Physical properties of Larimar

Larimar has been tentatively identified as pectolite, but there is doubt among several mineralogists about this determination.

Because the material is of volcanic origin, and it is often a replacement, great variability exists in the composition. Undoubtedly there is some pectolite, but chemical analysis indicates minimal amounts of sodium. Some needle growth, within the blue areas, has high aluminum content, suggesting natrolite. Much of the material is calcium silicate with occasional native copper flecks. Recent finds include “scenic” pieces with red plumes (iron) on a blue background and white “clouds”. Other samples contain pieces resembling thomsonites, some talc and several micro-minerals.

Hardness varies, but most dark blue material is from 5 to 7 on the Mohs scale. The color sometimes produces reticulate patterns of blue and white and there is often a pronounced silky chatoyance. Pectolite is often splintery in a literal sense: The long needles can be hazardous to a collector’s fingers. This is not true of Larimar, where the material is much more compact and therefore cuttable. X-ray diffraction is suggestive of pectolite, but with some unusual peaks.

Some chemical analysis has been performed with scanning X-ray probe analysis (through the kindness of Steve Speck). Analysis has shown a lack of sodium in dark blue Larimar and the presence of aluminum and sodium in natrolite-like crystals growing into the Larimar.

Though it has yet to be precisely identified, there is no doubt that Larimar, whatever it is mineralogically, is a captivating and valuable material to the lapidary.

Chemical composition

Larimar with rock impurities

SiO2 Silica 40.27%
CaO Calcium oxide 26.93%
MgO Magnesium oxide 14.71%>
Fe2O3 Ferric Oxide 14.54%
Al 2 OR 3 Alumina 2.65%

The high content of iron and magnesium in this sample shows the ferromagnesian character of the rock that houses the Larimar, an unusual content in minerals of this category but usual in basic rocks such as basalt.

Chemical composition

almost pure larimar

SiO2 Silica 54.05%
CaO Calcium oxide 33.72%
Na2O Sodium oxide 8.69%
H2O Water 2.62%
Fe2O3 Ferric Oxide 1.00%
MgO Magnesium oxide 0.66%
Al 2 OR 3 Alumina 0.44%
K2O Potassium oxide 0.013%

This quantitative analysis clearly shows that the essential constituents of Larimar are silica and calcium oxide, with sodium oxide and water as secondary elements, while iron, magnesium, alumina and potassium appear only as impurities.