How to identify Howlite

To correctly identify Howlite (Ca2B5SiO9(OH)5), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically opaque white to off-white, sometimes grayish, with characteristic gray, black, or dark brown veining.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Sub-vitreous to earthy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often porous, sometimes chalky or porcelain-like when massive. Can feel slightly rough or granular.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Poor to indistinct on {001}.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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)

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.

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