The Arsenate and Arsenide Collection

Arsenate and Arsenide Minerals

Arsenic-bearing minerals are broadly categorized into two distinct groups based on their chemical composition and formation environments:

1. Arsenates

These minerals are structurally related to phosphates.

• Structure: Characterized by the presence of the $[AsO_4]^{3-}$ tetrahedral unit.


• Formation: Almost exclusively secondary minerals. They form near the Earth’s surface through the oxidation and weathering of primary arsenic-bearing minerals (such as arsenic sulfides).


• Appearance: Due to the variety of associated elements, arsenates are often vibrant and aesthetic, making them highly prized by mineral collectors.


• Key Examples:


• Erythrite: Known for its striking purple-red or pink color (often referred to as “Cobalt Bloom”).


• Mimetite: Exhibits a spectrum of yellow, orange, or brown hues.


• Adamite: Frequently found in shades of green, yellow, or colorless crystals.

2. Arsenides

Unlike arsenates, these minerals contain no oxygen and are formed by the direct chemical bonding of arsenic with metals.

• Structure: Based on metal-arsenic bonds. Chemically and structurally, they are similar to sulfides, where arsenic effectively replaces sulfur in the crystal lattice.


• Formation: Primary minerals typically formed in hydrothermal environments deep within the Earth’s crust.


• Appearance: Characterized by a metallic luster, high specific gravity, and often displaying silver-white, grey, or brassy colors.


• Key Examples:


• Niccolite ($NiAs$): Displays a copper to brassy-red metallic hue.


• Skutterudite ($(Co,Ni)As_3$): A primary ore for cobalt and nickel.

Note on Arsenopyrite: Arsenopyrite ($FeAsS$), the most abundant arsenic mineral, is technically classified as a sulfarsenide (containing both sulfur and arsenic). However, in many geological classifications, it is studied alongside arsenides due to its mode of occurrence and metallic nature.

Quick Comparison Table