Vol. 35, pp. 1541-1549 (1997)
a new group of carbonate minerals related
to the hydrotalcite-manasseite group
Ottawa-Carleton Geoscience Centre, Department of Earth Sciences,
Carleton University, Ottawa, Ontario K1S 5B6
Robert A. Gault1
Research Division, Canadian Museum of Nature,
Ottawa, Ontario K1P 6P4
ABSTRACT
Charmarite-2H, charmarite-3T, quintinite-2H, quintinite-3T and caresite-3T are a group of new carbonate minerals with the general formula M2+4Al3+2(OH)12CO3 · 3H2O (4:2 minerals). They are structurally and chemically related to the hydrotalcite- manasseite group (6:2 minerals). Superlattice reflections in the single-crystal X-ray photographs suggest ordering of cations in the 4:2 minerals, whereas the cations of the 6:2 minerals are disordered. 2H and 3T designate the 2-layered hexagonal and 3-layered trigonal structures, respectively. All five species are late-stage hydrothermal minerals in alkaline rocks, and are associated with many rare mineral species. Quintinite-2H occurs at Jacupiranga mine, São Paulo, Brazil; charmarite-2H, charmarite-3T, and quintinite-3H occur at Mont Saint-Hilaire, Quebec; caresite-3T occurs at Mont Saint-Hilaire and at the Corporation Quarry, Montreal, Quebec.
In habit, they range from thin plates to thick tabular crystals, in some cases forming pyramidal or rosette-like aggregates. They generally range from yellow to brown, occasionally colorless to pale blue. All are transparent, with a white streak and vitreous luster, are non-fluorescent, and have a Mohs hardness of approximately 2. All are brittle, with a perfect {001} cleavage and uneven fracture. The measured densities are: charmarite-2H 2.47(1), charmarite-3T2.48(1), quintinite-2H and quintinite-3T 2.14(1), caresite-3T 2.59(1) g/cm3. Charmarite-2H is uniaxial negative, w 1.587(1) and e 1.547(1); charmarite-3T is uniaxial negative, w 1.587(1) and e undetermined; quintinite-2H is uniaxial negative, w 1.533(1) and
e 1.533(1); quintinite-3T is uniaxial positive or negative, w 1.533(1) and
e 1.533(1); caresite-3T is uniaxial negative, w 1.599(1) and e 1.570(1).
All are generally pleochroic, with only the near-end-member quintinite-3T being non-pleochroic.
Charmarite-2H | is hexagonal, space group P6322, a 10.985(5), c 15.10(2) Å, V 2366(4) Å3, Z = 4; |
Charmarite-3T | is trigonal, P3112 or P3212 a 10.985(3), c 22.63(3) Å, V 2366(4) Å3, Z = 6; |
Quintinite-2H | is hexagonal, P6322 a 10.571(1), c 15.139(7) Å, V 1465(1) Å3, Z = 4; |
Quintinite-3T | is trigonal P3112 or P3212 a 10.558(2), c 22.71(3) Å, V 2192(3) Å3, Z = 6; |
Caresite-3T | is trigonal P3112 or P3212 a 10.805, c 22.48(3) Å, V 2273(4) Å3, Z = 6 |
The following ideal formulae were developed from electron-microprobe analysis coupled with TGA-EGA data in some cases:
Charmarite-2H and charmarite-3T:
|
Mn4Al2(OH)12CO3 · 3H2O;
|
Quintinite-2H and quintinite-3T:
|
Mg4Al2(OH)12CO3 · 3H2O;
|
Caresite-3T: | Fe4Al2(OH)12CO3 · 3H2O |
IR spectra are given for all five species.