Mineralogy

Grossular

Structural info:

Grossular data

Name origin: Named after botanical name of gooseberry (Ribes grossularium) because of its resembelance in colour.

Description:

Grossular Ca3Al2Si3O12 is a calcium-aluminium nesosilicate mineral that belongs to a garnet group of general formula X3Y3Si3O12. It was named for the first time by Abraham Gottlieb Werne in 1803 (who originally named it cinnamon stone), and it was approved by IMA in 1959 [1]. Grossular forms a series with several minerals of garnet group (Andradite, Uvarovite, Katoite, Pyrop and Hibschite) [1]. Its colour varies from white, colourless or yellow to  red, brown or even green. Lustre of grossular is vitreous. This mineral has brittle tennancy, irregular or sub-chonchoidal fracture and no cleavage. Usually the mineral forms well-shaped dodecahedral crystals but it can be occasionally found in trapezohedral form [1]. In the structure six {SiO4} tetrahedrons enclose one Al atom that is octahedrally coordinated with chromophore {AlO6}. Remaining sites are occupied by calcium(II) atoms. There are 96 oxygen atoms per until cell. Each one is bonded to one tetrahedron, one octanhedron and two of the divalent dodecahedral sites [2].

Grossular occurs in contact or regionally metamorphosed limestones. At pressures of 23–25 GPa and temperatures of 1000–1600 K it dissociates into CaSiO3 (perovskite) and Al2O(corundum) [3]:

Ca3Al2Si3O12  →  3CaSiO3 + Al2O3

grossular1

Grossular, Maglovec quarry, Vyšná Šebastová, Prešov Co., Slovakia [photo: Michal Hegedus]

Literature:

[1] http://www.mindat.org/min-1755.html
[2] http://ruby.colorado.edu/~smyth/min/garnet.html
[3] Takafuji, N., T. Yagi, N. Miyajima, and T. Sumita (2002), Study on Al2O3content and phase stability of aluminous-CaSiO3 perovskite at high pressure and temperature, Phys. Chem. Miner., 29, 532537, doi:10.1007/s00269-002-0271-5.