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Howlite is a calcium borosilicate hydroxide mineral known for its distinctive white or off-white color with characteristic gray or black veining, resembling a spiderweb or matrix. It is relatively soft and porous, making it easy to carve and dye. Its appearance often leads to its use as an imitation for more expensive gemstones.
How to Identify
- Color
- Typically opaque white to off-white, sometimes grayish, with characteristic gray, black, or dark brown veining.
- Luster
- Sub-vitreous to earthy.
- Texture
- Often porous, sometimes chalky or porcelain-like when massive. Can feel slightly rough or granular.
- Crystal Form
- Rarely forms distinct crystals; usually found as irregular nodules, massive aggregates, or cauliflower-like concretions. When crystals do occur, they are typically small and tabular.
- Cleavage
- Poor to indistinct on {001}.
- Geological Environment
- Evaporite deposits, particularly in arid lacustrine (lake) environments, often associated with borax and other borate minerals. Can be found in sedimentary rocks that have undergone hydrothermal alteration.
Key Facts
- Hardness: 3.5 on the Mohs scale
- Specific Gravity: 2.53 - 2.59
- Crystal System: Monoclinic
- Color: White, grayish-white, with gray or black matrix/veins
- Luster: Sub-vitreous, earthy
- Transparency: Opaque
- Fracture: Uneven to conchoidal
- Cleavage: Poor to indistinct on {001}
- Composition: Calcium borosilicate hydroxide (Ca2B5SiO9(OH)5)
Quick Check
- Color: White to off-white with gray/black veining
- Luster: Sub-vitreous to earthy
- Streak: White
Physical Characteristics
- Crystal Habit: Typically massive, nodular, or in cauliflower-like concretions; rarely as small, tabular monoclinic crystals.
- Cleavage Type: Poor to indistinct, observed on {001}.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Sub-vitreous to earthy.
Formation
Howlite is a borate mineral that forms in evaporite deposits, often in arid regions where boron-rich waters evaporate. It is typically found in nodules or masses within sedimentary rocks, particularly shales and clays, that have been subjected to hydrothermal alteration or diagenesis. Its formation is linked to the precipitation of boron from solutions, often in association with volcanic ash or other boron-bearing minerals. It can also occur as encrustations or in veinlets.
Usage
Primarily used as an ornamental stone, particularly in jewelry (beads, cabochons), carvings, and decorative objects. Due to its porous nature and white color, it is frequently dyed to imitate other more valuable minerals such as turquoise, lapis lazuli, or coral. It is also used in metaphysical practices.
Age Distribution
Formed in relatively young geological settings, typically associated with evaporite deposits and volcanic activity, thus not tied to specific ancient geological ages but rather to ongoing or geologically recent processes.
Where to Find
Nova Scotia, Canada
The type locality, where it was first discovered in 1868 by Henry How. Found in gypsum and anhydrite deposits.
California, USA
Significant deposits, particularly in the Kramer Borate Deposit in Boron, Kern County, where it is associated with other borate minerals like borax and ulexite.
Nevada, USA
Found in borate-rich evaporite basins.
Turkey
Known for its extensive borate deposits, including howlite.
Germany
Occurrences reported in some evaporite formations.
Finding Tips
Geological Context
Look for howlite in arid regions associated with evaporite deposits, particularly near ancient lake beds or areas with volcanic activity that introduced boron into the environment. It often occurs alongside gypsum, anhydrite, and other borate minerals.
Appearance
Search for white, opaque nodules or masses with distinctive gray or black spiderweb-like veining. The material may feel relatively light for its size due to its porosity.
Hardness Test
Howlite is relatively soft (Mohs 3.5). A simple scratch test with a copper coin (Mohs 3.0) or a steel knife (Mohs 5.5) can help differentiate it from harder minerals. It should be scratched by a knife but not easily by a fingernail.
Specific Gravity
Its relatively low specific gravity (2.53-2.59) can be a distinguishing feature compared to denser minerals.
Similar Rocks
Magnesite
MgCO3
Also known as: White Buffalo Turquoise (misnomer)
Turquoise
CuAl6(PO4)4(OH)8·4H2O
Also known as: None
Dolomite
CaMg(CO3)2
Also known as: None
Scientific Classification
- Mineral Class
- Borate mineral
- Group
- Borate
- Crystal System
- Monoclinic
- Chemical Formula
- Ca2B5SiO9(OH)5
- Composition
- Calcium borosilicate hydroxide
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