minerals are naturally occurring crystalline chemical compounds.
rocks are bonded aggregations of minerals.
rocks may contain several different types of minerals, which have very different chemical compositions and strength properties contributing to slope failures.
physical properties of minerals
the main physical properties of minerals are the following:
1 colour
2 streak (colour in powder)
3 shape of the crystal (crystal habit)
4 size of the crystal
5 cleavage
6 lustre
7 transparency
8 hardness
9 tenacity (can be bent)
10 specific gravity
11 odour
12 taste
13 others
the colour of the mineral depends upon the presence of the colouring elements such as iron (yellow, black, red, and green), copper (green, blue), and chromium (green).
the colour of the mineral powder (streak) is generally different from the colour of the crystal. some of the minerals have very characteristic streak.
the shape of the mineral grain (crystal habit) is defined by its internal crystal structure. the crystals follow certain symmetry elements. the terms: needle-shaped means that the grains are elongated in one direction, flaky means very thin layers such as micas, whereas tabular means that the grains have the form similar to a table. the prismatic grains are like prisms or rods and the equi-dimensional grains are like a cube or a ball.
similarly, the grain size of the mineral can be grouped into the following divisions.
1 less than 0.1 mm (very fine-grained)
2 from 0.1 mm to 1 mm (fine-grained)
3 from 1 mm to 2 mm (medium-grained)
4 from 2 mm to 5 mm (coarse-grained)
5 greater than 5 mm (very coarse-grained)
cleavage is the property of the mineral grain to split into parallel planer surfaces. there could be one, two, and three sets of cleavages. the cleavage may be excellent, distinct, and weak in appearance.
hardness refers to the resistance of the minerals to scratching. some are so hard that they readily scratch glass, while others themselves are readily scratched with a fragment of glass, the point of knife, or even the fingernail. the minerals can be grouped into the following way:
1 hardness more than 5: those, which scratch the glass.
2 hardness between 5 and 3.5: those, which scratch the hammer
or knife but not the glass.
3 hardness between 3.5 and 1.5: those, which are scratched by hammer
but not by fingernail.
4 hardness less than 1.5: those, which are scratched by the fingernail.
lustre refers to the property of the mineral to reflect light. some minerals reflect much light and show metallic lustre, others have the appearance of glass and are said to have the vitreous lustre. they also may have pearly lustre, and some are dull like earth.
transparency is the property of the mineral to refract light. the minerals can be transparent, translucent, and opaque.
tenacity refers to the property of the mineral grain to bend (such as mica), and malleability is the property of the minerals to be scratched like a metal.
the specific gravity of the mineral grains also varies widely and it is widely used to identify them.
the other properties such as electrical conductivity, magnetism, radioactivity, and are other properties characteristic of some minerals.
thus, minerals, which are the constituent parts of rocks, can be easily identified if enough attention is given to their physical properties.
mineral identification table
| physical properties |
mineral 1 |
mineral 2 |
mineral 3 |
| colour |
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| streak |
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| crystal habit |
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| grain size |
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| cleavage |
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| lustre |
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| transparency |
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| hardness |
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| tenacity |
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| other properties |
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| name of the mineral |
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rock classification and weathering
principal rocks types and their weathering
minerals are naturally occurring crystalline chemical compounds.
rocks are aggregations of minerals.
the mineral constituents of a rock may have very different chemical compositions and properties.
a fresh granite sample may contain the following mineral groups:
- dark minerals;
- light minerals (milky);
- white mica (platy, translucent);
- quartz (sugary, translucent but can be milky).
rocks are affected by weathering. Weathering is defined as 'the physical and chemical alteration of rock by the action of heat, water, and air'. note that high temperature and high water content increase the rate of weathering.
a weathered rock sample will show some or all of the following features:
- softness ( i.e. minerals can be rubbed off by hand);
- discoloration;
- loosening of grains;
- intact white mica;
- intact quartz.
the relative order of susceptibility to chemical alteration in the common mineral groups is as follows:
- dark minerals l least resistant to alteration
- light minerals l
- white mica l
- quartz ▼ most resistant to alteration
in a soil, weathering has proceeded much further and the following features may be observed:
- particles are much smaller;
- clay minerals (fines) are present - these are new minerals derived
from the weathering products of rock;
- quartz and white mica remain, as they are the most resistant
ones to weathering.
the effect of weathering is to soften and weaken rock.
rock weathering grades
| weathering grade |
description |
| 1a |
fresh rock. no visible sign of weathering. |
| 1b |
faintly weathered. discoloration on major joint surfaces. |
| 2 |
slightly weathered. discoloration of all discontinuity surfaces or throughout rock. |
| 3 |
moderately weathered. up to 50% of rock material decomposed and/or disintegrated to soil. rock can be a continuous mass, or core stones. |
| 4 |
highly weathered. more than 50% of rock material decomposed or disintegrated to soil. rock mass is discontinuous. |
| 5 |
completely weathered. all rock material decomposed and/or disintegrated to soil. original mass structure still largely intact. |
| 6 |
residual soil. all rock material converted to soil. mass structure and material fabric destroyed. |
Geological Society Engineering Group Working Party, 1977. The description of rock masses for engineering geology, Engineering Geology 10 (4) pp. 355 - 388.
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