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Fluid inclusions can be found in virtually all crystals. They range in size from less than 1 μm to a few centimeters, although inclusions larger than 1 mm are uncommon. Most contain a solution which represents a sample of the original waters from which the crystal formed, plus a gas or solid phase which may have separated during cooling.

Careful petrographic study (commonly using heating or freezing stages) can determine the composition and original temperature of the fluids involved in crystal formation. This can provide useful information on the timing and conditions of cementation or mineralization, although care must be taken to determine the exact time relations of the fluid inclusions and the host mineral.

The top photo shows two phase (fluid and liquid) inclusions from the fluorite-zinc district of Illinois (0.40 mm).

The middle photo illustrates a three phase (solid, liquid, and gas) inclusion in a quartz geode from Iowa (10 μm).

The bottom photo shows a two phase immiscible mixture of oil and water within a fluorite crystal from the same locality as the top photo. The colorless fluid is a strong brine; yellow fluid is oil; gas bubbles are methane associated with oil. All photos by Edwin Roedder.

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