Guided Exploration: Mineral Forming Environments

GM Tour Map: Mineral Environments

 

Overview: Minerals are the building blocks of rocks, and form as rocks form. Mineral crystals are typically small, but sometimes grow to large sizes. In this section, explore minerals that form in different environments within Earth as well as at or near its surface, and see, for example, that the same mineral may form in different environments. Specimens that may be familiar to you are indicated at each stop.
 

Within Earth: When Minerals Form from Other Minerals

1. Metamorphic Environment

Conditions of high temperature, high pressure, or both can rearrange the atomic structures of minerals within rocks, transforming them into other minerals. In this environment, the minerals never become a liquid. Note minerals that formed in this way, such aspyrite (#9, 27), talc (#13), and graphite (#18).

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Hall Section

Metamorphic Environment

High temperature and/or pressure conditions within Earth’s crust alter the crystalline (atomic) structures of minerals without involving a liquid, or magma, phase. 


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Exhibit Object

Pyrite (#9)

This mineral formed in a metamorphic environment. 


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Exhibit Object

Talc (#13)

This mineral formed in a metamorphic environment.


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Exhibit Object

Graphite (#18)

This mineral formed in a metamorphic environment. 


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Exhibit Object

Pyrite (#27)

This mineral formed in a metamorphic environment.


Within Earth: When Minerals Form in Igneous Rock Deep in Earth

2. Magmatic Environment

Mineral crystals grow as magma cools and hardens into solid rock. The particu­lar minerals depend on the chemical composition of the magma, its depth, and the temperature when it crystallizes. Observe minerals that make up gabbro rock (#11, on left), rich in calcium and magnesium, which form at higher temperatures. Minerals that make up granite rock (#3, on right), rich in silicon and aluminum, form at lower temperatures.

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Hall Section

Magmatic Environment

High temperature and pressure conditions within Earth’s crust provide for minerals to develop when molten, silicate-rich liquids (magma) harden, or crystallize, upon cooling.


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Exhibit Object

Gabbro (#11)

This rock formed in a magmatic environment. 


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Exhibit Object

Granite (#3)

This rock formed in a magmatic environment. 


3. Pegmatite Environment

When magma becomes almost entirely crystalized, rare crystals can form and grow to large sizes. This happens because certain elements normally in lower abundance become con­centrated, and also because volatile components build up. The volatiles create vapor pockets that allow larger crystals to form. Take a look at quartz (#9, 10, 14 in the first case); zircon (#8 in the second case), magnetite (#15), and hematite (#33); beryl (#10-12 in the third case) and rose quartz (#17); and stunning examples of topaz (#3) and beryl (#8) in the last case of large crystals.

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Hall Section

Pegmatite Environment

Mineral materials (elements), once widely distributed throughout the original magma, sometimes become greatly concentrated in the residual liquid at a late stage in the cooling magma.


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Exhibit Object

Quartz (#9)

This typical mineral formed in a simple pegmatite environment.


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Exhibit Object

Quartz (#10)

This typical mineral formed in a simple pegmatite environment.


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Exhibit Object

Quartz (#14)

This typical mineral formed in a simple pegmatite environment.


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Exhibit Object

Zircon (#8)

This unusual mineral formed in a simple pegmatite environment.


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Exhibit Object

Magnetite (#15)

This unusual mineral formed in a simple pegmatite environment.


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Exhibit Object

Hematite (#33)

This unusual mineral formed in a simple pegmatite environment.


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Exhibit Object

Beryl (#10)

This lithium replacement mineral formed in a complex pegmatite environment.


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Exhibit Object

Beryl (#11)

This lithium replacement mineral formed in a complex pegmatite environment.


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Exhibit Object

Beryl (#12)

This lithium replacement mineral formed in a complex pegmatite environment.


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Exhibit Object

Rose Quartz (#17)

This lithium replacement mineral formed in a complex pegmatite environment.


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Exhibit Object

Topaz (#3)

This mineral illustrates the large-size crystals typical of pegmatites. 


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Exhibit Object

Beryl (#8)

This is an unusually large crystal that developed in a pegmatite environment.


Within Earth: When Metals are Deposited in Rocks

4. Hydrothermal Environment

As water with high concentrations of metals moves through rock, it can leave rich deposits. This creates veins containing met­als such as gold, copper, and zinc, both deep in Earth’s crust and closer to its surface. Hotter temperatures at greater depth result in certain types of deposits, like pyrite (#16 in hypothermal, #25 in mesothermal), while shallower, cooler temperatures (epithermal case) result in deposits of gold (#2) and silver (#21).

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Hall Section

Hydrothermal Environment

A hot, fluid residue, usually of magmatic origin, that is chiefly water contains a rich supply of metallic elements.


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Exhibit Object

Pyrite (#16)

This mineral formed in a hydrothermal environment.


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Exhibit Object

Pyrite (#25)

This mineral formed in a mesothermal environment.


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Exhibit Object

Gold (#2)

This mineral formed in an epithermal environment.


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Exhibit Object

Silver (#21)

This mineral formed in an epithermal environment.


5. Hydrothermal and Metamorphic Environment

Certain minerals result when two different environments overlap. Explore what minerals formed under these conditions near New York City. Look at different specimens such as copper (#15), pyrite (#20), and magnetite (#38).


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Exhibit Object

Copper (#15)

This mineral formed in a hydrothermal-metamorphic environment


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Exhibit Object

Pyrite (#20)

This mineral formed in a hydrothermal-metamorphic environment.


b.5.03_magnetite_38_DF.jpg

Exhibit Object

Magnetite (#38)

This mineral formed in a hydrothermal-metamorphic environment.


At or Near Earth's Surface: When Minerals Crystalize in Surface Water

6. Evaporite Environment

Lakes in warm and dry climates may contain water with a high mineral content because elements dissolved from surrounding rock formations wash into them. As the water evaporates, the concentration of dissolved elements becomes so high that they crystalize into minerals. Note halite (#1, 6) and gypsum (#13).

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Hall Section

Evaporite Environment

In warm and arid climates, water evaporating from landlocked, or playa, lakes may result in the development of various minerals.


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Exhibit Object

Halite (#1)

This alkali mineral formed in an evaporite environment.


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Exhibit Object

Halite (#6)

This alkali mineral formed in an evaporite environment.


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Exhibit Object

Sylvite (#9)

This alkali mineral formed in an evaporite environment.


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Exhibit Object

Gypsum (#13)

This alkali mineral formed in an evaporite environment.


At or Near Earth's Surface: When Minerals Form in Igneous Rocks at the Surface

7. Volcanic Environment

When lava erupts onto Earth’s surface, it typically forms mineral crystals as it cools and hardens. In addition, secondary minerals often grow in the numerous pockets and holes that form in the rock when it cools. (All of the minerals in the first volcanic case are from a basaltic rock 20 miles west of New York City.) Minerals are also often deposited around active volcanoes. For example, sulfur (#4 in the right-hand case), crystallizes from escaping sulfur dioxide and hydrogen sulfide gases. Check out hematite (#2) and calcite (#18).

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Hall Section

Volcanic Environment

When molten, silicate-rich fluids erupt onto the Earth’s surface, minerals often form in the numerous pockets and holes that develop in the cooling lava. 


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Exhibit Object

Hematite (#2)

This mineral formed in a volcanic environment.


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Exhibit Object

Sulfur (#4)

This mineral formed in a volcanic environment.


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Exhibit Object

Calcite (#18)

This mineral formed in a volcanic environment.


At or Near Earth's Surface: When Minerals Form in Flowing Water:

8. Sedimentary Environment

Water and wind weather surface rock and mineral materials into small­er pieces, which are then transported and deposited at new sites. When these sediments react chemically with water, new minerals develop. Heavier minerals like gold will sink, separate, and concentrate, forming a placer deposit. In the left-hand sedimentary case, look at gold (#3, 10) and zircon (#6, 13); in the center, sulfur (#7) and calcite (#10); and on the right, hematite (#15), pyrite (#20), and magnetite (#24).

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Hall Section

Sedimentary Environment

Dislodged by the action of water and wind, surface rock and mineral materials are transported and deposited at new sites. These sediments often change chemically in the water to allow for a variety of new minerals to develop.


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Exhibit Object

Gold (#3)

This placer mineral formed in a sedimentary environment.


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Exhibit Object

Gold (#10)

This placer mineral formed in a sedimentary environment.


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Exhibit Object

Zircon (#6)

This placer mineral formed in a sedimentary environment.


b.8.04_zircon_13_DF.jpg

Exhibit Object

Zircon (#13)

This placer mineral formed in a sedimentary environment.


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Exhibit Object

Sulfur (#7)

This non-metallic mineral formed in a sedimentary environment.


b.8.06_calcite_10_DF.jpg

Exhibit Object

Calcite (#10)

This non-metallic mineral formed in a sedimentary environment.


b.8.07_hematite_15_DF.jpg

Exhibit Object

Hematite (#15)

This metallic mineral formed in a sedimentary environment.


b.8.08_pyrite_20_DF.jpg

Exhibit Object

Pyrite (#20)

This metallic mineral formed in a sedimentary environment.


b.8.09_magnetite_24_DF.jpg

Exhibit Object

Magnetite (#24)

This metallic mineral formed in a sedimentary environment.