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The UK58 Group: Quintinite-3T, Charmarite-2H, Charmarite-3T, and Caresite-3T Abstracted by Malcolm Back
Charmarite-2H occurs as tabular hexagonal crystals to 0.5 mm. across, commonly stacked along [0001] to form pagoda-like aggregates up to 1 mm. Long, and as thin mica-like plates in some cases forming star-shaped aggregates. The stacked tabular crystals can show parallel-growth features along [0001]. The most common forms observed are {0001} and {10(bar1)0}. It is transparent, with a vitreous lustre, orange-brown, pale brown, pale blue or colourless, and the streak is white. There is one perfect cleavage {0001}, and an uneven fracture. The Mohs hardness is about 2, and it is non-fluorescent in ultraviolet light. The mineral is attacked by 1:1 HCl (hydrochloric acid) and HNO3 (nitric acid) with effervescence, and dissolves in H2SO4 (sulfuric acid) with effervescence. Charmarite-2H is uniaxial negative, w 1.587(1) and e 1.547(1), with weak pleochroism in the pale-blue varieties, w = colourless e = pale-blue, and strong pleochroism in the orange-brown varieties, w = orange-brown e = very pale brown. It is hexagonal, space group P6322, a 10.985(5) c 15.10(2) Å, V 1578(3) Å and Z = 4. Dcalc. = 2.50 g/cm3 and Dmeas. = 2.47(1) g/cm3.
Quintinite-3T occurs as hexagonal tabular crystals on {0001} to 1 mm across, commonly forming rosettes, in some cases in an epitactic relationship with donnayite-(Y). Common forms observed are {0001} and {10(bar1)0}. It is transparent, with a vitreous lustre, bright yellow to pale yellow and the streak is white. There is one perfect cleavage {0001}, and an uneven fracture. The Mohs hardness is about 2, and it is non-fluorescent in ultraviolet light. The mineral is attacked slightly by 1:1 HCl, strongly by HNO3, and dissolves in 1:1 H2SO4. Quintinite-3T is uniaxial positive or negative, w 1.533(1) and e 1.533(1), both the same within the error of measurement (0.0001), and most are non-pleochroic. One sample with Mg:Fe of ~1 is pleochroic with w = dark green, e = light green. It is trigonal, space group P3112 or P3212, a 10.558(2) c 22.71(3) Å, V 2192(3) Å and Z = 6. Dcalc. = 2.14 g/cm3 and Dmeas. = 2.14(1) g/cm3.
Caresite-3T occurs tabular crystals on {0001} and pyramidal crystals to 0.5 mm across, forming irregular aggregates. Forms observed are {0001} and {10(bar1)0}, generally with a hexagonal pyramid which is not consistent with the symmetry. This apparent form may be caused by a combination of an {h0(bar h)1} trigonal dipyramid and an {hh(bar2)(bar h)l} rhombohedron or by a diminishing pinacoid during crystal growth. It is transparent, with a vitreous lustre, green-black, but it is normally coated with a fine-grained smectite or with chamosite. There is one perfect cleavage {0001}, and an uneven fracture. The Mohs hardness is about 2, and it is non-fluorescent in ultraviolet light. The mineral dissolves in 1:1 HCl, is strongly attacked by HNO3 and H2SO4, with effervescence. Caressite-3T is uniaxial negative, w 1.599(1) and e 1.570(1). It is trigonal, space group P3112 or P3212, a 10.805(3) c 22.48(3) Å, V 2273(4) Å and Z = 6. Dcalc. = 2.59 g/cm3 and Dmeas. = 2.59(1) g/cm3.
Source:Chao, G.Y. & Gault, R.A.(1997). Quintinite-2H, Quintinite-3T, Charmarite-2H, Charmarite-3T and Caresite-3T, a new group of carbonate minerals related to the Hydrotalcite - Manasseite group. Can. Min. 35:1541-1549.
This article may not be copied, distributed or reprinted in any form without the author's permission. To contact the author, please use the e-mail address provided. If you are unable to contact the author, please contact the Canadian Rockhound. Authorized reprints must acknowledge the author, original source and the Canadian Rockhound, and include the website URL address of the Canadian Rockhound. Document Number: CR9802109
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