Thin section of glauconite filling rotaliid foram chambers

Two samples from Chatham Rise. The dominant morphology of glauconite is peloidal, with a few composite aggregates and some fossil infills (mainly benthic and planktic forams). The range of colours, from yellow-brown to grass green reflects variation in their chemical maturity – darker greens are higher in K (an average 7.2% K) and have greater crystallographic ordering determined from X-ray diffractometry. Those tending towards more brown hues have higher Fe2O3. Many peloids have a brown rim of limonite (Fe2O3). Individual peloids may also show internal colour gradation from lighter green or brown exteriors to darker green interiors. Both images plain polarized light. Images courtesy of Cam Nelson.
– Peloids are ovoid to tabular with smooth surfaces.
– Most peloids have quasi-radial or interconnected networks of cracks (dark brown – Fe oxide?). This is a common feature that mechanically weakens the grains such that they are unlikely to survive vigorous reworking or abrasion.
– Both examples contain planktic and benthic forams that show the beginnings of Fe-oxide-glauconite chamber linings.
– Image left contains fragmented echinoderm plates and spine cross-sections where perforations are partly filled with red-brown Fe oxide-glauconite.

Left: An irregular peloid with cracked, vermiform glauconite. The dark brown-black, opaque (and isotropic) material, that along its edges is yellow-brown, may be limonite. Right: Peloidal glauconite in shades of green-brown and yellow-brown. The cracks typically taper towards the peloid centre (arrows). The cracks may have formed by syneresis (a process of fluid expulsion typically associated with the transformation of gels to crystalline materials – I think the jury is still out on this interpretation). Cracks will render a peloid more vulnerable to disintegration from vigorous reworking; thus, the present examples have probably moved very little. Both images in plain polarized light. The bar scale applies to both images.

The microcrystalline habit of glauconite presents as a speckled appearance, best seen under crossed polars. The three grains (arrows) are the same as in the plain polarized image above. Grains with first-order grey interference colours are quartz. The bioclasts are probably molluscs and barnacles.

Left: Three things to note here: 1. The planispiral foram chambers have incipient glauconite fill, that along the inner margin of the test wall are transforming to pale green (red arrow). The radial crystallite wall structure is still visible. 2. Three ovoid glauconite peloids to the left of the foram display variations in colour that probably correspond with a range of maturities. The larger grain has developed a few cracks (black arrow). 3. The grain top left resembles barnacle plications; here too glauconite has begun to form with the outer grain rim developing green hues (arrow). Plain polarized light.
Right: A rotaliid foram chamber (radial wall structure) partly filled by yellow-green glauconite and a few silt-sized fragments of quartz. The left side of the fill shows mammillated morphology. Plain polarized light.

An oblique section through an echinoderm spine where the internal perforations are filled with immature, brownish glauconite. Left – plain polarized light. Right – crossed polars. The bar scale applies to both images.

The emphasis in this image pair is glauconite filling invertebrate microporosity: on the left the perforations in an echinoderm plate (e); at centre-right a longitudinal section through bryozoa zooids (b). An irregular, composite mass of glauconite has also formed along the echinoid plate margins also contains silt-sized quartz grains. The bioclasts are cemented by coarse calcite spar. Left – plain polarized light; Right – crossed polars.

The outer chambers of this rotaliid foram are filled with yellow-green glauconite; the inner chambers with calcite spar. The radial wall structure is still visible although much of the calcite has been replaced by limonite and glauconite. The outer margins of the foram may have an isopachous calcite rind (arrows). The remainder of the cements are coarse calcite spar. Plain polarized light.

– Glauconite peloids up to 0.5mm across (10%). Some glauconite grains are deformed around stronger quartz and feldspar grains (by compaction).
– Quartz grains (70-75%) are mostly monocrystalline (there are a few polycrystalline grains) and show a mix of unstrained and strained extinction. Grains are angular to well rounded, but some of the angularity is due to calcite replacement of quartz (some examples shown by white arrows).
– Untwinned K-feldspars (5-10%) show varying degrees of sericite alteration. Some appear superficially like lithic grains.
– Lithics (I) (~5-8%). Be careful to distinguish actual lithics from altered feldspar grains.
– White micas (1-2% – red arrows) show some alteration to chlorite (bluish interference colours).
– The framework clasts and micas have a crude alignment along the top left/bottom right diagonal.
– Cement is almost entirely coarse calcite. Contact between the calcite cement and framework grains is irregular, indicating silica replacement.

The sandstone framework here is compositionally similar to the arenite above, but note the changes in texture and fabric:
– The grains are more densely packed, with significantly less calcite cement and decreased calcite replacement of framework grains.
– The original peloidal shape of the glauconite grains has been distorted by compaction around stronger quartz-feldspar and squeezed between some grains (red arrows for some examples).
– Compaction has fractured some quartz grains – fractures are filled by calcite and possibly clays.