Did you know?
The Desert rose is made of gypsum crystals in connection with sand. The chemical formula is
CaSO4 * 2H2O. The hardness is about 1.5 to 2. (on the Mohs Scale of Hardness. )
Desert roses crystallize, because of the gypsum crystals in the monoclinical crystal system. There
are sulfatic solutions which crystallize near the surface and form these aggregates with the desert
sand. Source: http://desertrose.visitoran.com/
The Desert Rose has several alternate names, including barite rose, gypsum rose, gypsum rosette,
rose rock, sand rose and selenite rose. Source: http://suite101.com/article/the-desert-rose-naturescrystal-flower-a334447
Source: http://tatjana-mihaela.hubpages.com/hub/crystal-remedies#
Also see: http://www.trekearth.com/gallery/Middle_East/Saudi_Arabia/East/Ar_Riyad/Al_Khobar
%28Dhahran/photo1144154.htm
source: http://www.albarnseries.com/two/2_1_10.jpg
The following from Wikipedia:
The Mohs scale of mineral hardness was created in 1812 by the German geologist and
mineralogist Friedrich Mohs. The Mohs scale of mineral hardness is based on the ability of one
natural sample of matter to scratch another mineral. The Mohs scale is a purely ordinal scale. For
example, corundum (9) is twice as hard as topaz (8), but diamond (10) is four times as hard as
corundum. The table below shows comparison with absolute hardness measured by a sclerometer.
The Vickers hardness test was developed in 1921 by Robert L. Smith and George E. Sandland at
Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials.The Vickers
test is often easier to use than other hardness tests since the required calculations are independent of
the size of the indenter, and the indenter can be used for all materials irrespective of hardness. The
basic principle, as with all common measures of hardness, is to observe the questioned material's
ability to resist plastic deformation from a standard source. The Vickers test can be used for all
metals and has one of the widest scales among hardness tests. The unit of hardness given by the test
is known as the Vickers Pyramid Number (HV) or Diamond Pyramid Hardness (DPH). The
hardness number can be converted into units of pascals, but should not be confused with a pressure,
which also has units of pascals. The hardness number is determined by the load over the surface
area of the indentation and not the area normal to the force, and is therefore not a pressure.
To do the calculation directly, the following equation can be used:
where F is newtons and d is millimeters and A is the surface area of the resulting indentation in
square millimeters. A can be determined by the formula
Other Hardness tests include:
Brinell scale, Knoop hardness test and Meyer hardness test.
Mohs hardness
Mineral
Chemical formula
Absolute hardness
1
Talc
Mg3Si4O10(OH)2
1
2
Gypsum
CaSO4·2H2O
3
3
Calcite
CaCO3
9
4
Fluorite
CaF2
21
5
Apatite
Ca5(PO4)3(OH–,Cl–,F–
48
)
6
Orthoclase Feldspar KAlSi3O8
7
Quartz
SiO2
100
8
Topaz
Al2SiO4(OH–,F–)2
200
9
Corundum
Al2O3
400
10
Diamond
C
1600
72
Image
Assignment
Class 3A and 4A
1. What does a lapidist (or lapidarist) do?
2. Also look at these pages:
http://www.jewelrymakingdaily.com/blogs/jewelryartistmagazine/default.aspx
http://www.gemsociety.org/info/lap20.htm
Try and find out what tools you need to do this job.
3. Can diamonds be used other than as decoration? Where and why?
4. Experiment: design and do an experiment to test if hardness is related in any way to
melting point of products that you find in your kitchen and house. Write up a short
(2 pages) report. Remember you need
Aim/goal
Hypothesis
Method
Results (tables and/or graphs)
Discussion of results and errors
Conclusion
Class 5A
1. What is the difference between electronegativity and electron affinity?
2. Briefly explain what is meant by covalent and ionic bonding.
3. What are Van der Waals forces?
http://www.chemguide.co.uk/atoms/bonding/vdw.html
4. Hydrogen bonds are found between water molecules in solid phase. Why is the
volume of an amount of water smaller than the same amount of ice?
5. What is the relation between strength of bonds and melting/boiling point?
6. What is the relation between hardness and the strength of bonds? Give examples to
support your answer.
Class 6A
1. Describe what is meant by the term „pressure“, use formulas with units to support
your answer..
2. Explain the differences between pressure as described in 1, and the hardness test
described in this assignment.
3. What is meant by „gas pressure“ and „liquid pressure“ ?
4. Describe how a diamond is formed in nature and how a man-made diamond is
different. Make sure to include hardness, shape, optical properties and value.
Class 7A
1. Discuss, in-depth, the energy conversion during the process of evaporation, on macro
as well as molecular level. Use the formation of desert rose as example and consider
all the variables that influence the size and structure of this phenomenon. (min 300
words) Should include thermodynamic formulas and constants. List all references
used.