Ameghinite is a new hydrous sodium borate found at the Tincalayu borax deposit Salta, Argentina. It occurs in small nodules embedded in borax associated with ezcurrite and rivadavite.

The new mineral is monoclinic, 2/m or m; space group C2/c or Cc; a = 18.454 Å, b = 9.895 Å, c = 6.322 ± 0.005 Å, α = 104°20′; a:b:c = 1.8650:1:0.6389; cell volume 1118.1 Å3; cell contents: 4[Na20·3B2O3·4H2O], Crystals elongated on [010] with a maximum length of 5 mm are frequently bent. Faces of eight forms were observed; those best developed are: c{100}, a{100}, p{011}, and m{110). The strongest lines in the X-ray powder photograph in Å are: 3.065 (100), 3.147 (76), 2.548 (29), 3.352 (17), 2.914 (17).

Ameghinite is colorless but under ultraviolet light shows a strong pale blue fluorescence and phosphorescence. The hardness is 212; the specific gravity 2.030 + 0.006 (meas.) 2.037 (calc.). Cleavage: {100} good, {010} and {001} poor. Optically biaxial (−), α = 1.429, β = 1.528, γ = 1.538; 2V = 33°; r < v; Z = b, X Λ c = 9°.

The chemical analysis recalculated to 100 percent: B2O3 59.60, Na2O 19.60, H2O 20.80. The mineral is soluble in cold water. DTA shows major endothermic peaks at 245° (strong) and 780°C and an exothermic peak at 635°C.

The new mineral is named in honor of Florentino and Carlos Ameghino, Argentine geologists of late 19th and early 20th centuries.

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