Goldmanite Garnet: Gemstone Information
Name approved by the CNMMN of the IMA. Named after Marcus Isaac Goldman (1881–1965), sedimentary petrologist, US Geological Survey.
Found in vanadium-bearing sedimentary rocks or contact-metamorphosed carbonaceous and calcareous shales and sandstones. Also in magnetitebearing skarns. Type locality is Laguna District, Cibola Co., New Mexico. It occurs at Otjosondu, Namibia, and also at Usakos, Namibia. In Katkamsandi, Hazaribagh district, India, this rare garnet occurs as brownish vitreous masses scattered in matrix. Additionally it occurs at Bihar, and at Netra, Balaghat district, Madhya Pradesh, India. Also reported as dark yellow to reddish yellow.
Solid solutions exist between grossular, andradite and goldmanite. Manganese can replace calcium and a manganoan goldmanite exists tending towards the hypothetical end-member ‘yamatoite’ [Mn3V2(SiO4)3]. Vanadium can also replace the aluminium in spessartine and pyrope, producing colour change garnets. Idiochromatic vanadium gives rise to dark green to brownish green or grass-green colours. Refractive Index – 1.792–1.834 (natural) and 1.834 (synthetic); for manganoan material RI is 1.855. SG – 3.765 (calculated). Reported as a minor component in some garnets, e.g. vanadium-bearing tsavorite. Not reported as gem material to date.