Garnets from Val d’Ala Rodingites, Piedmont, Italy: An Investigation of Their Gemological, Spectroscopic and Crystal Chemical Properties

Valeria Diella,Maria Iole Spalla,Rosangela Bocchio,Franca Caucia,Ilaria Adamo,Antonio Langone,Lucia Mancini,Nicoletta Marinoni

doi:10.3390/min9120728

Abstract

In Val d’Ala (Piedmont, Western Alps, Italy), the more interesting rocks for the mineralogical research are represented by rodingites (rich in mineralized veins and fractures) associated with serpentinites in the eclogitized oceanic crust of Piemonte Zone, south of Gran Paradiso Massif. Among the vein-filling minerals, garnets are the most appreciated as mineral specimens and, in less degree despite their vivid and rich colors, for their potential as gem-quality materials. This study provides a complete gemological characterization of five faceted samples and offers new information by means of Synchrotron X-ray computed micro-tomography imaging gem features. Electron-probe microanalysis (EMPA) and laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) established that the chemical composition of garnets from different localities, resulted both close to pure andradite, enriched in light rare earth elements (LREE) with a positive Eu anomaly, and grossular-andradite solid solution (grandite), enriched in heavy rare earth elements (HREE). X-ray powder diffraction analyses indicate the possible coexistence of almost pure grossular and andradite. A spectroscopic approach, commonly used with gem-like material, by Raman and diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, completes the characterization of the samples. The new data on the textural and geochemical features of the grandite and andradite garnets suggest local growth processes under various chemical and oxidation conditions of metasomatic and metamorphic fluids interacting with the host-rocks. Garnets represent long-lasting mineral records of the complex geological history of the Val d’Ala rodingitic dikes during their oceanic- and subduction-related metamorphic evolution.

Highlights

Val d’Ala (Piedmont, Italy) is internationally renowned for mining activity of minerals used for practical and decorative purposes
The measured gravity ranges from 3.63 g/cm3 to 3.66 g/cm3, in veinlets, or fractures
All the analyzed gems are gravity ranges from 3.63 g/cm3 to 3.66 g/cm3, in agreement with the literature data for solid solution inert to long and short UV

Summary

Introduction

Val d’Ala (Piedmont, Italy) is internationally renowned for mining activity of minerals used for practical and decorative purposes. The orange–brown grossular-garnets, known with the variety name “hessonite”, were used in the past as precious gemstones in the traditional costumes of the valley and as a sign of engagement. 2 of 192 of 18 grossular‐garnets, known with the variety name “hessonite”, were used in the past as precious gemstones up to 2costumes cm in size, are valley generally in of rodingites and show a color from dark toin incrystals, the traditional of the and found as a sign engagement. The grossular crystals, upbrown to 2 cm red orange with large variation in the habit and complex micro crystal faces. Most of the deposits size, are generally found in rodingites and show a color from dark brown to red orange with large variation crystals, that havefaces

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