A fluid inclusion is a bubble of liquid and/or gas that is trapped within a crystal. As minerals often form from a liquid or aqueous medium, tiny bubbles of that liquid can become trapped within the crystal, or along healed crystal fractures. These inclusions usually range in size from 0.01 mm to 1 mm and are only visible in detail by microscopic study, however specimens of fenster or skeletal quartz may include thin sheet-like inclusions that are many millimetres in length and breadth within their lamellar voids.
Photomicrographs from Pea Ridge, MO, USA of secondary fluid inclusions in apatite (image A) and quartz (images B–H).
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Quartz is, therefore, classified structurally as a framework silicate mineral and compositionally as an oxide mineral. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar.
Quartz crystal cluster from Brazil
Sceptered quartz (as aggregates: "Elestial quartz")
Prismatic quartz with black hematite
Druse quartz