METEORITE OR METEORWRONG?

density & specific gravity

Density

Density is the term for how heavy an object is for its size. Density is usually expressed in units like grams per cubic centimeter (g/cc or g/cm3), kilograms per cubic meter, pounds per cubic inch (cubic foot or cubic yard), or pounds per gallon.

Rocks vary considerably in density, so the density of a rock is often a good identification tool and useful for distinguishing terrestrial (Earth) rocks from meteorites. Iron meteorites are very dense, 7-8 g/cm3. Most meteorites are ordinary chondrites, and ordinary chondrites have a density half as much. Most ordinary chondrites are in the range 3.0 to 3.7 g/cm3, which is denser than most terrestrial rocks. For example, limestone (2.6 g/cm3 or less), quartzite (2.7 g/cm3), and granite (2.7-2.8 g/cm3) are all common low-density rocks. Some meteorites have low densities (<3.0 g/cm3), but such meteorites are rare among meteorites. The density of basalt, one of the most common kinds of terrestrial volcanic rocks, can be as high as 3.0 g/cm3. The only types of terrestrial rocks that are more dense than meteorites are ores - oxides and sulfides of metals like iron, zinc, and lead. For example, rocks composed of hematite or magnetite (iron oxides) are often mistaken for meteorites (see concretions). Such rocks have high densities, 4.5-5g/cm3, which is greater than that of any kind of stony meteorite.

 

Relative Abundance and Densities of Meteorite Types
 
how common?
density (g/cm3)
density mean and
range based on
meteorite type
% of all meteorites*
average
minimum
(high porosity)
maximum
number of pieces
number of meteorites
STONY
95.6
         
    Chondrites
67.5
Ordinary
63.4
H
30.9
3.40
2.80
3.80
265
157
L
27.6
3.35
2.50
3.96
277
160
LL
4.7
3.21
2.38
3.49
149
39
other
0.2
Carbonaceous
2.49
         
CI
0.02
2.11
-
-
1?
1?
CM
0.72
2.12
1.79
2.40
33
11
CR
0.35
3.10
-
-
1?
1?
CO
0.38
2.95
2.79
3.09
22
8
CV
0.22
2.95
2.69
3.25
51
10
CH
0.05
3.44
1
1
CK
0.32
3.47
3.46
3.49
4
2
other
0.44
-
-
-
   
Enstatite
0.89
         
E
0.17
-
-
-
-
 
EH
0.56
3.72
3.71
3.73
8
5
EL
0.17
3.55
3.48
3.62
15
7
  Other
0.72
-
-
-
   
    Achondrites
2.71
         
aubrites
0.20
3.12
2.97
3.33
10
6
diogenites
0.42
3.26
3.11
3.44
8
3
eucrites
0.89
2.86
2.74
2.95
18
9
howardites
0.41
3.02
2.80
3.16
8
5
ureilites
0.41
3.05
2.81
3.21
7
3
Martian - shergottites
0.04
3.10
3.07
3.12
3
2
Martian - chassignites
0.004
3.32
1
1
Martian - nahklites
0.013
3.15
3.10
3.20
3
1
total Martian
0.07
lunar
0.08
2.7-3.8**
       
other
0.23
-
-
-
   
    Ungrouped & Unclassified
25.4
         
STONY-IRONS
0.52
         
    Pallasites
0.22
4.76
4.64
4.89
10
5
    Mesosiderites
0.29
4.25
4.23
4.27
8
3
IRONS
3.84
7-8
       
 
         
Density data primarily from Britt and Consolmagno (2003).  *Relative abundance data from Grady (2000). **Estimate. Feldspathic meteorites will have lower density, basaltic meteorites with have higher density. See Chemical Classification of Lunar Meteorites.

Specific Gravity

In order to measure density, it is necessary to measure the volume of a rock. That's hard to do accurately. Just as useful as density, however, is the specific gravity. Specific gravity is the ratio of the mass (weight) of a rock to the mass of the same volume of water. Water has a density of 1.0 g/cm3, so the numeric value of specific gravity for a rock is the same as that for density. Because specific gravity is a ratio, it has no unit.

Specific gravity is easier to measure than density. In order to measure specific gravity you need a balance or scale with a hook on the bottom. The technique is described in most high school physics books and most high schools (general science and physics labs) would have a single-beam or triple-beam balance that could be used for measuring specific gravity. It may be difficult to obtain an accurate measure for a small rock, e.g., <10 grams.

Bottom Line:

If you have a rock that is not metallic and it has a specific gravity greater than 4.0, it is not a meteorite.

If you have a rock that has a specific gravity in the range 3.0 to 4.0, it might be a meteorite. That's the good news. The bad news is that if you collect 1000 rocks with specific gravities in that range, they're probably all Earth rocks because some kinds Earth rocks are in the 3-4 range.

If you have a rock that has a specific gravity of less than 3.0, it is almost certainly not a meteorite. Most Earth rocks have specific gravities of less than 3.0.

References:

Britt D. T. and Consolmagno G. J. (2003) Stony meteorite porosities and densities: A review of the data through 2001. Meteoritics and Planetary Science, volume 38, number 8, pages 11611180.

Grady M. M. (2000) Catalogue of Meteorites, With special reference to those represented in the collection of the Natural History Museum, Fifth Edition, Cambridge University Press, Cambridge, 689 pages and CD-ROM.

Warren P. H. (2001) Porosities of lunar meteorites: Strength, porosity, and petrologic screening during the meteorite delivery process. Journal of Geophysical Research - Planets, volume 106, number E5, pages 10,10110,111.

Wilkison S. L., McCoy T. J., McCamant J. E., Robinson M. S., and Britt D. T. (2003) Porosity and density of ordinary chondrites: Clues to the formation of friable and porous ordinary chondrites. Meteoritics and Planetary Science, volume 38, number 10, pages 15331546.


www.catchafallingstar.com
www.catchafallingstar.com


Prepared by:

Randy L. Korotev


Department of Earth and Planetary Sciences
Washington University in St. Louis


Please don't contact me about the meteorite you think you’ve found until you read this and this.

e-mailkorotev@wustl.edu