Biotite
Biotite is a silicate mineral recognized among collectors for its crystal form and distribution, with several world-class Chinese localities.
About Biotite
Biotite is a silicate mineral in the mica group — biotite series and has the chemical formula K(Mg,Fe)3AlSi3O10(OH,F)2. It crystallizes in the monoclinic system and is relatively soft, requiring careful handling. Its combination of structural character and global distribution make it a recognized species in both systematic and aesthetic collections.
Identification & care
Crystals commonly develop as tabular, platy, pseudo-hexagonal, massive, as flakes and books in rocks. Its color is typically black, dark brown and dark greenish black. The luster is vitreous to pearly, splendent on cleavage faces, the streak is white to gray, and specimens are typically translucent in thin flakes. The cleavage is perfect {001} — one direction, produces flexible elastic sheets. The fracture is irregular, which aids identification.
Collector context
How it forms
Biotite forms in primary mineral in granites, granodiorites, tonalites, syenites; common in schists, gneisses, and amphibolites; pegmatites produce large 'book' crystals. It is commonly found in association with muscovite, quartz, feldspar, hornblende, magnetite.
Classic Chinese localities
**Huanggang Fe-Sn deposit** is an important Chinese source for the species.
Why collectors care
Collectors pursue Biotite for the clarity of its crystal form and, in good material, saturated color that reads instantly across a display case. A well-terminated biotite on clean matrix photographs well, identifies quickly, and anchors a cabinet piece. Top Chinese specimens over the last two decades have reset the bar for what biotite looks like at collector grade.
What affects value
Value in Biotite is assessed, in typical order of weight, against: (1) locality provenance; (2) size relative to the species norm; (3) crystal form and termination sharpness; (4) color saturation and zoning; (5) transparency and internal clarity; (6) matrix quality and aesthetic balance; (7) condition (absence of damage, chips, or repair). Cleaning quality and verified locality documentation act as multipliers across the above.
Naming history
The name Biotite has a specific etymological and historical context — see Mindat's reference entry for provenance details. We have retained naming data at the record level; published prose is paraphrased from factual fields rather than copied from source.