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The Mohs Hardness Scale
A relative scale for measuring mineral hardness
by Andrew Alden
Mohs hardness scale was devised in 1812 by Friedrich Mohs and
has been the same ever since, making it the oldest standard scale in geology.
It is also perhaps the most useful single test for identifying
and describing minerals.
You use the Mohs hardness scale by testing an unknown mineral
against one of the standard minerals.
Whichever one scratches the other is harder, and if both scratch
each other they are the same hardness.
Understanding Mohs Hardness
Scale
The Mohs scale of hardness uses half-numbers, but nothing more
precise for in-between hardnesses.
For instance, dolomite, which
scratches calcite but not fluorite, has a Mohs hardness of 3½ or 3.5.
Mohs Hardness
|
Mineral Name
|
Chemical Formula
|
1
|
Mg3Si4O10(OH)2
|
|
2
|
CaSO4·2H2O
|
|
3
|
CaCO3
|
|
4
|
CaF2
|
|
5
|
Ca5(PO4)3(F,Cl,OH)
|
|
6
|
KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8
|
|
7
|
SiO2
|
|
8
|
Al2SiO4(F,OH)2
|
|
9
|
Al2O3
|
|
10
|
C
|
There are a few handy objects that also help in using this
scale.
A fingernail is 2½, a penny (actually, any current U.S. coin) is
just under 3, a knife blade is 5½, glass is 5½ and a good steel file is
6½.
Common sandpaper uses artificial corundum and is hardness 9; garnet
paper is 7½.
Many geologists just use a small kit featuring 9
standard minerals and some of the above-mentioned objects; with the
exception of diamond, all of the minerals on the scale are fairly common and
inexpensive.
If you want to avoid the rare chance of a mineral impurity
skewing your results (and don't mind spending some extra money), there are sets
of hardness picks available specifically for the Mohs scale.
The Mohs scale is an ordinal scale, meaning that it is not
proportional.
In terms of absolute hardness, diamond (Mohs hardness 10) is
actually four times harder than corundum (Mohs hardness 9) and six times harder
than topaz (Mohs hardness 8).
For a field geologist, the scale works great.
A professional mineralogist or metallurgist, however, might
obtain absolute hardness by using a sclerometer, which microscopically
measures the width of a scratch made by a diamond.
Mineral Name
|
Mohs Hardness
|
Absolute Hardness
|
Talc
|
1
|
1
|
Gypsum
|
2
|
2
|
Calcite
|
3
|
9
|
Fluorite
|
4
|
21
|
Apatite
|
5
|
48
|
Feldspar
|
6
|
72
|
Quartz
|
7
|
100
|
Topaz
|
8
|
200
|
Corundum
|
9
|
400
|
Diamond
|
10
|
1500
|
Mohs hardness is just one aspect of identifying minerals.
You also need to consider luster,
cleavage, crystalline form, color, and rock type to zero in on an exact
identification.
See this step-by-step guide to mineral
identification to learn more.
A mineral's hardness is a reflection of its molecular structure
— the spacing of the various atoms and the strength of the chemical bonds between
them.
The manufacture of Gorilla Glass used
in smartphones, which is nearly hardness 9, is a good example of how this
aspect of chemistry is related to hardness.
Hardness is also an important consideration in gemstones.
Don't rely on the Mohs scale to test rocks; it is strictly for
minerals.
The hardness of a rock depends on the exact minerals that make
it up, particularly the mineral that cements it together.
Edited by Brooks Mitchell
Brooks
Mitchell
Education
Coordinator for the Space Science Institute in Boulder, Colorado
Active
member of the Atlanta Geological Society and the National Center for
Interactive Learning (NCIL)
Assists
in managing STAR Net's STEM Activity Clearinghouse coordinating with multiple
governmental and non-governmental agencies including NASA and Project BUILD
Active
NCIL representative at American Library Association (ALA), Mountain Plain
Library Association (MPLA), and Young Adult Library Services Association
(YALSA) conferences and symposiums
Experience
Brooks
Mitchell is a former writer for ThoughtCo and has contributed articles in
geology, paleontology, and marine biology.
After
graduating from the University of Alabama with a Bachelor of Arts degree in
Geology in 2012, Mitchell has used his training to create educational
curricula, interactive exhibits, and interpretive panels for numerous
museums and scientific institutions throughout the southeast. Although he
specializes in geology, he has experience developing paleontology, marine
biology, and anthropology-based content as well.
Mitchell
is an active member of the Atlanta Geological
Society and
has been featured on "CNN Newsroom with Brooke Baldwin." Mitchell has
also served as the Programs Educator of Earth Science for the Fernbank Museum
of Natural History and the Environmental Educator for the Georgia Aquarium in
Atlanta, Georgia.
Education
Brooks
earned his Bachelor of the Arts degree in Geology (with a minor in
Anthropology) from the University of Alabama in Tuscaloosa, Alabama in
2012.
Awards
and Publications
The
primary designer for the Georgia Aquarium's "Aquanaut Adventure: A
Discovery Zone" gallery
A
geological and paleontological specialist for the Fernbank Museum's
"WildWoods" exhibit
The
featured speaker for the Toccoa-Stephens County Public Library during the 2017
Solar Eclipse
Brooks
Mitchell
ThoughtCo
and Dotdash
ThoughtCo is a premier
reference site focusing on expert-created education content. We are one of the
top-10 information sites in the world as rated by comScore, a leading Internet
measurement company. Every month, more than 13 million readers seek answers to
their questions on ThoughtCo.
For more
than 20 years, Dotdash brands have been helping people find answers,
solve problems, and get inspired. We are one of the top-20 largest content
publishers on the Internet according to comScore, and reach more than 30% of
the U.S. population monthly. Our brands collectively have won more than 20
industry awards in the last year alone, and recently Dotdash was named Publisher
of the Year by Digiday, a leading industry publication.
Andrew
Alden
Professional
geologist, writer, photographer, and geological tour guide
Thirty-seven
years of experience writing about geological subjects
Six
years as a research guide with U.S.
Geological Survey (USGS)
Experience
Andrew
Alden is a former writer for ThoughtCo who contributed hundreds of
articles for more than 17 years. Andrew works as a geologist, writer, editor,
and photographer. He has written on geological subjects since 1981 and participates
actively in his field. For example, Andrew spent six years as a research guide
with the U.S. Geological Survey, leading excursions
on both land land and at sea. And since 1992, he has hosted the earthquakes
conference for the online discussion platform, The Well, which began as a
dialogue between the writers and readers of the Whole Earth Review.
In
addition, Andrew is a longtime member of the member of the Geological Society of America — an
international society that serves members in academia, government, and
industry; and the American Geophysical
Union — a community
of earth and space scientists that advances the power of science to ensure a
sustainable future.
Andrew
lives in Oakland, California; and though he writes about the whole planet
and beyond, Andrew finds his own city full of interest too and blogs about its
geology.
Education
Andrew
Alden holds a bachelor's (B.A.) degree in Earth Science from the
University of New Hampshire, College of Engineering and Physical Sciences, in
Durham, N.H.
Awards
and Publications
Andrew
Alden on Earthquakes (The Well Group, Inc.,
2011)
Assessment
of River — Floodplain Aquifer Interactions (Environmental
and Engineering Geoscience, 1997)
Andrew
Alden on Hosting (The
Well Group, Inc., 1995)
ThoughtCo
and Dotdash
ThoughtCo is a premier
reference site focusing on expert-created education content. We are one of the
top-10 information sites in the world as rated by comScore, a leading Internet
measurement company. Every month, more than 13 million readers seek answers to
their questions on ThoughtCo.
For more
than 20 years, Dotdash brands have been helping people find answers,
solve problems, and get inspired. We are one of the top-20 largest content
publishers on the Internet according to comScore, and reach more than 30% of
the U.S. population monthly. Our brands collectively have won more than 20
industry awards in the last year alone, and recently Dotdash was named
Publisher of the Year by Digiday, a leading industry publication.
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