(via Gemmology Queensland, Vol.4, No.1, January 2003) Hiroshi Kitawaki writes:
Forsterite is one of the end member minerals in the olivine group of minerals. It was named after a British mineral collector Jacob Forster.
Many solid solutions of olivine minerals are known, among which forsterite and fayalite form an isomorphous series. A yellow green crystal, with intermediate composition in this isomorphous series is called peridot. This gemstone is a birthstone of August and is one of the popular gemstones. On the other hand, forsterite is not common a as gemstone.
A forsterite that a chemical formula of, or close to, the end member is rare because one element (Mg) in the formula is generally replaced easily by Fe in nature. The mineral, forsterite, is found in ultrabasic rock or dolomitic limestone that had gone through thermal metamorphism.
Natural Forsterite
The green stone described in this report is a natural forsterite that we recently investigated and it is said to be from Sri Lanka according to its client. Its RI measured 1.635 – 1.670 with DR 0.035. The SG was 3.29 and the stone was inert to UV light. Directionally oriented minute needle-like inclusions were observed under magnification. Its compositional analysis by X-ray fluorescence detected considerable amount of Fe and very small amount of Ca and Mn as well as the main elements of Mg and Si.
Crystals within the olivine group have been extensively studied, and used in heat resistant materials, insulators or lasers. Among the crystals used in industry, single crystal forsterite of high quality and large size have been synthesized by the crystal pulling method for use as crystals that laze the near infrared.
Synthetic forsterite
Synthetic forsterite, which is marketed as Tanzaniod has been synthesized in Russia for gem use. As you can easily imagine from its name, it is meant to imitate tanzanite. The RI, DR and SG of the Tanzaniod are consistent with those of natural forsterite, with Tanzaniod fluorescing a weak orange to yellow and greenish yellow under long and short wave ultraviolet lights respectively. Prominent pleochroism of blue and violet is also recognized. Under the hand-held spectroscope, absorption bands are seen on 490nm, 520nm and 580nm. Dot-like and short needle-like inclusions are observed under magnification. The compositional analysis by fluorescent X-ray detected considerable amount of Co and V, other than the main elements of Mg and Si.
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