GEOS 334 Sedimentology & Stratigraphy- Laboratory Exercise 2
This laboratory exercise has three parts. In Part I, you will practice a basic technique for
determining grain size (>63um) using the RoTap (sieving) method. In Part II you will create a grain
size chart. Part III is a set of questions that everyone answers as part of the laboratory report.
The goals of this series of laboratory exercises are as follows:
 Learn particle size analytical techniques for sands;
 Graph data on arithmetic and logarithmic scales, both by hand and by computer;
 Perform graphical statistical analyses using a spreadsheet;
 Interpret results;
 Develop a well- written laboratory report following the guidelines.
You will work in groups of no more than 3 people- Make sure you take the time to observe what
the other groups are doing. Since you will be writing up a short lab report and interpretation based
on other’s data so it is crucial that you understand what errors are possible, and when they are
most likely to be introduced.
The procedures for Parts I and II will be best if finished during lab time. Each group will complete a
spreadsheet using Excel. Have a group representative e-mail your group’s spreadsheet to me. I
will check the data then e-mail it back to you a couple of days later. Every student will write an
individual laboratory report based on all datasets. You are responsible for obtaining all the data
sets. Finish graphing by hand at home. Turn these in with your lab report by a week from the start
of the lab. As part of your laboratory report, determine the statistics for each of the datasets
graphically, USING A SPREADSHEET. Statistical calculations can be found on p. 41-52 of Folk
and in handout. Be sure to check your formulas. The laboratory report is due no later than Sept.
20th.
Format for the laboratory report.
Your report should be a Max of 2 pages TYPED, plus amendments (graphs).
Purpose. A short paragraph explaining why you performed the experiments (saying “because I
had to” does not count).
Methods: Materials and Equipment. A brief description of the methods and equipment used (Do
not copy this lab step by step). Describe the materials used.
Experimental Procedure. Detail any deviations from the procedure outlined in the instructions.
(Specially true if you have more greater than 2mm fragments than others).
Results. Results must include: 1) All the datasets, 2) a table of graphic statistics for all the
datasets; 3) histogram and cumulative frequency plot for each of the datasets. Describe the results
in a short paragraph basically reiterating what is already shown by the graphics. It is ESSENTIAL
that you refer to the graphs of the data- these are your primary means of presenting the data. Don’t
worry about embedding graphics in your report- it’s a waste of your time. However, you must
attach the labeled (with captions) graphs with your report and refer to them by number in your text
(use your textbook as a guide if you are not sure how to properly reference material).
Discussion. Discuss the analysis; then interpret. Use ALL the questions of Part III provided as a
basis for your discussion. For Analysis, discuss any experimental error, strengths or weaknesses
of the design, and relationship to your objective. For Interpretation, discuss your data in terms of
potential depositional environment. Think in terms of sorting, as determined by skewness, standard
deviation, etc. Justify your interpretation.
Conclusion. A short summary of what you now know.
Part I. Coarse Grain Size Analysis (Sieving)
RoTap. Sieving is appropriate to particle sizes down to 1/16 mm (63m), when electrostatic
attraction interferes.
Equipment and Materials
Mechanical sample splitter
Balance
Wire- mesh sieves
Sieve Shaker
Brush for cleaning sieves
Binocular microscope
Determination of particle- size distribution.
1. Obtain ~100 to 200 g sample. Split it mechanically to reduce it to an appropriate size (~40-70 g).
2. Weigh it to the nearest 0.01 g and record the weight on the form.
3. Assemble the CLEANED sieves plus a pan and lid.
4. Pour the sample into the top, coarse sieve. Cap it and RoTap for 10 minutes.
5. Remove nested sieves and carefully put contents of each sieve onto a sheet of white paper
6. Clean sieve screen by quickly inverting it and slamming it sharply and evenly on the paper
(hitting it at an angle will damage the mesh). NEVER force a grain through (some are supposed to
be lodged on the screen). NEVER touch the screen with anything other than the brush.
7. Weigh each fraction and record the weight. Save the fine fractions.
8. Determine the total weight retained. Compare it to the starting weight to determine sieve loss.
9. Determine weight percent (weight retained per class/ total weight) * 100.
10. Plot as a histogram by hand.
11. Determine cumulative weight percent data. Plot by hand.
12. Transfer data to a spreadsheet. Complete and email to me
13. Obtain data from the other groups. Using the instructions attached, determine graphical
statistics for all datasets using a spreadsheet.
14. Complete laboratory report by answering questions first.
Part II. Grain Size Chart
Estimating grain size is difficult until you get a feel for it. For this reason you are going to construct
a grain size chart with RoTap samples.
1. Obtain a cardboard template.
2. Label the holes as coarse silt, v. fine sand (4 phi), fine sand (3 phi) medium sand (2 phi), coarse
sand (1phi) and very coarse sand (0 phi).
3. Glue the bottom of the circles; add a thin layer of sand. Let it dry.
Fine grained particles
Because of their small size, silt and clay particles (finer than 4or 62 m diameter) cannot be
measured by sieving. Most methods developed to measure small particle diameters involve
“sedimentation” in which particle size is estimated from the rate at which particles sink in fluid
medium. This rate can be determined by measuring the weight or volume of accumulated
sediment, the decrease in fluid density, or the decrease in turbidity of the suspension.
There are three methods of analysis of silts and clays: pipette, hydrometer, and decantation
methods. The pipette method is more common and more accurate than the other two. All are
based on the settling velocity of the particles, computed on the basis of Stoke’s Law.
1. the pipette method: a small volume of suspension is obtained, evaporated and the residue is
weighed. The residue represents the range of grain sizes suspended at the given time;
2. the hydrometer method: the density of the suspension is measured, which depends on the
amount of sediments in suspension;
3. decantation method: all the grains still suspended after a given time are poured off, dried and
weighed.
For pipette analysis, all coarse material (> 4or 62.5 m diameter) must be removed from the
sample either by wet or dry sieving. If wet sieving, be sure not to exceed a total volume of 1000 ml
of suspension. The optimum amount of sediment to work with is 15 g (although it is possible to run
an analysis on a sample of 5 – 10 g). Too large a sample leads to grain interference and possible
flocculation, and too small a sample leads to very small residues and greater experimental error
during weighing. Also, any organic matter should be removed with an oxidizing agent (hydrogen
peroxide).
The pipette method is based on the idea that fine sediment is uniformly distributed throughout the
1000 ml column, and we draw off exactly 20 ml at the stated times, then the amount of mud in
each withdrawal is equal to 1/50 of the total amount of mud remaining in the column at that given
time and at that given depth (i.e., the amount of mud finer than the given diameter; all particles
coarser than the given diameter will have settled past the point of withdrawal). The first withdrawal
is made so quickly after stirring and at such a depth that particles of all sizes are present in
suspension; therefore if the initial withdrawal weight (minus the dispersant weight) is multiplied by
50, then you can obtain the weight of the entire amount of mud in the cylinder. Then, if you draw
off a sample at a settling time corresponding to a diameter of 6, and multiply it by 50, then we
know that the product represents the number of grams of mud still in suspension at this new time,
therefore the grams of mud finer than 6.
Table for Settling Times computed according to Stokes Law for temperatures near 20°C.
Diameter
Velocity
Depth
()
(mm)
(cm/s)
4
1/16
0.349
5
1/32
0.0872
6
1/64
0.0217
7
1/128
0.00545
8
1/256
0.00136
9
1/512
0.00034
10
1/1024
0.000085
11
1/2048
0.000021
Times of Settling
(cm) (hours)
(minutes)
20
0
00
10
0
01
10
0
07
10
0
31
10
2
3
7
5
43
7
22
53
5
65
25
(seconds)
20
55
41
As you can see this procedure is time and labor consuming, so we will no perform it, but we will
need to compute calculations based on Stoke’s Law.
PART III Questions:
1. What is the difference between accuracy and precision?
2. What factors in these methods might affect accuracy?
3. What factors might affect precision?
4. What do the data indicate about the energy of the system?
5. What is a likely environment of deposition for your sample? Justify it.
6. Does the composition / mineralogy of the sediment change by size fraction?
7. What assumptions are made in the RoTap method of mechanically sieving the sample?
Given Stoke’s Law:
Settling velocity assumes a spherical shape and is valid for particles 3and smaller:
Vs = [d2(1-2)g]/18
1= density of sphere (Quartz = 2.65 g/cm3)
2= density of liquid water (at 20°C, = 0.998 g/cm3)
g = gravitational constant (980.17 cm/sec2)
d = grain diameter
= viscosity of water at 20°C = 1.00 centipoises
1 centipoise = 0.01 gm/cm-sec
8. What would be the effect on the settling velocity for analyses conducted on the moon where the
gravitational constant is only 1/6 of that of earth? CALCULATE AND SHOW WORK.
9. In very cold water (~ 1°C), the viscosity is 1.75 centipoises. How would this effect settling
velocity? CALCULATE AND SHOW WORK.
10. Compare the settling velocity of quartz vs. magnetite (density = 5.2 g/cm3) sand grains in 20°C
water. Compare grains of 63m (4) CALCULATE AND SHOW WORK.