow Latera

VT 75a 32T 732182 4705103 Yellow tuff Latera

VT 77 32T 725129 4704228 Welded pumice flow ‘‘pipernoid’’ Bolsena

VT 78 32T 717653 4710446 Yellow tuff from Monte Calvo vent Bolsena

a Key samples

Environ Earth Sci (2011) 63:861–871 865

123

tephritic in composition) locally shows strong alteration

and oxidation with secondary crystal growths in vesicles.

Fallout deposits, several meters thick, mainly composed by

loose pumice with dark lava grains, were deposited in

stratigraphic succession with both lavas and pyroclastic

flows. White, yellow or gray centimetric juvenile pumices

show strong vesiculation (often filled with secondary

minerals) and partial recrystallization.

X-ray diffraction

XRPD analysis (Fig. 2) revealed that 27 out of 41 samples

contain zeolites above the detection limit (about 1 wt%)

(Fig. 3).

Zeolites were mainly represented by analcime (23/41

samples), but also by chabazite (11/41) and phillipsite (9/41)

(Figs. 2a, 3). Phillipsite usually occurred in small amounts and

in strict association with chabazite.

As shown in Fig. 3, the most relevant mineral phases

in terms of relative frequency were K-feldspars (85%),

mainly sanidine, and micas (biotite and/or phlogopite)

(70%). The relative frequency of plagioclases and

pyroxenes was about 61 and 46%, respectively. XRPD

patterns of pyroxenes are consistent with augite or diop-

side, as these isomorphs are almost undistinguishable by

XRPD when present in relatively low quantities. Clay

minerals were present in 12/41 samples (39%), principally

as halloysites (5/41) and montmorillonite (4/41), but also

as illite and kaolinite. Kaolinite and illite were found

associated in two samples. Quartz and calcite were found

in 20 and 15% of cases, respectively. Quartz was rarely

associated with zeolites and frequently associated with

clay minerals, while calcite was often associated with

analcime and chabazite.

Microscopical dispersion staining

The dispersion staining analysis revealed mineral fibers

with refractive index consistent with those of zeolites

(n = 1.48) in five key samples numbered as VT 13, VT 34,

VT 35, VT 43 and VT 75. VT 13 is a Lower Pleistocene

fine-grained, strongly welded, dark-gray tephrite outcrop-

ping beneath the southwestern margin of the upper pleist-

ocenic travertine, in the distal part of the VVD (30 km SW

from the Bolsena caldera vent). VT 13 belongs to a 1-m-

thick, white lacustrine fine ash deposit sitting on top of the

main (20 m thick) black basaltic lava flow. VT 34, VT 35

and VT 43 belong to the Vico volcanic centre and represent

Upper Pleistocene tephritic–phonolitic ignimbrites with

yellowish and reddish pumice mixed with black scoria

clasts showing large (centimetric) leucite and sanidine

phenocrysts. Samples VT 34 and VT 35 represent hydro-

thermally altered pumice fallout layers (VT 35) interbed-

ded with a 30-m-thick high-temperature pyroclastic flow

sequence (VT 34). Sample VT 43 belongs to a several-

meter-thick high-temperature pyroclastic flow sequence.

VT 75 was sampled from a welded 20-m-thick ignimbrite

belonging to the Upper Pleistocene ‘‘Lithoid Yellow Tuff’’

and outcropping in the median part of the Latera Volcanic

District. This ignimbrite is brown-reddish in color with

abundant small lavic and juvenile (pumice) inclusions and

discontinuously outcrops among the quaternary cover in

the southern area of Bolsena Lake.

The subsequent DIP analysis allowed the relative

quantification (% vol.) of zeolite fibers, whose results are

reported in Table 2. Samples VT 13, VT 34, VT 35, VT 43,

VT 75 (Table 2) showed a fiber-particle area amount

ranging between 0.54% (VT 43) and 1.61% (VT 35).

Supposing the fibers as cylinders, the fiber-particle volume

amount was included between 0.21% vol. (VT 43) and

1.66% vol. (VT 75).

SEM–EDS analysis

The five key samples were then investigated by SEM.

Inosilicate (Fig. 4a) and zeolite fibers (Fig. 4b–d) were

identified on the basis of their distinctive elemental

composition. Fibrous zeolites were found in four key

samples (VT34, 35, 43 and 75). The other fibers occurring

Fig. 2 Examples of X-ray powder spectra, showing the mineral

phase identification with regard to the most intense Bragg reflections.

a Zeolitized tuff, b non-zeolitized lava flow. Anl analcime, Cpx clin-

opyroxenes, Cbz chabasite, Kf K-feldspars, Mca micas, Phi phillip-

site, Pl plagioclase, Ttn titanite

866 Environ Earth Sci (2011) 63:861–871

123

in key samples VT 13, VT 43 and VT75 were ascribed to

inosilicates (amphibole or pyroxene group), due to the

relative higher abundance of Mg and Fe cations

([7.5 wt%).

Typical morphologies are depicted in Fig. 4a (inosili-

cate fiber) and Fig. 4b–d (zeolites). Corresponding EDS

spectra are reported in Fig. 4e–h.

Twelve zeolite fibers in the key samples were subjected

to semi-quantitative EDS analysis: all fibers were consti-

tuted by K between 4.0 and 5.2 wt%, while Fe was not

detected in most cases; when present, it occurred at con-

centrations of 0.5 wt% (Table 3). Fibrous zeolites were

also constituted by a non-negligible amount of Ca

(\2.6 wt%).

The key samples were characterized by a mineralogical

association having zeolites as main components, except

one (VT 35), in which only analcime was present in

quantities close to XRPD detection limit.

Zeolite fibers were found with apparent orthorhombic or

tetragonal prismatic habit (Fig. 4b), sometimes in radial

aggregates (Fig. 4c), and hexagonal prismatic habit

(Fig. 4d). In general, fibrous zeolites were constituted by

low aspect ratios (min = 3.8; mean = 6.3; max = 13.1),

generated by lengths varying from 10.8 and 60.3 lm

(mean = 25.7) and diameters ranging from 2.1 to 9.2 lm

(mean = 4.2).

Discussion

Mineralogy

The mineralogical association in volcanic samples of

Viterbo area was in agreement with data reported in geolo-

gical literature, with petrographic composition varying

from latitic to tephritic and/or phonolitic. According to

Fig. 3 Mineralogical

associations of the 41 samples

analyzed by XRPD. Boldtext key samples, blackcells mineral phase

identification, Am amphiboles,

Anl analcime, Ank ankerite,

Cpx clinopyroxenes,

Cal calcite, Cbz chabasite,

Grs grossular, Gp gypsum,

Hal halloysites, Hem hematite,

Ill illite, Kf K-feldspars,

Kln kaolinite, Lct leucite,

Mca micas,

Mnt montmorillonite,

Ne nepheline, Phi phillipsite,

Pl plagioclase, Qtz quartz,

Sps spessartine

Table 2 Relative amounts of

fibrous zeolites by

microscopical DIP

Sample Fiber area

(lm2)

Total area

(lm2)

Area% Fiber volume

(lm3)

Total volume

(lm3)

% volume

VT 13 773 103,734 0.75 2,125 615,478 0.35

VT 34 578 73,710 0.78 1,274 268,001 0.48

VT 35 1,505 93,495 1.61 4,431 305,178 1.45

VT 43 665 122,435 0.54 1,295 618,741 0.21

VT 75 1,409 89,722 1.57 10,612 637,784 1.66

Environ Earth Sci (2011) 63:861–871 867

123

Amendolagine et al. (1963), leucite is often almost totally

transformed into analcime, which is consistent with our

observations (Fig. 3). Accessory minerals like amphiboles,

titanite and Fe-oxides were identified in some samples, in

accordance with the local mineralogical composition

described in literature (Orlando et al. 1994). Grossular,

clinopyroxene, quartz and spessartite association found

in sample VT 60C is consistent with the composition

of accessory minerals found in carbonate xenoliths (Di

Battistini et al. 1998).

Twenty-seven samples out of 41 contain zeolites,

mainly analcime, chabazite and phillipsite, which generally

do not show fibrous habit. Such result can be explained

considering that the sampling was primarily focused on the

selection of rocks subjected to zeolitization processes and

was not necessarily representative of the actual bulk min-

eralogical composition of the volcanic units outcropping in

the study area. It was suggested that extensive zeolitization

processes of volcaniclastic rocks may occur through

interaction between volcanic glass and percolating fluids at

a very low salinity degree and a temperature close to

200�C. Phillipsite was generated principally by alkali-

trachitic products, while chabazite could be more likely

associated with the phonolitic–tephritic series (Ghiara and

Petti 1996). However, in our samples, chabazite and phil-

lipsite are in most cases associated (Fig. 3) since they are

generally in thermodynamic equilibrium (Gibbs free

energy for the reaction chabazite?phillipsite & 0) and

tend to coexist (La Iglesia et al. 1991).

Chemical composition of fibrous particles

As already described, SEM–EDS analyses revealed that

either inosilicate (VT13, VT43, VT75) or zeolite fibers

Fig. 4 High-resolution SEM

pictures and respective EDS

spectra. a, e Fibrous amphibole

in sample VT 13. b, f Fibrous

zeolite in sample VT 34. c,

g Radial zeolite aggregates in

sample VT 34. d, h Fibrous

zeolite in sample VT 75

868 Environ Earth Sci (2011) 63:861–871

123

(VT34, VT35, VT43, VT75) were present in the key

samples.

Fibrous zeolites were K (3.99–5.19%) and Ca rich

(0.93–2.65%), with no detectable amount of Na and Mg.

The relative abundance of extra-framework cations was in

agreement with the low Na and Ca content reported in a

study about RCP phillipsites sampled in the black pumice

ignimbrite (Passaglia and Vezzalini 1985).

The identification of fibrous zeolite mineral species was

conducted on the basis of the rules and nomenclature pro-

posed by the Subcommittee on Zeolites of the International

Mineralogical Association (Coombs et al. 1997) and con-

sidering other crystallochemical information (Passaglia

1970; Passaglia et al. 1998; Dogan and Dogan 2008).

Crystal habit was also considered as another possible

diagnostic element to discriminate among different fibrous

zeolites. Although zeolite mineral species shall not be dis-

tinguished solely on the basis of Si:Al ratio, the combina-

tion between the tetrahedral Si content (TSi = 0.70–0.76)

and the elemental composition of fibrous zeolites in terms

of extra-framework cations with dominant K, lower Ca and

non-detectable Na, was consistent with Phillipsite-K spe-

cies, which usually exhibits TSi = 0.59–0.76. Nevertheless,

also offretite, erionite-K and chabazite-K framework com-

position generally showed a partial overlap with the TSi

observed range (Coombs et al. 1997). Phillipsite was not

included in the list of typical fibrous zeolites, as defined by

an IPCS monograph (WHO 1986), although it is well

known that this species can display a prismatic habit,

which can produce fibers with aspect ratio (L/D) C 3

(Novembre et al. 2004). In our samples, fibrous zeolites

often occur in tetragonal or orthorhombic prismatic habit

(Fig. 4b, c), and in only one case hexagonal prismatic habit

(Fig. 4d). The prevalence of tetragonal or orthorhombic

prismatic habit strengthens the hypothesis that fibrous

zeolites were mainly consistent with K-phillipsite. Phillip-

site is actually monoclinic but it is always twinned,

forming groups with an orthorhombic or tetragonal habit.

On the contrary, K-erionite (TSi = 0.74–0.79) or offretite

(TSi = 0.69–0.74) frequently exhibit hexagonal prismatic

or asbestiform habit, while chabazite crystallizes in the

rhombohedral system, developing equidimensional crystals

with pseudocubic predominant habit (Gottardi and Galli

1985).

Moreover, the balance error E = [(Al ? Fe3?) -

(Na ? K) ? 2(Ca ? Mg ? Sr ? Ba)]/[(Na ? K) ? 2(Ca ?

Mg ? Sr ? Ba)] 9 100 is considered a diagnostic element

to test if minerals are not erionite (E% [ 10%) (Passaglia

1970). 5/12 zeolites fibers showed E% [ 10% (Table 3),

including that having hexagonal prismatic habit. Thus, the

combination of balance error and habit analysis suggested

to exclude the presence of erionite in the considered sam-

ples. It is difficult to distinguish between erionite and of-

fretite only on a chemical basis, because of the extensive

compositional overlap that exists between the two species

(Dogan et al. 2008). Nevertheless, a comprehensive study

on the crystal chemistry of erionites and offretites (Passa-

glia et al. 1998) demonstrated that the most significant

compositional discrimination between erionite and offretite

is based on the Mg/(Ca ? Na) cation ratio. Offretites

generally showed Mg/(Ca ? Na) values close to 1.0, while

erionites presented values significantly less than 0.3. Thus,

it can be reasonably argued that fibrous offretite is not

present in our samples as Ca was detected in most fibrous

zeolites, while Mg was never revealed by SEM–EDS

analyses (Table 3).

Table 3 Chemical composition of fibrous zeolites, determined by EDS, and relative crystal habit

Sample Al Si K Ca Fe O TSi E% Habit L (lm) D (lm) L/D

VT 34 8.8 23.2 4.4 0.9 – 62.7 0.72 1 OTP 10.8 2.4 4.4

9.2 22.9 4.0 1.2 - 62.7 0.71 18 OTP 15.8 4.2 3.8

8.7 23.2 4.5 0.9 - 62.6 0.73 -6 OTP 19.2 4.0 4.9

8.7 23.3 4.2 1.1 - 62.8 0.73 6 OTP 15.3 2.5 6.1

VT 35 8.2 23.5 4.4 1.2 - 62.7 0.74 -9 ND 35.3 4.8 7.4

8.1 24.2 4.9 - - 62.9 0.75 -34 ND 13.5 2.5 5.4

7.8 24.3 5.1 - - 62.8 0.76 -46 OTP 60.3 9.2 6.6

VT 43 8.8 22.5 4.4 1.5 0.5 62.4 0.72 7 ND 41.2 5.7 7.3

8.4 22.2 4.8 2.7 - 62.0 0.73 -11 ND 10.8 2.1 5.1

VT 75 9.0 22.5 5.2 1.1 - 62.2 0.71 -19 HP 27.4 6.1 4.5

9.1 21.8 4.5 2.5 - 62.1 0.71 0 OTP 32.8 2.5 13.0

9.5 21.7 4.5 2.2 - 62.1 0.70 6 OTP 25.9 3.8 6.8

OTP orthorhombic or tetragonal prismatic habit, HP hexagonal prismatic habit, ND nondistinguishable. Fiber dimensions were also reported

(L length, D diameter, L/D aspect ratio)

Environ Earth Sci (2011) 63:861–871 869

123

Inosilicate fibers observed in sample VT 13, VT 43 and

VT 75 had an elemental composition consistent with K and

Ti rich clino-amphiboles.

Dimensional analysis of fibrous zeolites

Zeolite fibers were mainly characterized by prismatic habit

and low aspect ratios (arithmetic mean = 6.3; geometric

mean = 5.9), comparable with erionites from the Rome

(Oregon, USA) area (arithmetic mean = 6.02) (Fraire et al.

1997) but lower than those reported for erionite from

Yucca Mountain (USA) (Chipera and Bish 1989) and

for uncoated fibers (geometric mean = 15.8) and bodies

(geometric mean = 105) found in the bronchoalveolar

lavage fluid of environmentally exposed subjects

(Dumortier et al. 2001). It should be also noted that

diameters of fibrous zeolites were generally thicker than

3 lm (Table 3), which is considered the upper threshold

limit for the definition of fibers to consider in exposure

monitoring for risk assessment and health protection pur-

poses (WHO 1997).

Conclusion

Fibrous zeolites were preliminarily revealed in 5/41 sam-

ples by microscopical dispersion staining and confirmed by

SEM–EDS in 4 samples. They showed a chemical com-

position mainly consistent with phillipsite-K, that is not a

known health hazard at present. It should be also noted that

phillipsite was already revealed by XRPD in 3 (VT 34, 43

and 75) out of these 4 samples (Fig. 3). Zeolite fibers

showed prismatic habit with low aspect ratios and thick

diameters, which are factors negatively associated with the

toxicity and inhalability of fibers (Lippmann 1993; Bailey

et al. 2004).

The present work is the first step of a risk assessment

process: a proper risk characterization needs further

investigations on human exposure to fibrous zeolites.

However, the exposure to fibrous zeolites can be assumed

negligible where anthropic activities such as quarrying or

building/restructuring activities can be excluded, due to the

low levels of fibrous zeolites quantified in the collected

rocks and soils and to the dimensional properties of such

fibers.

This work presented a field sampling and a laboratory

analytical protocol, composed by a sequence of investiga-

tions, which can be useful to characterize the presence of

fibrous zeolites in a suspected geographical or geological

area and to estimate their relative amount, to achieve a

basis for risk assessment and risk management processes.

The sampling protocol should be representative of the

whole area of zeolitization, which can include different

lithological units, with possible chemical variations at

different degree.

The analytical part of the protocol is composed by a

dispersion staining analysis, to identify the presence of

fibrous minerals having refractive index consistent with

those of zeolites (IR = 1.47–1.50) in the collected sam-

ples. A proper quantification (wt%) of the zeolite mineral

species should be conducted by high-sensitivity XRPD

(high counting statistic or peak/background ratio), using

preferably the Rietveld method with an addition of a

known amount of internal standard to take into account the

amorphous component in the sample. If fibrous zeolites in

samples are below the XRPD detection limit (usually about

1 wt%), a SEM–EDS analysis should be used in order to

allow their proper identification and distinguish them from

other fibrous minerals. The identification of fibrous min-

erals present in quantities below the detection limit of

XRPD can be also achieved using more specific and

expensive particle-by-particle analytical approaches, such

as selected area electron diffraction using transmission

electron microscopy.

The quantification of fibers in amounts below the XRPD

detection limit can be conducted by OM (dispersion

staining method). DIP is a useful tool to identify zeolites as

a function of their refractive index, to discriminate between

fibrous and non-fibrous zeolites and to estimate the relative

amount of fibrous zeolites in bulk samples.

Acknowledgments Authors thank Dr. Fulvia De Palma for pro-

viding a suitable 4 9 4 vehicle, Geom. Sergio Checcacci for the

logistics and Massimo Ligorio Bisi for the precious help during the

laboratory sample preparation.

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