Dossi di Franscia, Franscia, Lanzada, Sondrio Province, Lombardy, Italyi

Regional Level Types
Dossi di Franscia	Group of Hills
Franscia	Village
Lanzada	Municipality
Sondrio Province	Province
Lombardy	Region
Italy	- not defined -

At Dossi di Franscia, a group of hillocks to the SSW of Campo Franscia (Franscia) village, the various mineral sites can be grouped into:

- ancient asbestos mines and prospecting works;
- serpentinite quarries;
- outcrops of mineralised veins and lenses in the country-rock.

Minerals occur in different formation environments inside serpentinite:
- asbestos-andradite clefts;
- carbonate-rich fissures;
- titanian hydroxylclinohumite-forsterite-calcite veins;
- rodingite dykes;
- carbonate veins (hydrothermalites) with Ti-Ni-Cu minerals;
- chromitite lenses and veins.

Asbestos-andradite clefts are the source of remarkable specimens of demantoid. For demantoid and fibrous chrysotile, the best finds are recorded from the ancient mines, locally named “Trona Granda”, located in front of the building of Ristoro di Franscia and below the present Mauri serpentinite quarry (now they are collapsed) (Sigismund, 1948; Gramaccioli, 1962 and other authors). Various other mines, quarries, prospecting works, and outcrops sporadically yielded good specimens of demantoid and fibrous chrysotile, such as:

- the serpentinite quarries (Mauri and Gerosa quarries) located in the proximity of the building of Ristoro di Franscia, below the spurs of Ciudé or Piodé (an elevation at the east foot of Motta Mt.);
- some old galleries in the little valley beneath the building of Ristoro di Franscia;
- the hillock overhanging the building of Ristoro di Franscia. Here, demantoid typically forms lenticular or spheroidal crystal nodules, attaining various centimetres in diameter, named “asbestos seed” (“semenza d’amianto”) by the miners/quarrymen.

Hydrated magnesium carbonates are sporadically found inside serpentinite fissures. Artinite was described as a new mineral species on specimens, consisting of white radiating veinlets, collected by Pietro Sigismund in the summer of 1902 at the old municipal asbestos mine, located behind the chapel of Dossi di Franscia (Brugnatelli, 1902 and 1906). A large fissure lined with artinite, as spherules of acicular crystals up to 2 cm in diameter, in association with hydromagnesite and probably coalingite, was found during the winter of 1987-88 at the Mauri serpentinite quarry. The first find of nesquehonite was recorded in 1921 from the “Trona Granda” asbestos mines (Artini, 1921).

Rodingite dykes crop out a few tens of meters from Ristoro di Franscia. Typical minerals are:
- colourless to white small crystals of diopside;
- colourless, pinkish or yellow rhombododecahedral crystals of grossular;
- green and yellow prismatic crystals of vesuvianite with red-brown terminations.
Associated minerals include clinochlore, calcite, chalcocite, bornite, and malachite.
White veins of fibrous pectolite, in association with creamy calcite, embedded in a chlorite-rich facies were found at the beginning of the 1960s to the west of Ristoro di Franscia. Acicular pectolite crystals up to 2 cm are known from the cavities in the compact mineral.

Magnetite, dolomite, pecoraite, and heazlewoodite in carbonate veins (hydrothermalites) have been found in the dump of an abandoned mine near Ristoro di Franscia (Bedognè et al., 1993).
Finds of copper minerals (native copper, bornite, chalcocite, and cuprite) and other species (rutile, quartz, apatite, brucite, etc.) were reported by Sigismund (1947).

Chromitite lenses and veins at Dossi di Franscia are known for a long time. A chromitite lens was reported from the Ciatùn asbestos mine, currently no longer identifiable on the territory, but whose location is probably in the area of the present Mauri serpentinite quarry. It was the source of bright emerald-green crystals of “uvarovite” (actually chromian andradite) in association with violet chromian clinochlore, asbestos, and probable sepiolite (Sigismund, 1948; Gramaccioli, 1962). Reevesite, zaratite, theophrastite, brucite, calcite, and prehnite have been found on the fractures of chromitite blocks from the dumps in the neighbourhood of Ristoro di Franscia. Chromitite lenses have also been found at the Mauri and Gerosa serpentinite quarries. A chromitite vein crops out in one of the abandoned adits to the southeast of Ristoro di Franscia.

NOTES

Artinite: Dossi di Franscia is the type locality for this mineral species, discovered by Pietro Sigismund (1874-1962), mineral collector and pioneer mineral seeker in Valtellina, in the summer of 1902 at the old municipal asbestos mine, located behind the chapel of Dossi di Franscia (Brugnatelli, 1902 and 1906). However, six years earlier, Luigi Brugnatelli already studied an identical hydrated magnesium carbonate (Brugnatelli, 1897 and 1899) which he personally collected in Valbrutta (Val Brutta). From the chemical analysis he deduced the formula MgCO3•Mg(OH)2•3H2O, but, due to the scarcity of the analysed material (0.171 g including impurities), Brugnatelli thought it was prudent to wait for a new analysis before recognising the mineral as a new species. The new analysis, that he performed on the abundant Sigismund's material forwarded through Ettore Artini, confirmed the two finds to be identical.

"Uvarovite": emerald-green crystals in chromitite fissures, traditionally considered to be "uvarovite" (Sigismund, 1948; Gramaccioli, 1962; De Michele, 1972, and other authors), are actually chromian andradite with a content in Cr2O3 varying inside the crystals from 9.6% in the core to 0.8% at the rim (Bedognè et al., 1993; Sciesa, 1993).

“Ti-bearing clinohumite; titanclinohumite”: according to the analyses performed, this mineral in the Malenco valley serpentinite is practically fluorine-free (Trommsdorff and Evans, 1980). Therefore, after the institution of the new species hydroxylclinohumite in 1999, all records of “titanclinohumite” in the antecedent literature and all specimens preserved in public and private collections consequently labelled are now Ti-bearing hydroxylclinohumite (Bedognè et al., 2006).

Mauri quarry: one of the two active serpentinite quarries at Dossi di Franscia is operated by the company Marmi Mauri S.r.l. As the same company operates three serpentinite quarries in the municipal territory of Lanzada (at Le Prese, Valbrutta, and Dossi di Franscia respectively), some confusion can arise about the exact provenance of specimens simply labelled as from “Mauri quarry, Lanzada”. Nevertheless, the name Mauri quarry has been generally used among the mineral collectors to indicate the one located at Le Prese (http://www.mindat.org/loc-233994.html), where vesuvianite-rich rodingite dykes have been evidenced.

Provided geographical coordinates refer to the still-active "Dossi di Franscia" Quarry. The whole collecting area, as here described, encompasses several sub-localities in a radius of about 1 km.

Other Regions, Features and Areas containing this locality

References