Geology Reference
In-Depth Information
Fig. 16. (a) Elongated cyanobacterial filaments covered with EPS, note small clots of anhedral calcite crystals, L ´ˇky E
point, March 2003; (b) three dimension network of EPS, H´j upper waterfall point, June 2003; (c) moss stem on the
tablet exposed between August 2002 and November 2003 at the H´j upper waterfall point, arrow indicates direction
of water flow; (d) cross section of moss stems, the same sample as in c; a, b - lyophilized sample, SEM images;
d - thin section.
tubes of Oocardium stratum are overgrown syntaxi-
ally and cemented together into a more compact
structure (Figs 14a, b & 21a). However, the bushy
shape is still retained. The Oocardium stratum
tubes inherit their crystal properties from their
parent tubes (Golubic et al. 1993). Hence, many
neighbouring tubes display uniform optical orien-
tation and they, while being overgrown, form
bigger sparite crystals (Fig. 21b, c). However,
some defects must occur, probably during the div-
ision of the cells, since the bush that is a progeny
of one tube, is composed not of one but a few
sparite crystals slightly differing in optical orien-
tation (Fig. 21d). Cementation of neighbouring
tubes is a rapid process. On a tablet exposed in the
course of 14 months, more than half of the tubes
were cemented together; however, some primary
inter-tube voids within bushes still remain empty.
The tube interiors survived empty as well.
however, their orientation becomes variable just
over the substrate because of competitive growth
fabrics (Figs 18c, 22, 23a, b, d; cf. Gonz´lez et al.
1992).
The sparry crystals occur in laminae or lenses,
the latter commonly surrounded by micrite. The
thicknesses of individual laminae range from a
few micrometres to more than one millimetre.
Some laminae are uniform in thickness, while
others vary in thickness as a result of their uneven
tops (Fig. 18d). The uneven tops are characteristic
mostly of thicker laminae, composed of fan-shaped
crystals. Such crystals display undulose extinction
and comprise several smaller radially arranged
subcrystals. The fan-shaped crystals may coalesce
laterally or are separated from each other. Some
sparry laminae exhibit high intercrystalline porosity
(Fig. 23a, b). The porosity is higher in sparry lamina
from the Karw ´w and H´j sites than in those formed
at the L ´ˇky site. Sparry crystals abound in moulds
of filamentous cyanobacteria, diatom stalks and
sporadic diatom frustules (Fig. 12a, b). The fila-
ments are mostly normal to the crystal substrate or
form upward radiating fans.
Laminae built of sparry calcite are common
components of tufa. Pedley (1992, 2000) noted
that
Sparry calcite. Sparitic crystals are common com-
ponents of the tufa formed during the experiment
(Fig. 22). The crystals are elongated, they range
from several micrometres to a few millimetres in
length. Their terminations are flat, convex-up or,
rarely, steeply sided (Figs 18e & 22a, b, f ). Most
of
they
form
mainly
in
high-energy
settings.
the
crystals
grew
normal
to
the
substrate;
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