Aquamarine is often confused with quartz because both minerals are popular in jewelry. However, aquamarine is definitively not a type of quartz; it belongs to the beryl mineral family. Understanding the unique chemical structure of each mineral clarifies why they are classified into distinct geological groups.
Defining Aquamarine The Beryl Family
Aquamarine is a variety of the mineral beryl, known for its six-sided prismatic crystals. Its chemical composition is beryllium aluminum silicate (Be3Al2Si6O18). The presence of beryllium is a defining characteristic of the beryl family. Aquamarine’s distinctive color, ranging from pale blue to greenish-blue, is caused by trace amounts of ferrous iron ions (Fe2+) incorporated into the crystal structure.
The crystal system for beryl is hexagonal, meaning its atomic structure grows into prisms that often feature flat terminations. Other highly valued members of the beryl family include emerald, morganite, and heliodor. Aquamarine forms within granite pegmatites and hydrothermal veins, requiring beryllium-rich fluids. With a Mohs hardness ranging from 7.5 to 8, aquamarine is considered a durable stone resistant to scratching.
Defining Quartz The Silica Structure
Quartz is one of the most abundant minerals found within the Earth’s crust. It is composed solely of silicon dioxide, or silica, with the simple chemical formula SiO2. Quartz is classified as a tectosilicate, or framework silicate, due to its structure where silicon and oxygen atoms form a continuous three-dimensional network.
Quartz crystals generally form in the trigonal crystal system, often presenting as a six-sided prism that terminates in a pyramid shape. The mineral is separated into two main categories based on crystal size: macrocrystalline (visible crystals like amethyst and citrine) and microcrystalline (chalcedony varieties like agate and jasper). Quartz sets the standard for the number seven on the Mohs hardness scale, making it slightly softer than aquamarine.
Key Differences in Mineral Classification
The primary difference between aquamarine and quartz lies in their chemical composition and classification. Aquamarine requires beryllium and aluminum in addition to silicon and oxygen, placing it in the cyclosilicate group. Quartz, being pure silicon dioxide, is categorized as a tectosilicate, based on how the silicon-oxygen tetrahedra are arranged. This structural difference also causes a slight variation in their specific gravity.
The specific gravity of aquamarine falls between 2.65 and 2.85, while quartz has a consistent specific gravity of 2.65. Both minerals exhibit a vitreous luster, but their crystal habits differ. Quartz crystals are typically terminated by a pyramid, whereas beryl crystals, including aquamarine, are often long prisms with a flat top. Aquamarine is also harder than quartz, scoring 7.5 to 8 compared to quartz’s fixed 7.