Multiple episodes of zeolite deposition in fractured silicic tuff Page: 4 of 9
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Table 1. Chemical analyses of zeolites.
Heulandite
Tabular Tabular Pris- Stellr-
core rim matic ite
SiO2 64.67 65.06 64.46 64.60
A1203 15.34 15.61 14.82 15.33
MgO 0.08 1.86 1.86 -
CaO 5.86 4.83 4.73 8.29
SrO 2.53 1.06 0.88 -
Na20 0.64 0.22 0.29 0.15
K20 0.41 0.17 0.17 0.08
Total 89.53 88.81 87.21 88.45
Si 28.18 28.12 28.32 28.12
Al 7.88 7.95 7.67 7.87
Mg 0.05 1.20 1.22 -
Ca 2.74 2.24 2.23 3.87
Sr 0.64 0.27 0.22 -
Na 0.54 0.18 0.25 0.13
K 0.23 0.09 0.10 0.04
0 72 72 72 72
Si/Al 3.58 3.54 3.69 3.58
UE-25a#1 1243
2USW G-2 1505
calcite has been dated.
Between deposition of the tabular heulandite and
prismatic heulandite, the fluid composition must
have changed. The compositional boundaries
between rims and cores of tabular crystals are not
sharp growth boundaries, but somewhat variable
(Fig. 2) and appear to indicate diffusion from the
edges and along cracks or imperfections in the
crystals. The zonation is not of equal width around
the edges of crystals and appears to have been
influenced by crystallographic orientation. A
sharper and more regular zonation boundary would
have resulted from overgrowths on the tabular
crystals. SEM images indicate that when moreFigure 2. Backscattered electron SEM image of
zoned tabular heulandite showing irregular
pattern of Mg enrichment (darker areas).magnesian heulandite was deposited on existing
tabular crystals, it formed prismatic crystals on the
surface of the tabular crystals. The intermediate
compositions in the visibly zoned areas between the
rims and the core, and the intermediate
compositions of some rims could indicate a gradual
change in composition of the fluid, but the pattern
of Mg distribution observed in BSE images and the
consistent relationship of chemistry with
morphology suggest two separate intervals of
headite deposition with different fluid
compositions and diffusion into the tabular crystals
rather than a gradual change in fluid composition
over time.
Results of thermodynamic modeling show that
silica activity has a strong control over, whether
stellerite and/or heulandite form relative to other
zeolites (e.g., clinoptilolite and mordenite). Figure
3 shows the effect ofsiica activ tyadthe
calculated 2equiJ'bria using log[(aK4) &aCa'1 vs.
log[(aNa) /aCa 1 for zeolites found at Yucca
Mountain. Present-day cation concentrations in
Yucca Mountain waters are plotted on the figure for
reference. At elevated silica activities (e.g.,
equilibrated with amorphous silica), clinoptilolite
and mordenite are the zeolites that are most likely to
form. At lower silica activities (e.g., equilibrated
with cristobalite or tridymite) stellerite and/or
heulandite are the zeolites most likely to form.
Stellerite and heulandite coexist in some fractures
and have similar Si/Al'ratios, so it is not surprising
that modeling of silica activities does not
differentiate between the two minerals. Silica
activities in the fractures are most likely in
equilibrium with cristobalite or quartz as indicated
by the coexistence of these silica polymorphs with
--- Amorphous SMca Saturated
- Cristobaflts saturated
Quartz saturated
10 - - - - - r - - - r r -
8 Analcime . - - -
Mordenite
2 l -
JYM Water ptilolite
- Stelleritel
- HeuOndite
-01 -8 -6 -4 -2 0 2 4 6 8 10
Log [(aK)2Iaca2+]
Figure 3 Lod 4)2/ ] vs.
log[(aNa) faCa I diagrams for zeolite phases
found at Yucca Mountain constructed at 75*C
and at variable silica activities.
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Carlos, Barbara A.; Chipera, Steve J. & Snow, Marjorie G. Multiple episodes of zeolite deposition in fractured silicic tuff, article, April 1, 1995; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc697700/m1/4/: accessed April 22, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.