A Note on the Chemical Composition and Origin of Peridot from the Harrat Kishb, Saudi Arabia

Abstract

Zabarjad (peridot in Arabic) is a common gemstone of valuable importance used in jewelry. It is the mineral olivine that has some gemological characters that classify it as peridot or gem olivine. Although it is not found in huge quantities in Saudi Arabia, there are some mining activities to extract peridot from the Harrat Kishb where the mineral is found scattered in alkali basalt or in the scree and wadi deposits. The basaltic flows at the Harrat Kishb include this gem olivine in the form of mantle xenoliths that comprise both dunite and peridotite. The size of collected peridot lies in the range of 0.4-0.8 cm and it occurs either as free olivine or in the form of cluster with components of host basalt. The collected crystals are transparent olive-green after washing and screening. Using the modern technique of analysis by the electron microprobe, the accurate chemical composition of peridot crystals was obtained. The studied peridot is a Mg-rich end-member of the foresterite (Mg) and fayalite (Fe) solid solution series where the forsterite content is Fo90.35-92.31. Such composition of olivine is similar to majority of olivine in the ultramafic mantle beneath the western Arabian Peninsula. The present work uses the concentrations of some elements such as Mn, Ni and Ca in addition to the forsterite content to characterize the Saudi peridot. It is typically terrsterial when compared to extraterrestrial (pallastic) olivine, and mostly xenocrystic and rarely magmatic. The forsterite content (Fo>90) suggests transition from a residual to cumulate origin.