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Mineralogy and crystal structures of barium silicate minerals from Fresno County, California
Laurel Basciano*, Lee A. Groat (The University of British Columbia)
6339 Stores Road, Vancouver, British Columbia V6T 2B4, [email protected]
Robert A. Gault, Joel D. Grice (The Canadian Museum of Nature)
Andrew C. Roberts (The Geological Survey of Canada)
Gail E. Dunning (773 Durshire Way, Sunnyvale, California)
The sanbornite deposits at Big Creek and Rush Creek, Fresno County, California host many rare barium
silicates. Fresno County is the type locality for nine of these minerals. Bigcreekite, UnKnown mineral 6
(UK6), verplanckite and walstromite are part of this series of barium silicates. Walstromite and verplanckite
were described in detail by Alfors et al. (1965) and their crystal structures were solved by Glasser & Glasser
(1968) and Kampf et al. (1973), respectively. Bigcreekite and UK6 have not been previously described.
As shown in Figure 1, the Fresno County localities occur along a narrow five km belt on the western slope of
the Sierra Nevada, approximately 90 km by road northeast of Fresno. The sanbornite-bearing rocks are in two
meta-sedimentary roof pendants along Rush Creek (Esquire 1) and on the eastern border of an area of metasedimentary rocks along Big Creek (Esquire 7). The sanbornite deposits consist of gneissic metamorphic
rocks, composed mainly of variable amounts of sanbornite and quartz, and minor amounts of pyrrhotite and
diopside. The sanbornite-quartz rock forms conformable tabular bodies up to 13 m thick within foliated
quartzite in a migmatite zone at or within 100 m of the contact with a large granodiorite pluton. The trend of
the outcrop of the sanbornite-bearing quartzite is sub-parallel to the granodiorite contact. Some fragments of
the sanbornite-bearing rock have broken off and are incorporated into the locally massive quartz veins at the
margin of the pluton. The quartzite is conformably inter-bedded with steeply dipping mica schist, amphibole
schist, and wollastonite-bearing quartzite (Alfors et al. 1965). The metamorphic rocks have undergone
multiple deformations resulting in at least three superimposed sets of folds (Hinthorne 1974).
Figure 1: Sanbornite localities in the Western Metamorphic Belt Subprovince (after Hinthorne 1974).
Mineralogy and crystal structures of barium silicate minerals from Fresno County, California
There is a gradational change in the quartz to sanbornite ratio in both the Big Creek and Rush Creek localities.
The changes are mainly dependent on local bulk compositions. Therefore, the sanbornite-quartz rock can be
mapped as units with high quartz and sanbornite content. This layered structure of the deposit indicates that
bedded barite deposits from an exhalitive sedimentary origin is the most probable protolith for the quartzsanbornite rocks, similar to the barite deposits in Nevada. Other sanbornite deposits are known from
Chickencoop Canyon, Tulare County and Incline, Mariposa County, both in California; and from Baja
California in Mexico.
Bigcreekite, ideally BaSi2O5·4H2O, occurs along very thin transverse fractures in well-laminated quartz-rich
sanbornite portions of the rock. It post dates the other associated barium silicates and represents either a later
primary phase from intruded fluids or a product of alteration of pre-existing Ba-rich minerals, possibly
sanbornite. It is colourless and forms poorly developed crystalline masses parallel to the fracture direction.
There are two perfect cleavages on {010} and {001}. Bigcreekite is biaxial (-), with indices of refraction α
1.537(2), β 1.538(2), γ 1.541(2); X = b, Y = a, Z = c and 2Vmeas = 59.2(5)°, 2Vcalc = 60.1°. It is orthorhombic,
space group Pnma, a 5.038(6), b 9.024(3), c 18.321(6) Å, and Z = 4. The empirical formula of bigcreekite
(based on nine O) is (Ba1.0Na0.01)å1.01Si1.99H8.02O9. Dcalc = 2.739 g/cm3 and Dmeas = 2.66(3) g/cm3. The
structure has been refined to an R index of 3.5%. As shown in Figure 2, bigcreekite is a hydrous chain silicate
containing four-membered rings that form chains of silica tetrahedra, parallel to [100] and staggered in the
[001] direction. Water molecules fill the large spaces between the rows of silicon tetrahedra. The structure has
similarities to those of sanbornite and gillespite. Bigcreekite was named for the type locality.
Figure 2: The crystal structure of bigcreekite.
UK6, ideally (Si, Al)4Ba3ClO9(Cl, H2O)4, is a newly identified mineral species that forms irregular masses up
to one cm enclosed in parallel-bedded sanbornite-quartz rock. It is light blue-grey, with one perfect cleavage on
{0001}. UK6 is uniaxial negative, with indices of refraction ε 1.594(2), ω 1.642(2). It is hexagonal, space
group P63mc, a 5.2432(7), c 29.859(6) Å and Z = 2. The empirical formula of UK6 (based on 7 cations and
two H2O per formula unit) is (Ba2.97Na0.03Ca0.01)å3.01(Si2.63Al1.34Ti0.03)å4.00ClO9(2H2OCl1.61F0.05)å3.66, and Dcalc =
3.709 g/cm3. The structure was refined to an R index of 4.9%. The UK6 structure, shown in Figure 3, is
based on double tetrahedral layers, [T4O8]∞∞, consisting of six-membered rings, with three layers of barium
polyhedra connecting the tetrahedral layers. UK6 is part of the monteregianite-(Y)-wickenburgite series
(Struntz classification) and is structurally and chemically similar to cymrite.
Mineralogy and crystal structures of barium silicate minerals from Fresno County, California
Figure 3: The crystal structure of UK6.
Glasser & Glasser (1968) solved the crystal structure of walstromite and refined it to an R index of 16% using
photographic methods. In this study it was refined in space group P1 to an R index of 3.8%. The refined cell
parameters are a 6.733(1), b 9.608(2), c 6.685(1) Å, α 69.64(3), β 102.29(3), γ 96.89(3)°. The empirical
formula of walstromite (based on nine anions per formula unit) is (Ca1.95Na0.01)å1.96Ba1.02(Si3.00Al0.02)å3.02O9
which is similar to that determined by Glasser & Glasser (1968). The walstromite structure consists of trigonal
rings of silica tetrahedra arranged in layers on (101) with barium and calcium atoms lying approximately
halfway between the silicon tetrahedral rings, perpendicular to the a axis. Walstromite is isostructural with
margarosanite, as predicted by Glasser & Glasser (1968).
The crystal structure of verplanckite was originally solved by Kampf et al. (1973) in space group P6/mmm to
an R index of 10.2%. In this study it was refined in P6/mmm with unit cell parameters a 16.300(2) c 7.169(1)
Å, Z = 3 to an R index of 4.0%. The empirical formula of verplanckite (based on 12 Si and Al per formula
unit) is (Ba12.76Ca0.01Na0.21)å12.98(Mn4.54Ti1.19Mg0.03Fe0.20)å5.96(Si11.66Al0.34)å12.00O31.78(Cl23.64F0.49)å24.13, which is
significantly different from those reported by Alfors et al. (1965) and Kampf et al. (1973). Verplanckite is a
framework structure consisting of four-membered tetrahedral rings and square pyramid polyhedra that form
open hexagonal rings, with barium polyhedra located along the inner edges of the rings. There is a large
degree of disorder in the verplanckite structure. Verplanckite is similar to the zeolite group of minerals with a
very open framework.
Mineralogy and crystal structures of barium silicate minerals from Fresno County, California
References
ALFORS, T. J., STINSON, M.C., MATTHEWS, R. A. & PABST, A. (1965): Seven new barium
minerals from eastern Fresno County, California. Am. Mineral. 50, 314-340.
GLASSER, L.S.D. & GLASSER, F.P. (1968): The crystal structure of walstromite. Am. Mineral. 53, 913.
HINTHORNE, J.R. (1974): The Origin of Sanbornite and Related Minerals. PhD. thesis, Univ.
California, Santa Barbara, U.S.A.
KAMPF, A.R., KHAN, A.A. & BAUR, W.H. (1973): Barium chloride silicate with an open framework:
Verplanckite. Acta Crystallogr. B29, 2019-2021.
Biographical Note:
Laurel Basciano
M.Sc., University of British Columbia, 1999
B.Sc. (Honours), Queen’s University, 1997