Biotite

Though biotite does not feature prominently in art or myth, its role in scientific history is notable. The mineral’s study by Jean-Baptiste Biot helped unlock the optical secrets of micas, influencing mineralogy and physics. In modern culture, biotite is sometimes referenced as "black mica" in educational contexts to contrast with muscovite, the "white mica." Its name endures as a tribute to scientific curiosity and discovery.

• Biotite is weakly magnetic due to its iron content—a rarity among common rock-forming minerals.
• It can alter to other minerals like chlorite or vermiculite through weathering.
• Biotite’s perfect cleavage allows it to split into ultra-thin, flexible sheets.
• It is a key mineral for geologists when interpreting metamorphic and igneous rock histories.

Biotite’s chemical formula is K(Mg,Fe)₃AlSi₃O₁₀(OH)₂, indicating a potassium-rich mica with significant magnesium and iron content, along with aluminum, silicon, oxygen, and hydroxyl groups.

Biotite crystallizes in the monoclinic system, forming layered sheets typical of the phyllosilicate structure. Its atomic arrangement allows for perfect basal cleavage, resulting in flexible, elastic flakes that are a hallmark of mica minerals.

Biotite is characterized by a Mohs hardness of 2.5–3, dark brown to black coloration, and a vitreous to pearly luster. It exhibits perfect basal cleavage, making it easy to split into thin sheets. The mineral is weakly magnetic and has a specific gravity that reflects its iron and magnesium content.

Biotite is widespread in igneous rocks such as granite, diorite, and gabbro, and in metamorphic rocks like schist and gneiss. It commonly coexists with other micas, feldspars, and quartz, and plays a role in the mineralogical evolution of these rocks. Biotite’s alteration products, such as chlorite and vermiculite, are also found in soils derived from weathered rocks.

• Black mica – the most common synonym, used to distinguish biotite from muscovite.
• Magnesian mica and iron mica – historical names reflecting its composition.

Biotite’s industrial uses are limited, but it holds scientific importance in petrology, serving as an indicator mineral for geological mapping and research. Its presence helps reconstruct the thermal and chemical history of rocks. Occasionally, biotite is considered as a minor source of potassium and contributes to soil fertility through weathering.

• Look for thin, dark brown to black sheets that are flexible and elastic.
• Test for perfect basal cleavage—the mineral should split easily into shiny, smooth layers.
• Biotite’s weak magnetism and association with granite or schist can help confirm its identity.

Handle biotite specimens gently to avoid breaking their thin, flexible sheets. Store in a dry environment to prevent weathering and alteration. Clean with a soft brush or cloth; avoid harsh chemicals, as biotite is sensitive to environmental changes and may degrade over time.

While biotite is abundant and does not have widely celebrated individual specimens, large, lustrous sheets from classic localities such as the Ural Mountains and Ontario’s Bancroft region are prized by collectors for their size and clarity. Notable samples are often featured in geological museums as representative mica minerals.