Howieite

Chemical formula: Na(Fe<sup>2+</sup>,Fe<sup>3+</sup>,Al,Mg)<sub>12</sub>(Si<sub>6</sub>O<sub>17</sub>)<sub>2</sub>(O,OH)<sub>10</sub>

Howieite is a rare, almost black sodium, iron, and manganese silicate, forming prismatic crystals with a vitreous luster.

## Characteristics Howieite is a complex chain silicate from the inosilicate group, belonging to rare rock-forming minerals. It forms elongated, prismatic or bladed crystals, often gathered into radial or fibrous aggregates. Its color is almost black, with subtle dark green, dark brown, or gray hues, especially visible on thin edges. This mineral is opaque, and on fresh fracture surfaces, it exhibits a strong, vitreous luster. ## Physical Properties Howieite is characterized by a hardness ranging from 5-6 on the Mohs scale, making it a relatively hard mineral. Its density is significant, approximately 3.38 g/cm³. It possesses perfect cleavage in one direction, which is visible as flat, shiny surfaces on the crystals. The luster is typically vitreous, and on cleavage surfaces, it can be close to pearly. ## Colors and Varieties The dominant color of howieite is black. Depending on impurities and crystallization conditions, it can take on shades from dark green, through dark brown, to black. No named color varieties or commercial varieties are distinguished. ## History and Name The mineral was discovered in 1964 and described in 1965 by a team of mineralogists: S. O. Agrell, M. G. Bown, and E. J. McKie. The name "howieite" was given in honor of Robert Andrew Howie (1923–2012), a British mineralogist and petrographer, co-author of the fundamental work "Rock-Forming Minerals". ## Uses Howieite has no industrial application. Its significance is limited to the scientific sphere as an indicator of specific metamorphic conditions and to collector interests due to its rarity and attractive, lustrous aggregates.

Properties

Mohs hardness
5-6
Luster
Vitreous
Streak
Grey to greenish-grey
Density
3.38
Cleavage
Perfect on {100}
Fracture
Uneven
Transparency
Opaque, Translucent in thin splinters
Crystal system
Triclinic

Diagnostic features

## Identification Howieite can be identified by its almost black color, strong vitreous luster, and characteristic elongated crystals forming radial aggregates. A key feature is its perfect cleavage in one direction, visible as flat, mirror-like surfaces. The mineral is relatively hard and dense. ## Distinguishing from Similar Minerals Howieite is sometimes confused with black tourmaline (schorl), hornblende, or aegirine. It is distinguished from schorl by its perfect cleavage (tourmaline does not have it) and often radial crystal arrangement. It differs from hornblende by a stronger luster and usually a darker, more uniform color. Aegirine forms similar prismatic crystals, but less often forms radial aggregates and has a slightly different hue (often greenish-black). ## Crystal Forms Howieite crystals are elongated, prismatic, often bladed or acicular. They usually occur as radial, stellate, or fibrous aggregates. Single, well-formed crystals are rarely found.

Geological environment

## Genesis Howieite is a characteristic mineral of metamorphic rocks that have undergone transformations under conditions of high pressure and relatively low temperature. It forms mainly in glaucophane schists (blueschist facies) and in some gneisses and quartz veins cutting these rocks. Its presence indicates a specific environment of subduction-zone metamorphism. ## Mineral Associations This mineral often co-occurs with other minerals typical of the blueschist facies, such as crossite, riebeckite, stilpnomelane, deerite, quartz, garnet (almandine-spessartine), epidote, albite, and pyroxenes. ## Localities The most important and classic locality, from which the best howieite specimens originate, is Laytonville Quarry in Mendocino County, California, USA. Other confirmed localities include the area around St. Marcel in the Aosta Valley, Italy, and some localities in Japan and the Kyushu Peninsula.

Rarity

Rare

For collectors

## Quality Criteria The most prized howieite specimens by collectors are those from Laytonville Quarry in California. An ideal specimen should consist of well-formed, lustrous, black crystals forming aesthetic, radial or stellate aggregates. Contrast with a light rock matrix (e.g., quartz) significantly enhances visual appeal. The size of aggregates and individual crystals also matters – the larger, the more valuable. ## Popular Localities By far the most desired and known source of collector specimens is Laytonville Quarry in California, USA. Specimens from this locality are considered exemplary for the species. Minerals from other places, such as Italy or Japan, are much rarer on the collector's market.

Care and storage

## Cleaning Howieite specimens should be cleaned very carefully. It is best to use a soft brush to remove dust. In case of heavier soiling, compressed air can be used or, as a last resort, the specimen can be quickly rinsed in distilled water and immediately thoroughly dried. Ultrasonic cleaners should be avoided, as they can damage crystals along cleavage planes. ## What to Avoid Howieite is sensitive to strong acids and bases. Contact with household and laboratory chemicals should be avoided. Prolonged exposure to moisture can lead to slow oxidation of iron and dulling of the surface. It should not be heated or exposed to sudden temperature changes. ## Storage Howieite specimens are best stored in stable conditions, in closed boxes or display cabinets, to protect them from dust and mechanical damage. Due to its perfect cleavage, avoid placing other minerals on it and protect it from impacts.

Sources

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