4.3: The Building Blocks of Rock
- Page ID
- 22622
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)A rock is a solid substance that is made of one or more minerals or mineraloids. As discussed elsewhere, there are three families of rock composed of minerals: igneous (rock crystallizing from molten material), sedimentary (rock composed of the products of mechanical weathering, [sand, gravel, etc.] and/or chemical weathering [minerals and mineraloids precipitated from solution]), and metamorphic (rock produced by the chemical and physical reorganization of other rock under conditions induced by elevated heat and/or pressure).
Mineral identification is the first step in understanding the formation of a rock and its history. Geologists learn to “read the rock” to understand Earth’s history at any given location where a rock is found in an outcrop. This allows geologists to understand what the environment was like at the moment the rock formed. Was there a volcano erupting or does the rock tell us that it formed deep inside a magma chamber? Was the rock formed by burial of an ancient beach? Was the rock formed by compressive forces deep within the crust as continents collided and new mountains were forming? The clues to these widely different environments of formation are “written in the rock.” The first step in understanding the rock’s history is being able to identify, characterize and quantify the minerals that compose the rock. Rocks are fascinating to a geologist because every rock has a story to tell. As we read the rock from one location to the next, it helps us piece together the fascinating story of the Earth.
As of 2018, there were over 5,000 minerals officially recognized by the International Mineralogical Association. Many of these formed under very specific chemical and geological conditions and may only occur in one location on Earth. Fortunately for geology students, only a small subset of these minerals are common and truly necessary for identifying Earth’s most common rocks. These minerals have been dubbed “the Big Ten” by the prominent American mineralogist Mickey Gunter (Dyar, M.D., Gunter, M.E. and Tasa D. Mineralogy and Optical Mineralogy: Mineralogical Society of America, Chantilly, Virginia, USA). These ten minerals, plus a handful of others, will be our focus and will recur in many discussions in this text.
“The Big Ten” minerals are: olivine, augite, hornblende, biotite, calcium-rich plagioclase (anorthite), sodium-rich plagioclase (albite), potassium-rich feldspar (commonly orthoclase), muscovite, quartz, and calcite.