Download PC Minerals

MINERALS AND MINERAL DIAGENESES
The presence of minerals such as gypsum suggests a dry and warm environment with high
evaporation rate. Saline lakes, when evaporated, typically leave behind minerals such as gypsum,
borate, halite, ulexite etc. The individual minerals may be identified from their shape in hand
specimens.
The rocks on Ping Chau probably formed in a saline lake environment. Gradual evaporation of the lake
water caused minerals to precipitate out of the saline water. Two commonest minerals that formed in
the evaporite are halite and gypsum. Since halite is easily dissolved, gypsum becomes the more readily
observed minerals in the rock. In the rocks near the pier, gypsum crystals form sub-rounded lumps or
rhomb-shaped crystals on the bedding surface.
(Note: The white grains are NOT oolitic grains as described on the display sign!)
White gypsum grains in shales near pier
Minerals precipitated directly from drying up of the saline water body formed tiny rhombic crystals of
gypsum.
Diageneses
Many chemical changes continue to occur to a sedimentary rock even long after the rock is formed.
Changes in temperature, pressure and chemistry of ground water permeating the rock formation
produce new minerals. Minerals formed earlier may be replaced chemically. New minerals may also
form in the interstitial space among sediment grains. Two particular minerals frequently observed in
Ping Chau are zeolite and acmite (also known as aegerine).
Zeolite – the white minerals are zeolite formed by replacement of gypsum at low (~100oC)
temperature during post-depositional stage. In some layers, the zeolite minerals form radiating
aggregates.
Acmite – greenish colored minerals belonging to the pyroxene mineral group formed by replacement
of minerals such as zeolite at relatively high temperature (~250oC or so)
Radiating mineral
aggregates of zeolites by
the pier
Oblique view of the zeolite mineral aggregates near the pier.
Acmite crystals formed by replacement of older radiating mineral aggregates of zeolite.
Acmite crystals that have replaced older crystals of gypsum or glauberite, retaining the rhombic shape.
Siliceous siltstone layer
The siliceous siltstone layer at Lung Lok Shui was probably formed by a hot-spring event during which
silica-rich spring water converted a formerly sequence of shale and siltstone in to a resistant siliceous
unit.
If you examine the siliceous unit carefully, you can find layers and contorted beds in it, suggesting a
siltstone that has been replaced by silica.
The presence of this unit is evidence for hot-spring or hydrothermal event, which also explains the
presence of acmite which only forms in a rather high temperature.
Silicified siltstone layer formed by hydrothermal replacement of shale and siltstone.