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5.1 Mineralogy of chloride-carbonate nodules
The chloride component of the nodules is dominated by halite, whereas individual grains of
sylvite are rare. Typically, sylvite is included in halite, making up to 30 vol% of the chloride
assemblage, and in places halite is sprinkled with minute sylvite crystals. Sometimes sylvite
inclusions in halite show crystallographic outlines, however, round, lens-shaped and
ameboid-like blebs of sylvite with different sizes and orientations are a prominent feature of
the chloride masses (Fig. 11). Sylvite domains are often extremely irregular in shape, with
curved re-entrances and attenuated swellings. Some domains are thin and elongated, and
they can be either subparallel or perpendicular to the contacts with the carbonate sheets
(Fig. 11 a-c). Chloride minerals also seal fractures in carbonates (Fig. 11).
The carbonate sheets are very heterogeneous in texture and composition (Figs. 11). In some
occurrences a patchy distribution of textures and compositions is observed, but commonly a
symmetrical zoning across carbonate sheets exists (Fig. 11b). The Na-Ca carbonate (shortite-
like) at the rims, near contacts with chlorides forms intergrowths of acicular crystals. The
interstitial space between these crystals (at polished surfaces) is either porous or filled with
chlorides and Na-K sulphates. The transition from rims to cores is very distinct (Fig. 11b), as
the cores do not show crystalline structure and are principally different in composition. On
average the carbonate core is characterised by Na-Ca composition with significant K
2
O and
SO
3
. Highly variable, but with good correlation, amounts of SO
3
(up to 13 wt%) and K
2
O (up
to 14 wt%) in the individual analyses of core carbonates suggest that Na-Ca carbonates are
intermixed with tiny K-(Na) sulphate phases, the presence of which can be identified at high
magnification. The Ca/Na in the core carbonate is higher than in the rim carbonate. Another
Na-Ca carbonate with the highest Ca/Na is developed along the cleavage planes in the core
and at the contacts with the rims.
An alkali sulphate, aphthitalite (Na
0.25
K
0.75
)
2
SO
4
, is a minor but widespread component of
the carbonate-chloride nodules. It is always associated with halite as irregular blebs,
fringing the outmost rims of carbonate sheets (Fig. 11d), and filling fractures and interstitial
spaces in carbonates (Fig. 11).
Anhydrous and hydrated Na-Ca carbonates with variable Ca/Na ratios are typical in all
nodules, but in one sample (UV-2-03, Fig. 10e) an end-member shortite composition
Na
2
Ca
2
(CO
3
)
3
was found in close association with Cl-bearing Na-Mg carbonate (northupite -
Na
3
Mg(CO
3
)
2
Cl). Unlike heterogeneous and thus barely transparent carbonates in other
nodules, well-formed crystals of shortite and northupite are clear and can be used for the
inclusion studies. The mineral assemblage in this nodule is very complex, and includes
euhedral crystals of apatite and phlogopite, as well as tetraferriphlogopite, djerfisherite, K-
Na and Na-Ca sulphates, Ba-, Ca- and Sr-Ca-Ba- sulphates and carbonates, calcite,
perovskite, and bradleyite Na
3
Mg(PO
4
)(CO
3
). The above minerals are present in aggregates
within the interstitial chloride cement and as inclusions in shortite.
Maas et al. (2005) concluded that Sr-Nd-Pb isotopic ratios for the silicate, carbonate and
halide components in the groundmass of the Udachnaya-East kimberlite support a mantle
origin for the carbonate/chloride components. This conclusion relies in part on accurate age
corrections to measured 87Sr/86Sr. However, the extreme instability of magmatic halides
and alkali carbonates in air, even on the timescale of hours and days (Zaitsev & Keller,
2006), means that Rb-Sr isotope systematics of these kimberlites may have been modified
since kimberlite emplacement in the late Devonian. An attempt to use Cl isotopes as a direct
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