Tobermorite Locality: Concepción del Oro , Municipio de Concepción del Oro , Zacatecas , Mexico ( Locality at mindat.org ) Size: miniature, 3.9 x 2.3 x 1.9 cm Tobermorite Brilliantly shimmering, lustrous, large balls of tobermorite crystals , forming a translucent "blob" over matrix of pyrite, make this the most visibly appealing example of the species I have seen myself. Its a display-quality miniature, from old Mexican finds (I am told 1960s). This was in the Marty Zinn collection for a long time (#1492). Apparently the small acicular tufts are also tobermorite. — Photo: Robert M. Lavinsky · CC BY-SA 3.0
Tobermorite
A calcium silicate hydrate — the natural analogue of the C-S-H that binds Portland cement.
Intro adapted from Wikipedia (CC BY-SA).
Tobermorite is an IMA-recognized mineral species in the silicate class, with the chemical formula Ca₄Si₆O₁₇(H₂O)₂·(Ca·3H₂O).
It crystallizes in the triclinic crystal system (space group C-1). It is a member of the Tobermorite structural group.
Key properties: Mohs hardness 2.5; specific gravity 2.42–2.46; Vitreous, silky in fibrous aggregates luster; refractive index 1.572; optically biaxial.
IMA symbol Tbm, IMA status: approved, named in 1882.
A hydrothermal alteration product of calcium carbonate rocks, due to contact metamorphism and metasomatism; filling vesicles and cavities in basaltic rocks.
It is found associated with Zeolites, ettringite, portlandite, calcite.
The name is taken from the type locality, Tobermory, Isle of Mull, Scotland.