Minerals: Quartz, Petroleum Fluid Inclusion
Locality: Madirobe, Antsalova District, Melaky, Madagascar
Dimensions: 38mm x 21mm x 19mm
Weight: 8.3g

Petroleum included quartz contains one of the more extraordinary forms of fluid inclusion: tiny sealed pockets of ancient hydrocarbons preserved inside the crystal as it grew.

Before the quartz itself formed, ancient rainforests and organic matter once captured energy from sunlight and, after being buried beneath layers of sediment, was gradually broken down over millions of years. Through this process, some of that organic material was transformed into hydrocarbons, including petroleum and natural gas.

Much later, those hydrocarbons migrated through fractures and cavities in the surrounding rock. As quartz crystallised in the presence of these fluids, tiny amounts could become enclosed within the growing crystal and sealed inside. The result is a fluid inclusion, a microscopic chamber preserving a sample of the environment that surrounded the quartz during its formation.

In Madagascar petroleum quartz these inclusions can be especially complex. Studies of the material have identified yellow to brown petroleum, gas bubbles thought to be methane, water and dark solid hydrocarbons such as asphaltite. The petroleum can also fluoresce dramatically under ultraviolet light, often glowing yellow, greenish yellow or blue depending on the material and lighting conditions.

There are three principal types of fluid inclusion:

1. One phase: a single fluid or gas occupies the cavity. In petroleum quartz this can appear as a small pocket of liquid hydrocarbon without an obvious bubble.

2. Two phase: liquid and gas occur together, commonly petroleum with a vapour bubble. Where there is enough space inside the cavity, the bubble may move when the crystal is tilted.

3. Three phase: liquid, gas and a solid particle are all enclosed within the same cavity. These are particularly fascinating, as an inclusion may contain petroleum, a mobile gas bubble and a dark solid hydrocarbon or other particle together. In exceptional examples, both the bubble and solid particle can move independently inside the sealed pocket.

These inclusions are effectively tiny preserved samples of fluids that were moving through the rock as the quartz formed. In the most sought after examples they're large enough to observe with the naked eye, with bubbles, petroleum and occasionally solid particles visibly moving inside a chamber that has remained sealed within the crystal for geological time.