Alstonite, also known as bromlite,[6] is a low temperature hydrothermal mineral[6] that is a rare double carbonate of calcium and barium with the formula BaCa(CO3)2, sometimes with some strontium.

The species was named Bromlite by Thomas Thomson in 1837 after the Bromley-Hill mine,[7] and alstonite by August Breithaupt of the Freiberg Mining Academy in 1841, after Alston, Cumbria, the base of operations of the mineral dealer from whom the first samples were obtained by Thomson in 1834.

Alstonite appears to have a superstructure based on paralstonite without long range order of the metal cations or the CO3 groups.

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

[4][6] At the type locality at Brownley Hill, alstonite occurs in low-temperature lead–zinc hydrothermal deposits associated with witherite, calcite, and baryte.

[5][6] The crystals are white to colourless or faintly pink acute pseudohexagonal pyramids or dipyramids up to 6 mm long.

Alstonite commonly encrusts compact crystalline white to pale pink baryte.

[9] It occurs typically in low-temperature hydrothermal lead–zinc ore deposits, as is the case at the type localities, and it has also been reported as a rare phase in carbonatites.

It occurs associated with calcite, baryte, ankerite, siderite, benstonite, galena, sphalerite, pyrite, and quartz.