Guided Exploration: Mineral Forming Environments main content.

Guided Exploration: Mineral Forming 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).

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.

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.

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).

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).

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).

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).

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).