Howlite

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Formula: Ca₂SiB₅O₉(OH)₅
Anhydrous borate containing hydroxyl
Crystal System: Monoclinic
Specific gravity: 2.62 measured, 2.61 calculated
Hardness: 6½
Streak: White
Colour: White, colourless, brown
Luminescence: Fluorescent bright sky-blue under long wave UV (Webmin)
Solubility: Readily soluble in dilute acids. Gelatinous silica results from the reaction upon evaporation
Common impurities: Na,K
Environments

Evaporite deposits

Howlite is a secondary mineral occurring in arid environments. It is an evaporite mineral, found in borate deposits. Associated minerals include anhydrite, colemanite, gypsum, studenitsite, ulexite and veatchite (Mindat).

Localities

At the Potash Corporation of Saskatchewan mine, Penobsquis, Cardwell Parish, Kings county, New Brunswick, Canada, howlite occurs rarely, associated with danburite, boracite, hydroboracite, hilgardite and volkovskite. It forms elongate, flattened and striated crystals up to 0.3 mm and nodules to 15 mm across. Crystals are transparent and colourless with a vitreous luster, and nodules are a porcelaneous white (CM 43.1469-1487).

At the Clover Hill deposit, Salt Springs evaporite deposit, Hammond Parish, Kings county, New Brunswick, Canada, howlite is the most common mineral in association with hydroboracite and volkovskite as well as rarer (although locally common) occurrences of hilgardite, danburite, szaibelyite, colemanite and veatchite. Howlite occurs as patches and bands of white nodules up to 1.5 cm in diameter, but generally ranging from 0.5 mm to less than 1 cm (CM 30-445-448 ).

At Fischell's Brook and Flat Bay quarry, Newfoundland, Newfoundland and Labrador, Canada, the anhydrite and gypsum beds contain small quantities of ulexite and howlite. The howlite forms composite framboidal nodules about 0.5 to 2 cm in diameter, consisting of thin platy crystals less than 0.1 mm long. The normal amounts of boron in seawater are sufficient to account for the observed quantity of borates. It is possible that similar small quantities of borates are present in other anhydrite gypsum deposits but have been overlooked. If they are truly absent from marine evaporite beds elsewhere, the difference may be caused by differences in pH, solubility, rate of evaporation etc. rather than by the presence or absence of external sources of boron (CM 13.370-376).

At the type locality, Black's quarry, Windsor, Hants county, Nova Scotia, Canada, howlite occurs as nodules embedded in anhydrite or gypsum, associated with ulexite, gypsum and anhydrite (Mindat).

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