Vol.:(0123456789) 1 3 Geoheritage (2021) 13:96 https://doi.org/10.1007/s12371-021-00622-3 ORIGINAL ARTICLE Giant Garnet Crystals in Wollastonite–Grossularite–Diopside‑Bearing Marbles from Tamarispa (NE Sardinia, Italy): Geosite Potential, Conservation, and Evaluation as Part of a Regional Environmental Resource M. Franceschelli 1  · S. Columbu 1  · F. M. Elter 2  · G. Cruciani 1 Received: 29 June 2021 / Accepted: 22 October 2021 © The Author(s), under exclusive licence to The European Association for Conservation of the Geological Heritage 2021 Abstract The wollastonite–garnet–diopside-bearing marbles cropping out in the Migmatite Complex west to the Tamarispa and San Lorenzo villages share a common metamorphic and deformational history with the surrounding migmatite, with metamor- phic peak conditions between 650 and 850 °C. Within the marble, there is an interesting and rare garnet mineralization. The peculiar characteristic and geological–cultural and touristic attraction of this geosite is the presence of large garnet crystals (up to 20 cm). The whitish rock matrix is characterized by coarse-grained rock-forming minerals (mainly wollastonite, cal- cite, diopside and subordinately pectolite, quartz, plagioclase, epidote, apatite, titanite) with a compositional layering and a weak foliation (S2 schistosity), parallel to that of the surrounding gneiss and migmatites. At the outcrop scale, the giant garnet crystals often show a brown core in high relief surrounded by a darker rim with less relief. Under conservation state, the wollastonite–garnet–diopside-bearing marbles show an evident diferential alteration with dissolution processes of the matrix and an increasingly pronounced enucleation of the garnet crystals. The Tamarispa outcrop with spectacular giant garnet crystals is here proposed as a new, potential geosite relevant for didactic, cultural, and touristic purposes. Conserva- tion and valorization aspects are discussed within the more general framework of the geological, natural, and environmental resources of the local territory. Keywords Geosite · Sardinia · Marble · Giant garnet crystals · Metamorphism · Geoconservation Introduction and Aims Sardinia island, located in the center of western Mediterra- nean Sea, is interesting and attractive from a geological point of view because rocks related to the igneous, metamorphic, and sedimentary petrogenetic process can be observed and investigated. The island shows a complex geological his- tory starting, at least, from about 500 million years. The ancient geological history of Sardinia is mainly related to the Variscan orogenic cycle and, subordinately, to the Alpine orogeny. The Island consists of a crystalline basement (intru- sive and metamorphic rocks) of Palaeozoic Age, above which post-Variscan sedimentary and volcanic deposits are superimposed. Its current location in the western Mediter- ranean is the result of a series of geodynamic events (rifting followed by counter-clockwise rotation) that occurred in the Miocene (Advokaat et al. 2014 and references therein), lead- ing to its detachment from the southern European continent (Southern France and the Iberian Peninsula). Thanks to this complex geological history, Sardinia hosts several areas of particular scientifc–environmental value, relevant for the understanding of the geological processes, also in the framework of the evolution of the South European region. Geological monuments with uniqueness and rarity for the stratigraphic, sedimentological, structural, geomor- phological, mineralogical, petrographic, and paleontological record are widespread in the Sardinia territory, in the form of a very wide open–air laboratory of Earth Sciences. * S. Columbu columbus@unica.it 1 Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, Cittadella Universitaria, 09042 Monserrato, Italy 2 Dipartimento di Scienze della Terra dell’Ambiente e della Vita, Università di Genova, Corso Europa 26, 16132 Genova, Italy