Merwinite
Merwinite is an IMA-recognized mineral species in the silicate class, with the chemical formula Ca₃Mg(SiO₄)₂.
It crystallizes in the monoclinic crystal system (space group P2_1/a). It is a member of the Aphthitalite structural group.
Key properties: Mohs hardness 6; specific gravity 3.15–3.32; Vitreous luster; optically biaxial.
IMA symbol Mw, grandfathered status — known before the IMA’s founding, named in 1921.
Found in siliceous dolomitic limestone in contact metamorphic zones, formed at relatively elevated temperatures, locally in substantial quantities.
It is found associated with Gehlenite, spurrite, monticellite, vesuvianite (Crestmore, California, USA); spurrite, gehlenite, spinel (Scawt Hill, Ireland).
The name refers to Dr. Herbert Eugene Merwin (1878–1963), American mineralogist and petrologist, Carnegie Institute, Washington, D.C., USA.