Is Selenite a Type of Quartz?

Selenite and quartz are two popular crystal varieties used for decoration, jewelry, and claimed metaphysical properties. But is selenite actually a type of quartz crystal or are they totally separate minerals?

Selenite, also known as satin spar, is a variety of gypsum. It forms in crystal masses, most often as tabular or bladed crystals with a pearly, satin-like luster. The word selenite comes from the Greek word for moon, referring to the soft white glow of the mineral.

Chemically, selenite is hydrated calcium sulfate with the formula CaSO4·2H2O. The water content in its chemical structure is what allows it to easily cleave into thin sheets. These translucent crystalline sheets are perhaps selenite’s most iconic visual characteristic.

Quartz, on the other hand, is composed of silicon and oxygen with the chemical formula SiO2. It’s the second most abundant mineral in the Earth’s continental crust, after feldspar. Quartz is found in many varieties, from the popular purple amethyst to rock crystal, citrine, smoky quartz and more.

While selenite and some quartz varieties can appear similar at first glance, especially colorless transparent quartz, the two minerals have very different chemical compositions and crystal structures. Selenite contains calcium, sulfur, oxygen, and water, while quartz only contains silicon and oxygen.

Selenite crystallizes in the monoclinic system, meaning its crystals have three unequal axis angles. Quartz crystallizes in the trigonal system, with three equal axis angles of 120 degrees. The differences in chemical makeup and crystal structure show that selenite does not belong to the quartz mineral family.

In addition to being chemically unrelated, selenite and quartz form under very different geological conditions. Selenite is an evaporite mineral that forms from the evaporation of salty lake and sea waters. Quartz can form from both igneous and metamorphic rocks and hydrothermal veins.

While not related to quartz, selenite does sometimes form interesting combinations with other minerals. Selenite crystals are often found with contrasting white veins of gypsum running through them. Selenite roses feature intersecting bladed crystals resembling rose blooms. It can also occur with contrasting brown limonite bands or clear calcite crystals.

So in summary, despite some visual similarities in crystal habit, selenite is definitively not a type of quartz. They are entirely separate mineral species with different chemical compositions and crystal structures. Selenite’s soft, satiny look and perfect cleavage makes it easy to differentiate from quartz and its glassy luster. Both are popular decorative and metaphysical crystals, but an understanding of their geological differences is key. So next time you come across a cluster of clear crystalline points, look closer – is its selenite or quartz gleaming back at you?