• Chemistry: Mg5(CO3)4(OH)2 - 4H2O , Hydrated Magnesium Carbonate Hydroxide.
  • Class: Carbonates
  • Uses: Only as mineral specimens.
  • Specimens

Hydromagnesite is one of those minerals that has a name that sounds more like a chemical than a mineral. It is either named for its chemistry or for being the hydrated relative of magnesite. Other hydrated magnesium carbonates include dypingite, giorgiosite and artinite. Artinite is often associated with hydromagnesite as both are commonly found as alteration products of serpentine, brucite and other magnesium rich minerals. Both minerals can form acicular aggregates although hydromagnesite's crystals are more platy. Hydromagnesite aggregate "puffballs" are sometimes found attached to the needle-like crystals of artinite specimens.

Hydromagnesite is also found in caves as a very unusual cave formation called "bubbles". The bubbles look exactly like chewing gum bubbles and are caused by magnesium rich fluids being forced into the openning of the cave and encountering a viscous film that is pushed outward forming the bubble. The film is a plastic-like, carbonaceous liquid called "moonmilk" that when it eventually dries will crack open and reveal the hydromagnesite precipitate within.


  • Color is colorless to white.
  • Luster is silky, vitreous to earthy.
  • Transparency crystals are transparent to translucent.
  • Crystal System is monoclinic; 2/m
  • Crystal Habits include acicular, lathlike and platy crystals. Tiny spherical aggregates called "puffballs" are found attached to artinite crystals. A type of cave formation called "bubbles" which looks like white chewing gum bubbles are quite unique. Also found as concretions, encrustations and massive.
  • Hardness is 3.5
  • Specific Gravity is 2.15 - 2.25 (very light)
  • Cleavage is perfect in one direction and distinct in another.
  • Fracture is uneven.
  • Streak is white.
  • Other Characteristics: Fluorescent green in shortwave UV and bluish white in longwave UV light and crystals can be striated.
  • Associated Minerals include serpentine, artinite, aragonite, calcite, periclase, brucite, pyrrhotite, talc and olivine.
  • Notable Occurrences include Alameda County; New Idria, San Benito County; Fresno County and other localities in California; Staten Island and Long Island, New York; Jewel Cave, South Dakota and the Grand Canyon, Arizona, USA; Soghan Mine, Kerman, Iran; Eastern Pyrenees, France; Mt. Vesuvius, Italy and British Columbia, Canada.
  • Best Field Indicators are crystal habits, associations, softness and density.
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HYDROMAGNESITE specimen hym-1
$ 30.00
Dims: 2.4 x 1.2 x 0.9" (6.1 x 3.1 x 2.3 cm)
Wt: 0.7 oz. (20 g)
San Benito County, California, U.S.A.
Dozens of tiny Hydromagnesite nodules rest on the gray/white base of this hand specimen. These nodules do not tend to exceed 1 mm in diameter and are generally in excellent condition. They are too small for me to see any definite crystals with my loupe, and do not appear to be fibrous in habit. All have a dull, waxy luster and are translucent. Many rest on much larger artinite orbs, which in turn rest on the green-gray base rock.
no photo
hym-1 ($ 30.00)
San Benito County, California, U.S.A.


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