Dc. Kamineni et al., PALYGORSKITE FROM FRACTURE-ZONES IN THE EYE-DASHWA LAKES GRANITIC PLUTON, ATIKOKAN, ONTARIO, Canadian Mineralogist, 31, 1993, pp. 173-183
The mode of occurrence, X-ray, chemical, uranium series and stable iso
tope characteristics of palygorskite found in fracture zones to a dept
h of 25 m in the Eye-Dashwa Lakes granitic pluton, located near Atikok
an, Ontario, are documented. The palygorskite formed as a product of a
lteration of epidote according to the reaction: epidote + chlorite + w
ater --> palygorskite + goethite +/- smectite-group mineral. The palyg
orskite has d values of 10.48 to 2.28 angstrom, is orthorhombic (Pbmn)
, and is compositionally uniform, with cation values of Si 4.07 to 4.1
5, Mg 0.62 to 0.68 and Al 1.05 to 1.17, based on 11 atoms of oxygen, a
nd minor Fe. Ca and K contents. There are significant abundances of Sr
(557 mug/g), U (10 mug/g) and Th (20 mug/g), inherited from the paren
t epidote. U is preferentially incorporated into palygorskite, whereas
Th is incorporated into goethite. It is fractionated in light rare-ea
rth elements (SIGMALREE = 1974; La(N)/Yb(N) = 142) inherited from the
parental epidote (SIGMALREE = 2844; La(N)/Yb(N) = 56). Most of the ana
lyzed samples of palygorskite from greater depths have U-234/U-238 rat
ios close to unity, indicating secular equilibrium, whereas palygorski
te samples located closer to the surface have Th-230/U-234 ratios grea
ter than one, suggesting geochemical disturbance in recent times (<0.3
5 Ma). Stable isotope characteristics (deltaD between -75 and -28 part
s per thousand deltaO-18 between 18.4 and 23.4 parts per thousand) ind
icate that palygorskite formed in equilibrium with modern groundwaters
at temperatures less-than-or-equal-to 25-degrees-C, but in some cases
, it experienced selective depletion in D accompanied by loss of U.