FERMO

On September 25, 1996 a stony meteorite fell in Central Italy at a site (13° 45' 12" E, 43° 10' 52" N) close to a field, some 3-4 km North-East of the town of Fermo and a few kilometres from yhe Adriatic coast. The meteorite was recovered two days later as a single stone (size 19 X 24 X 16 cm)weighting 10.2 kg within a crater of 30-40 cmm and now is housed at Palazzo dei Priori and, after january 20th, will be at the Polar Museum of Villa Vitali in Fermo.

Abstract: On September 25, 1996 a stony meteorite fell in central Italy at a site (13° 45' 12" E, 43° 10' 52" N) close to a field, some 3-4 km north-east of the town of Fermo and a few kilometres from the Adriatic coast . The meteorite has the form of a single 10.2 kg stone and exhibits the characteristic fusion crust. The body is classified as a H3-5 chondrite breccia.

Introduction: description of the event.

On September 25, 1996 a stony meteorite fell in central Italy at a site (13° 45' 12" E, 43° 10' 52" N) close to a field, some 3-4 km north-east of the town of Fermo and a few kilometres from the Adriatic coast (fig.1). At 15:30 UT a farmer, Mr.Luigi Benedetti, heard the sound of an explosion followed by a loud noise similar to that of "an approaching helicopter". After a few seconds a crash was heard, about 200 meters away from the nearest farm-house. Two days later, on September 27 at about 06:00 UT, Mr. Giuseppe Santarelli discovered the stone at approximately the point described by the first witness, at the margin of a narrow country path. The meteorite was recovered as a single stone on a wet and soft clay bedrock within a crater of 30-40cm and now is housed at the Polar Museum of Villa Vitali in Fermo.

Stony meteorites are well represented by the very numerous chondrites, which most closely approximate primitive solar nebula condensate and contain spherical millimetre- to centimetre-sized chondrules, i.e. silicates that rapidly melted and immediately cooled early in the Solar System's history. Objects responsible for bright fireballs from which meteorites may drop occupy the low mass end of the asteroid size spectrum. Recent studies (Chyba et al., 1993 ; Hills and Goda, 1993) have considered the fragmentation of large meteoroids during atmospheric flight, although comparison of these models with observational data has not been possible. Low-density cosmic bodies with very weak structural strength and higher velocities, such as cometary debris and soft stony bodies, are able to produce very brilliant fireballs accompanied by episodes of catastrophic fragmentation and explosion at the path end (Cevolani, 1994). So far, only four meteorites have been recovered for which detailed data on atmospheric trajectory and orbit exist. At present, we are unable as yet to calculate the Fermo meteorite trajectory as Italy does not possess an all-sky photographic camera network of the kind coordinated by the European fireball network and those operating in the 1970s in the USA and Canada. The only four photographed fireball trajectories with the associated falls - Pribram (Ceplecha, 1961), Lost City (McCrosky, 1970), Innisfree (Halliday et al., 1978) and Peekskill (Beech et al., 1995), relate to ordinary chondrites with low-inclination orbits (between about 5 and 12 degrees), low pre-atmospheric velocities (between 14 and 21km/s), eccentricities ranging from 0.40 and 0.67 and aphelia in the asteroidal belt.

So far the eye-witness accounts have allowed only a rough reconstruction of the true fall path of the meteorite through the Earth's atmosphere. In addition, few accounts are available concerning the light phenomena associated to the fireball before reaching the retardation point (when the decelerated meteorite usually bursts into parts and its cosmic velocity is overcome). As a partial explanation, the event took place during daylight (15:30 UT) and the fireball was travelling south-southeast. After the retardation point, light phenomena ceased and the dark body fell only under the influence of its weight and the resistance of air, almost vertically (10-20 degrees with respect to the vertical).

Fermo is the 12th meteorite find in Italy in this century, but is the third most important in terms of weight, after Vigarano (a carbonaceous chondrite of two pieces, 11.5kg and 4.5kg, recovered in 1910) and Bagnone (an iron body of 48kg found in 1904). The stone (size 19x24x16 cm), weighing 10.2 kg, has an irregular, angular, prismatic shape with rather sharp corners (Fig.2). It was almost completely covered by a very thin black fusion crust. Depressions similar to thumb prints (remaglypts) are evident on two faces of the stone. Due to the impact after fall small pieces of the corners are broken. After about twenty days after the fall a 2-3 cm thick slab (weight 800 g) was cut parallel to the outer surface of the meteorite. This slab is now under measurement for the detection of cosmogenic nuclides at the Istituto di Cosmogeofisica del CNR and Dipartimento di Fisica Generale dell‚Università di Torino (Bonino et al., personal communication).

Experimental Techniques, Petrography and Mineral Chemistry

Two chips of the Fermo meteorite, coming from opposite broken edges of the stone and weighing respectively 4.2 and 15.1 g, were examined visually and under a low-magnification stereomicroscope. One polished thin section (pts) was examined under a polarizing microscope in transmitted and reflected light. The chemical composition of mineral phases and glass were determined using a CAMECA Camebax microprobe operating at 15 kV and 15 nA sample current. Petrologic type was determined using the criteria of Van Schmus and Wood (1967).

The exposed surfaces of the two broken chips display areas of varying grey also characterized by subtle differences in texture and grain size. Centimetric to millimetric dark and light clasts are cemented by a grey matrix, suggesting that Fermo is a brecciated meteorite. Round millimetric to submillimetrc inclusions of troilite may be observed inside the light clasts and matrix. Although similar inclusions have not been observed in dark clasts, this preliminary observation may be of no significance, as dark clasts are less abundant, at least in the broken chips examined. Metal and troilite are homogeneously distributed, except for some areas where they gather to constitute centimetric "clouds". Troilite also makes up thin, gently waved, opaque veins up to 3-4 cm long. Rusty brown halos are evident on exposed surfaces.

The examined pts is romboid in shape (diagonals of about 30x13 mm) and was prepared from the smaller chip. This chip is made up of three different fragments, visually recognizable on a macroscopic scale thanks to their different colours. These fragments, hereafter referred to as zones 1, 2 and 3, represent respectively dark clasts (1), grey matrix (2) and light clasts (3) respectively. The boundaries among these three zones are discordant with respect to the outer surface which has a thin (0.2 mm) fusion crust. On microscopic examination, zone 1 is clearly distinct from the adjacent zone 2, whereas the boundary between zones 2 and 3 is faint. The three areas have different chondritic textures. Several grains of olivine and low-Ca pyroxene were analysed to determine compositional variations in each zone. Both cores and rims of several crystals were investigated to check intracrystalline compositional uniformity. In each zone, mean chemical compositions and standard deviation of silicates were calculated including core values only.

 Zone 1 - Dark clast.

Examination of the pts revealed abundant heterometric (150-1200µ) chondrules, some fragments of chondrules, and angular, broken mineral grains. The matrix is microcrystalline to semi-opaque. The chondrules are of various types: granular, porphyritic (Fig. 3), excentroradial, microcrystalline and barred, with very clearly defined borders. Some of them contain very pale brown, clear, isotropic glass (with K2O contents up to 6.8 wt%); others contain turbid brown glass (with lower K2O contents). Low-Ca pyroxene (mean En87.2Fs12) is mainly monoclinic, displaying thin polysynthetic twinning. Mean olivine composition is Fa12.1. Both olivine and low-Ca pyroxene have a wide compositional range varying from Fa2.1 to Fa26.5 and from En97.8Fs2.1 to En75.3Fs22.2 respectively. The metal-sulfide ratio is about 70:30. Kamacite (Ni6.8) is much more abundant than taenite (Ni53.7). Metal is finer grained than in zones 2 and 3, and makes up small drops (80-240 µm), sometimes rimmed by corroded sulfide. Melt pockets containing abundant and very fine-grained metal and sulfide globules are common. Fractures in silicates are cemented by troilite. Small amounts of chromite were observed.

 Zone 2 - Grey matrix.

Chondrules are scarce in the matrix, whereas fragments of chondrules and broken mineral grains are more abundant. One of these grains, a low-Ca pyroxene crystal, is even faulted, with a measured displacement of 300 µ. The chondrules are granular, porphyritic and well defined. Average grain size is generally finer than in the adjacent zones 1 and 3. The matrix appears to have undergone a grain size reduction, and some bigger clasts of slightly different petrologic type are evident inside it. Clear, light brown glass was found inside one porphyritic chondrule, whereas another chondrule contains small interstitial patches of plagioclase. Both olivine (mean Fa17.4) and low-Ca pyroxene (mean En82.7Fs15.8) have uniform compositions, except for the fragments of small chondrules of different (H3) petrologic type. Low-Ca pyroxene is sometimes twinned. Nickel-iron occurs as irregular grains and consists of equal amounts of kamacite (Ni7) and taenite (Ni54), often intergrown. The metal-sulfide ratio is about 70:30. Troilite makes up polycrystalline interstitial patches (200-400µm) and is also the main component of some thin melt (shock) veins. Apatite and chromite are minor constituents.

Zone 3 - Light clast.

The chondrules in this area are not abundant; their boundaries towards the matrix are generally poorly defined. Only porphyritic and excentroradial chondrules are observed. The matrix is microcrystalline to well recrystallized. Both olivine (mean Fa18.2) and low-Ca pyroxene (mean En82.5Fs16.0) have quite uniform compositions. Twinning in low-Ca pyroxene is rare. Plagioclase is found as very small microcrystalline interstitial patches. Nickel-iron occurs as irregular grains and consists of equal amounts of kamacite (Ni4.6) and taenite (Ni51.5), often intergrown. The metal-sulfide ratio is about 55:45. Troilite makes up polycrystalline, interstitial patches (200-400µ) and is also the main component of some thin melt (shock) veins running subparallel. Minor apatite and chromite are also present.

 Classification

The relatively low FeO/(FeO+MgO) ratios of olivine and low-Ca pyroxene allow the three areas of the Fermo meteorite to be assigned to chondritic group H (Fig. 4).

On the basis of the presence of clearly defined chondrules, sometimes containing clear isotropic glass, and the variability of olivine, the dark clasts (zone 1) are assigned to petrologic class H3. Smaller amounts of polysynthetically twinned low-Ca pyroxene, the presence of microcrystalline plagioclase, and the uniform olivine and pyroxene composition all mean that the light clasts (zone 3) are assigned to petrologic class H5.

Zone 2 apparently makes up the matrix of dark clasts H3 and light clasts H5. It is mainly composed of clasts and small fragments of petrologic type 5 (and possibly 4); fragments of petrologic type 3 are less abundant.

Following the definition of Wasson (1974) and the classification of Bunch & Rajan (1988), the Fermo meteorite may be classified as a genomictic, regolith breccia of chondritic group H, with fragments of petrologic types 3 to 5.