GARNIERITE

Garnierite, an important nickel (NiO) ore, is not a single mineral, but a generic, “umbrella” term for a mixture of varying shades of green of more than six different hydrated phyllosilicates of Ni and Mg.

This mixture includes two or more of the following minerals: serpentine, talc, sepiolite, smectite, chlorite, and vermiculite. In addition, Ni-Mg silicates occur, with or without alumina, whose X-ray diffractometry patterns are typical of serpentine, talc, sepiolite, chlorite, vermiculite, and mixtures thereof. Determinations of the chemical compositions of garnierite vary within certain limits, and several compositions have been proposed, including rare minerals such as falcondoite, stevensite, willemseite, and others.

A synonym for garnierite is “noumeaite” or “noumeite”. Varieties include Fe-garnierite, Fe-halloysite-garnierite, and Zn-garnierite. Chrysoprase is a green variety of quartz with less than 2% Ni by weight. Some varieties of garnierite dissolve easily in water, adhere to the tongue, and have a taste. But please, never lick minerals or rocks, do not inhale dust, and do not swallow anything. Wash your hands after handling minerals and rocks.

1. Characteristics

Crystal system: No.  

Color: Intense green colors, ranging from light to dark. Darker colors have more Ni.

Habit: Massive, coloform, crustiform. 

Cleavage: No.       

Tenacity: No.        

Twinning: No.       

Fracture: No.       

Mohs Hardness: 2 – 3

Parting: No.         

Streak: White to greenish.         

Lustre: Resinous.          

Diaphaneity: Transparent to opaque.           

Density (g/cm³):  2.5 – 3

 

2. Geology and Deposits

Garnierite is formed by the lateritic alteration of ultramafic rocks such as serpentinites, dunites, and peridotites. It forms pocket and vein deposits of great economic importance. 

Hand specimen of garnierite with its typical green color.
Garnierite 01: In PPL, garnierite. Garnierite presents itself as an aggregate of very fine grains with some more developed mica lamellae. Opaque elements are present in regular numbers, always in very small grains. Fractures filled with opaque material also occur. The texture resembles that of agalmatolite, steatite ("soapstone"), and jade. Macroscopically, when polished, this garnierite exhibits a texture and color very similar to jade.
Garnierite 01: The same section as of the previous image, now in XPL. The interference colors of the flakes are between the 1st and 2nd order, but the grains are always too small to characterize them individually.
The image is of the same sample as the two previous images. Here, the opaque particles, always very small and seemingly polygonal in shape, accompany the larger mica lamellae that are arranged in a parallel fashion. PPL, 8x eyepiece, 50x objective.

5. Reflected Light Microscopy

Using the Reflected Light technique, it is possible to characterize the opaque minerals of garnierite, but it should be considered that these opaque minerals will always be very small and their identification will not be easy.

In PPL, garnierite with two different opaque minerals, pyrrhotite (yellowish) and magnetite (gray). In XPL the image is very dark, black.