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Sandstone
Sandstone is the indurated equivalent
of unconsolidated sand.
Sandstone
Sand includes clast with diameters of 2 mm
to 1/16 (0.0625) mm.
 Also referred to as “psammites (Greek), and
as “arenites” (latin).
 Arenites name still in use.
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Sandstone
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Sand grains constitute the framework of
sandstones, and the pore may be entirely filled,
partially or empty.
Pore filling may be a combination of:
 Finer-grained primary or secondary clastic
matrix
 Cement (typically calcite, quartz, chert, or
hematite)
 Fluids such as gas, air, oil, and groundwater.
Sandstone composition
 Sandstone composition is analyzed
using a petrographic microscope and
thin section.
 This allows for discriminating
individual mineral species and rock
fragments.
Sandstone composition

Quartz
 Sand grains can be composed of any mineral
but monocrystalline quartz grains are by far the
most abundant type of sandstone grain.
 Although monocrystalline quartz grains
typically constitute 60-70% of sandstone, some
sandstone are almost 100% quartz; other
contains none.
 This is not surprising; quartz is common of
granites, gneiss, schist that are abundant rocks
in continents. Quartz is also resistant to
weathering.
Quartz sand in Puerto Rico
Deposits in the north coast but also some
deposits around the island.
 In the north coast they are up to 99.8% pure
qtz.
 No sedimentary structure.
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Sandstone composition
A
Monocrystalline quartz typically
Constitute 60% to 70% of sandstone.
B
C
Med-sand size quartz grains sandstone: What
appears to be pressure solution contacts are actually
secondary qtz. overgrowth abutting into original pore
space.
A- under crossed nicols.
B- under cathodoluminiscence
Feldspars
Feldspars is generally less abundant than
quartz in sandstone, a 10% to 15% of
sandstone composition.
 This is because, although they are more
abundant in crystalline rocks such as
granites and gneiss, they are less resistant to
weathering.

Implications for the presence of
feldspars

High feldspar content in a sandstone caries
specific implications about source area
climate and topography. Feldspar rich sands
are produced in:
 Not extensive chemical weathering either
because climate and/or high source relief.
 Low precipitation (arid setting)
 Artic climate where precipitation occurs
as snow and ice rather than rain (limits
hydrolysis).
Topographic Relief

Even in climates that usually promotes
decomposition to clays (warm tropical
climate), feldspars can survive if
topographic relief is high because fastmoving streams will erode feldspar before it
can be decomposed.
K-feldspar (potassium) vs. plagioclase

Potassium and plagioclase feldspar differ in
abundance in sedimentary rocks. Potassium
feldspar are more prevalent because they are more
common in continental crust and more resistant to
decomposition.
Microcline (K-feldspar) shows
the characteristic grid, twining
Orthoclase (a K-feldspar)
usually untwined.
Rock (lithic) fragments
Abundance of rocks fragments varies.
 Lithic fragments provide the most specific
information about provenance.
 Granites and gneiss decompose as
individual crystals grains. For that reason it
is more common to find volcanic rocks,
phyllite, shale, and chert.

Texture of lithic
fragments in SST
B- chert
A- volcanic
C-plutonic
D-metamorphic-foliated
E- sedimentary
Grain size
Phi units
 Fixed ratio between each successive size
clast boundary, each size is twice as large:
2, 1, 1/2, 1/4, 1/8, 1/16 = 2, 1, 0.5, 0.25,
0.125, 0.0625

Phi uses a logarithmic-based unit

8, 4, 2, 1, 1/2, 1/4, 1/8, 1/16, 1/32 mm
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23, 22, 21, 20, 2-1, 2-2, 2-3, 2-4 25
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Grain size diameter in phi units = -log2 grain diam
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2mm= 21 = -log2 2 = -1 phi
Phi uses a logarithmic-based unit
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8, 4, 2, 1, 1/2, 1/4, 1/8, 1/16 mm
23, 22, 21, 20, 2-1, 2-2, 2-3, 2-4
-3, -2, -1, 0, 1,
2,
3, 4
ø
Simple Histogram
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Bar diagrams plotted on
graph paper using two
arithmetic scales, one
horizontal and the other
vertical
The height or length of the
bars plotted on the vertical
axis corresponds to the
proportion of grains in each
class (weight percent from
sieve).
Frequency curve
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It is a smooth curve
that can be fitted to the
bar diagram.
It can be
superimposed directly
onto the bar a
histogram simply by
joining the mid point
marks of each size
class bar.
Cumulative frequency
curve

It is produce by plotting
the cumulative weight
percent on the vertical
axis, beginning with the
coarsest size class.
Probability graph
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Cumulative percentage are
plotted on probability
paper. The vertical axis is
a log probability scale
A standard horizontal axis.
Normal bell-shaped
distribution paper as
straight lines when plotted
on probability paper.
Mean size = average size
Median size = the diameter that splits the distribution into
precisely equal halves.
Modal size = most frequently occurring size, that is half of all
the data are below the median, and half are above.
Variation in grain size or Sorting

Sorting express the number of significant size
classes in a population. The implied significance
of sorting is that transporting agents differ in their
ability to entrain, transport, and deposit grains of
different sizes.
Skewness

Skewness is a statistical measure of the symmetry of a
distribution.In a normal bell-shaped distribution, mean,
median, and mode coincide; the two halves of the
distribution are mirror images.
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Size-frequency curve of three
sediments having an identical
sorting but different skewness.
A- sediment is unskewes and
shows sym. Dist.
B- Positively skewed; the
coarser half of the population
shows better sorting than the
finer half, so both the median
and the mean are shifted
toward finer grain size.
C- negatively skewed finer half
is better sorted than the coarser
and the median and the mean
are shifted toward coarser grain
size.
Modern beach sand tend to have
Four segment distribution.
Several ancient beach sand
have the same four segment.
Dune sand, better sorted and with
A single large saltation population
And only minor traction and
suspension pop.
Turbidites sands are very poorly sorted
(shallow slope), with a single population
Ranging from very coarse to very fine.
Shape and roundness
Sand grain shape (form) and roundness
(angularity) are useful properties for
describing and differentiating sandstone
units.
 They are of limited value in identifying
provenance, dispersal, and depositional
mechanism (different environments will
contain grains of similar shape).

Zingg (1935)
He used a caliper to measure the long,
intermediate, and short axes of pebbles and
defined four shape categories: equant,
tabular or oblate, bladed and prolate.
 These categories are more useful for loose
sediments than for cemented rocks.
 Visual comparison using a standard
reference silhouettes is quicker and simpler.
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Shape

Sand particles shape do not identify
transport history or depositional
environment definitively. Transport agents
do not shape sand grains; rather, shape is
inherited and is controlled by composition
(mineralogy).
 Ex. biotite, slate, and phyllite = flakes
Roundness or angularity
It is a function of the curvature of the
corners of a clasts rather than overall grain
dimensions.
 Grain roundness represent abrasion history.
 It is a function of:
 Clast size
 Mineralogy - hardness and cleavage
 Transport agent - velocity and viscosity

Grain surface
The surface of sand grains display a variety
of small-scale features ranging from pits,
scratches, and ridges to polish or frosting.
 Usually require a Scanning Electron
Microscope.
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Classification of Sandstone