Bromlite is a member of the aragonite group of minerals. It consists of an isomorphous mixture of calcium and barium carbonates in various proportions, (Ca,Ba) C0₃, and thus differs chemically from barytocalcite , which is a double salt of these carbonates in equal molecular proportions. Being isomorphous with aragonite, it crystallizes in the orthorhombic system, but simple crystals are not known. The crystals are invariably complex twins, and have the form of doubly terminated pseudo-hexagonal pyramids, like those of witherite but more acute; the faces are horizontally striated and are divided down their center by a twin suture.

The examination in polarized light of a transverse section shows that each compound crystal is built up of six differently orientated individuals arranged in twelve segments.

The crystals are translucent and white, sometimes with a shade of pink. Specific gravity is 3.706, and hardness is 4 - 4.1. The mineral has been found at only two localities, both of which are in the north of England. At the Fallowfield lead mine, near Hexham in Northumberland, it is associated with witherite; and at Bromley Hill, near Alston in Cumberland, it occurs in veins with galena. The species was named bromlite by T. Thomson in 1837, and alstonite by A. Breithaupt in 1841, both of which names, derived from the locality, have been in common use.