Abenakiite-(Ce)

Abenakiite-(Ce) is a mineral of sodium, cerium, neodymium, lanthanum, praseodymium, thorium, samarium, oxygen, sulfur, carbon, phosphorus, and silicon with a chemical formula Na₂₆Ce₆(SiO₃)₆(PO₄)₆(CO₃)₆(S⁴⁺O₂)O. The silicate groups may be given as the cyclic Si₆O₁₈ grouping. The mineral is named after the Abenaki, an Algonquian Indian tribe of New England. Its Mohs scale rating is 4 to 5.^[3]

Abenakiite-(Ce)
General
Category	Silicate, cyclosilicate
Formula	Na26Ce6(SiO3)6(PO4)6(CO3)6(S4+O2)O
IMA symbol	Abk-Ce[1]
Strunz classification	9.CK.10
Crystal system	Trigonal
Crystal class	Rhombohedral (3) H-M symbol: (3)
Space group	R3
Unit cell	a = 16.02, c = 19.76 [Å], Z = 3
Identification
Colour	Pale brown, to dark brown
Crystal habit	Euhedral Crystals - Occurs as well-formed crystals showing good external form.[2]
Cleavage	{0001}, poor
Fracture	Conchoidal
Mohs scale hardness	4–5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent[2]
Specific gravity	3.32 gm/cc.
Density	3.21
Optical properties	Uniaxial (−)
Refractive index	nω=1.59, nε=1.57
Other characteristics	Radioactive
References	[3][4]

Occurrence and association

Abenakiite-(Ce) was discovered in a sodalite syenite xenolith at Mont Saint-Hilaire, Québec, Canada, together with aegirine, eudialyte, manganoneptunite, polylithionite, serandite, and steenstrupine-(Ce).^[3][4]

Notes on chemistry and relation to other species

Combination of elements in abenakiite-(Ce) is unique. Somewhat chemically similar mineral is steenstrupine-(Ce).^[4][5] The hyper-sodium abenakiite-(Ce) is also unique in supposed presence of sulfur dioxide ligand. With a single grain (originally) found, abenakiite-(Ce) is extremely rare.^[3]

Crystal structure

In the crystal structure, described as a hexagonal net, of abenakiite-(Ce) there are:^[3]

• chains of NaO₇ polyhedra, connected with PO₄ groups
• columns with six-membered rings of NaO₇, and NaO₇-REEO₆, and SiO₄ polyhedra (REE – rare earth elements)
• CO₃ groups, NaO₆ octahedra, and disordered SO₂ ligands within the columns

See also

• List of minerals