Andesine

Chemical formula: Na<sub>0.7-0.5</sub>Ca<sub>0.3-0.5</sub>Al<sub>1.3-1.5</sub>Si<sub>2.7-2.5</sub>O<sub>8</sub>

Andesine is an important rock-forming mineral from the plagioclase group, an intermediate member of the isomorphic series between albite and anorthite.

## Characteristics Andesine is a sodium, calcium, and aluminum silicate, belonging to the plagioclase group within the feldspars. It is an intermediate member in the isomorphic series between sodium-rich albite and calcium-rich anorthite, containing 30-50% anorthite. It rarely forms well-developed, tabular or prismatic crystals. Most often, it occurs as granular aggregates, embedded in rock, where its identification with the naked eye is difficult. Collector specimens are usually fragments of larger crystals or granular aggregates. ## Physical Properties This mineral is characterized by a hardness of 6-6.5 on the Mohs scale, which is typical for feldspars. Its density ranges from 2.66 to 2.69 g/cm³. It exhibits perfect cleavage in two directions, intersecting at an angle close to 90 degrees (approximately 94°), which is a characteristic feature of plagioclases. The luster of andesine is vitreous, and on cleavage planes, it can be pearly. ## Colors and Varieties Andesine is most commonly white, gray, or colorless. It can also take on yellowish, greenish, or reddish-pink hues. Some specimens, particularly from Oregon, USA, exhibit aventurescence – an optical phenomenon of sparkling in light, caused by the presence of tiny copper inclusions. Red and green gem-quality andesines, often subjected to color enhancement processes, are sometimes sold under the trade name "Oregon sunstone," although they are not always andesine in the strict sense of the word. ## History and Name The mineral's name, given in 1841 by Wolfgang Sartorius von Waltershausen, comes from the Andes mountain range in South America. It occurs abundantly there in andesitic lavas, from which the name of this volcanic rock was derived. ## Uses Andesine is primarily a rock-forming mineral, a key component of intermediate igneous rocks such as andesite, diorite, and syenite. Transparent, colored varieties are sometimes used as ornamental and jewelry stones, although their application is limited. It also has significance in the ceramic industry.

Properties

Mohs hardness
6-6.5
Luster
Vitreous, pearly
Streak
White
Density
2.66-2.69
Cleavage
Perfect on {001}, good on {010}
Fracture
Uneven, conchoidal
Transparency
Transparent to translucent
Crystal system
Triclinic

Diagnostic features

## Identification Macroscopic identification of granular andesine within a rock is very difficult and often requires laboratory analysis. For better-formed crystals, a key feature is the presence of two directions of perfect cleavage, intersecting at an angle of approximately 94°. On cleavage surfaces, delicate, parallel striations (grooves) are often visible, resulting from polysynthetic twinning (so-called albite twinning). This is one of the most important features distinguishing plagioclases from potassium feldspars (e.g., orthoclase), which do not have such striations. ## Distinguishing from Similar Minerals Andesine is practically impossible to distinguish from other plagioclases (such as albite, oligoclase, labradorite) without specialized optical or chemical analyses. It is distinguished from potassium feldspars (orthoclase, microcline) by the aforementioned cleavage angle (for orthoclase, it is exactly 90°) and the presence of striations on the cleavage planes. ## Crystal Forms Andesine crystals are rare, and if they occur, they are tabular or short-prismatic. It usually forms granular, massive aggregates or porphyroblasts in metamorphic rocks. It often occurs in twinned forms, mainly according to the albite law, which manifests as the aforementioned striations.

Geological environment

## Genesis Andesine is a typical mineral of igneous rocks, both extrusive and intrusive, of intermediate character. It crystallizes from magma in a medium temperature range. It is a main component of rocks such as andesite, diorite, and syenite. It also occurs in metamorphic rocks formed under amphibolite and granulite facies conditions, for example, in gneisses and granulites. ## Mineral Associations This mineral often co-occurs with other rock-forming minerals. In igneous rocks, these are typically hornblende, biotite, pyroxenes (augite, diopside), magnetite, and ilmenite. In metamorphic rocks, it is accompanied by garnets, cordierite, hornblende, and biotite. ## Localities As a rock-forming mineral, andesine is widely distributed worldwide. Classic localities, from which its name originates, are in the Andes (e.g., Marmato volcano in Colombia). It occurs in many places in Europe, including France (Auvergne), Germany (Baden), Italy, and the Czech Republic. Significant occurrences are also found in the USA (California, Oregon, New York), Canada, and Japan.

Rarity

Common

For collectors

## Quality Criteria Collector specimens of andesine are valued primarily for well-formed, sharp, and undamaged crystals, which are rare. Transparent fragments of gem quality are also highly prized, especially those with unusual colors (red, green) or exhibiting optical phenomena such as aventurescence. The value of a specimen increases with its size, clarity, and intensity of color. Association with other contrasting minerals (e.g., dark hornblende) also enhances its appeal. ## Popular Localities Although andesine is common, specimens of collector significance come from selected localities. Well-formed crystals have been found in France and the Czech Republic. Gem-quality varieties, including those exhibiting aventurescence, primarily come from Oregon, USA, although their precise classification as andesine is sometimes debated.

Care and storage

## Cleaning Andesine specimens should be cleaned with lukewarm water and a mild soap, using a soft brush. After washing, they should be thoroughly rinsed with clean water (preferably distilled) and left to air dry completely or gently dried with a soft cloth. ## What to Avoid Avoid contact with strong acids and bases, which can damage the mineral's surface. Andesine is sensitive to sudden temperature changes, which can lead to the formation of internal cracks. Ultrasonic cleaners or steam cleaners should not be used, as they can enlarge existing fractures or damage cleavage planes. ## Storage Due to its perfect cleavage, andesine is susceptible to mechanical damage. Specimens should be stored in separate, padded boxes or on stands to prevent scratches and impacts from harder minerals. It should be protected from dust, which can accumulate in crevices.

External references

Sources

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