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Cement Types (Kaolin Cement (Hexagonal plates stacked end to end,…
Cement Types
Kaolin
Cement
Hexagonal plates
stacked end to end
Commonly forms from
feldspar weathering
Layered aluminosilicate
Microscopic porosity between kaolin clays
PPL - black & white speckle filling pores (some porosity btwn grains poss)
XPL - low birefrincence
Quartz
Cement
interlocks
grains together
decreases porosity
by filling porespace
euhedral overgrowth
angular crystal growths
superimposed atop remnant quartz grains
optical continuity
- goes extinct with grain it surrounds
dusty rough pitted rim
= remnant of rounded original sand grain (detrital grain)
Formation
qtz cement
Qtz dissolved during chemical compaction
pressure solution at long/suture grain contacts
Dissolved quartz diffuses away
Reprecipitates as cement nearby
Quartz sourced from surrounding rock
Illite Clay
Potassium silicate
- hydrated muscovite
Form yellow birefringent rims
around quartz grains
Fibrous (tho too fine to see in thin section)
Matrix formed during diagenesis
Cement Types
Carbonate
dolomite CaMg(CO3)2
calcite CaCO3
Silica (quartz) SiO2
Clay e.g. kaolinite / illite / clay coats
Cement
derived
often from surrounding rock
advection of material from elsewhere
Depositional setting = control
quartz = grain-grain disolution qtz - reprecipitation
calcite - local detrital grains
Dissolution & Replacement
Minerals may be altered
during dissolution to other (more stable) minerals
Calcite cement
calcite cement = alkaline
calcite dissolves quartz grains away at contacts
v erratic / wiggly edge of calcite
rainbow birefringence effect
where qtz dissolved flat
does not create porosity
Kaolin Clay
biotite mica alters to kaolin clay
brown/pleochroic/fibrous ---> layered white biref silicate
occurs along
v flat contacts
mica fans out in all directions
creates
micro-porosity
Clay Coats
Iron stained brown rims
around grain surfaces / pores
common on edge clay grains
less common in pore space
Probably illite composition
( too small to resolve in t/s)
Form after deposition & before burial
do not rim entire detrital grain
do not rim secondary pore
Cement
=
Precipitate filling space between grains
Mineral in solution in pore water
Crystallises out during burial
Prevents further compaction
closes porosity