Sodalite is a mineral distinguished by a high sodium content. While it is most recognized for its deep, royal blue color, it also occurs in shades of gray, yellow, green, or pink. Identifying this mineral involves a series of observations and simple tests. The following methods offer a practical guide for accurate identification of Sodalite specimens.
Initial Visual Assessment
Sodalite is typically opaque, exhibiting a deep blue hue that often appears mottled or streaked with other colors. Often, the dark blue matrix is prominently veined or patched with white or grayish material, which is typically calcite inclusions.
The luster of Sodalite is described as vitreous (glassy) to somewhat greasy in appearance. While it can form rare dodecahedral crystals, Sodalite is most commonly found in a massive or granular habit, appearing as large, dense, and irregularly shaped masses. The visual presence of these coarse white veins against the deep blue is often the first distinguishing feature.
Key Physical Identification Tests
Determining the hardness of a mineral is one of the most practical field tests for identification. Sodalite has a hardness that falls between 5.5 and 6 on the Mohs scale. This range indicates that a specimen can be scratched by a steel file or glass, but it cannot be easily scratched by a common knife blade.
A second reliable test is the streak test, which involves rubbing the mineral across an unglazed porcelain plate to observe the color of its powder. Despite the dark blue body color, Sodalite produces a white or pale blue streak. This result is an important characteristic, as some similar-looking blue minerals yield a blue streak. Sodalite also displays poor or indistinct cleavage, typically exhibiting a conchoidal or uneven fracture.
Unique Optical and Chemical Signatures
One of Sodalite’s most defining features is its reaction to ultraviolet (UV) light, which provides a strong optical signature. Under longwave UV light, most Sodalite exhibits a bright orange fluorescence. This distinctive orange glow is a reliable test for confirming Sodalite, especially in darker specimens.
Sodalite also has a relatively low specific gravity, ranging from approximately 2.13 to 2.33. This means Sodalite feels lighter than many other blue minerals of a similar size, a subtle difference that can be noted with experience. Furthermore, Sodalite is chemically soluble in hydrochloric acid, where it will slowly gelatinize, a chemical reaction that can serve as a definitive identification clue.
Sodalite vs. Common Look-Alikes
Sodalite is most frequently confused with Lapis Lazuli, which shares a similar deep blue color. They are differentiated by examining the inclusions: Lapis Lazuli almost always contains visible, brassy flecks of pyrite (iron sulfide), which Sodalite lacks. The white inclusions in Sodalite are typically coarse veins of calcite, while the calcite in Lapis Lazuli is often more finely dispersed.
Sodalite can also be distinguished from other blue minerals like Azurite and Dumortierite by comparing streaks and hardnesses. Azurite produces a blue streak, unlike Sodalite’s white streak. Dumortierite is a harder mineral than Sodalite, often registering 7 or higher on the Mohs scale.