Vol. 36, pp. 1295-1301 (1998)

Thomasclarkite-(Y), a new sodium - rare-earth-element bicarbonate mineral species from Mont Saint-Hilaire, Quebec

Joel D. Grice¹and Robert A. Gault
Research Division, Canadian Museum of Nature,
P.O. 3443, Station D, Ottawa, Ontario K1P 6P4

¹E-mail address: jgrice@mus-nature.ca

ABSTRACT

Thomasclarkite-(Y), ideally Na(Y,REE)(HCO₃)(OH)₃ · 4H₂O, is a newly identified mineral species from Mont Saint-Hilaire, Quebec. It occurs as individual, well-formed, blocky, white-to-yellow crystals up to 2 mm in length. Associated minerals include aegirine, albite, analcime, ancylite-(Ce), astrophyllite, calcite, catapleiite, dawsonite, elpidite, epididymite, franconite, gaidonnayite, galena, genthelvite, microcline, molybdenite, natrolite, petersenite-(Ce), polylithionite, pyrochlore, rhodochrosite, rutile, sérandite, siderite, sphalerite, zircon and an unidentified NaREE carbonate. The transparent to translucent mineral has vitreous luster and a white streak. It is soft (Mohs hardness 2-3) and brittle, with an uneven fracture,
a perfect {010} cleavage and good {1010} parting. Merohedral twinning on {101} is ubiquitous. Thomasclarkite-(Y) is pseudo-uniaxial negative,
w = 1.540(4) and e = 1.40(2). It is monoclinic, space group P2, a 4.566(1), b 13.018(6), c 4.566(2) Å, B 90.15(3)°, V270.2(2) Å³ and Z = 1.

The strongest X-ray powder-diffraction lines [d in Å (I)(hkl)] are:

12.97(10)(010)

6.52(3)(020)

4.57(3)(100,001)

4.32(5)(110,011)

3.223(3)(-101,101)

3.133(5)(-111,111)

2.593(3)(-131,131)

2.035(3)(201,102)

2.016(4)(230,032)

1.844(3)(231,132)

The infrared spectrum is given. Two electron-microprobe analyses and a thermogravimetric analysis gave:

Na₂O - 6.72
(6.20 - 7.24)

CaO - 0.33
(0.35 - 0.32)

Y₂O₃ - 14.52
(14.97 - 14.07)

La₂O₃ - 8.63
(8.54 - 8.72)

Ce₂O₃ - 17.11
(18.44 - 15.79)

Pr₂O₃ - 1.92
(1.94 - 1.90)

Nd₂O₃ - 5.63
(5.60 - 5.66)

Sm₂O₃ - 1.22
(1.27 - 1.17)

Gd₂O₃ - 1.22
(1.13 - 1.31)

Dy₂O₃ - 2.04
(2.08 - 2.00)

Er₂O₃ - 2.20
(2.20 - 2.20)

Yb₂O₃ - 1.32
(1.45 - 1.19)

ThO₂ - 0.54
(0.48 - 0.60)

F - 0.17
(0.35 - 0.00)

CO₂ - 12.00

H₂O - 26.60

O = F - -0.07

for a total of 102.10 wt.%.

The empirical formula, based on 10 anions and one atom of C, is :

(Na_0.80Ce_0.18Ca_0.02)_S1.00(Y_0.48La_0.20Ce_0.21Pr_0.04Nd_0.12Sm_0.03

Gd_0.03Dy_0.04 Er_0.04Yb_0.03)_S1.22(HCO₃)[O_6.97F_0.03]H_9.93.

D_calc. = 2.34 g/cm³ and D_meas. = 2.30(2) g/cm³. The structure has been refined to R = 0.055 using a twinned crystal. The structure is layered on {010}. One layer consists of [HCO₃]^- groups oriented perpendicular to (010) and H-bonded to a H₂O layer. A second layer consists of (Yr₈) polyhedra and (Nar₆) polyhedra which are H-bonded to the H₂O layer. In other bicarbonate minerals, the H-bonding is between the CO₃ groups forming chains, whereas thomasclarkite-(Y), the H-bonding is directed toward the H₂O groups.

Logo ™