Mcallisterite is a rare magnesium borate mineral typically found as small, clear, tabular crystals in borate-rich evaporite deposits. Collectors primarily find it associated with other borate minerals in the Kramer deposit of California. It is notable for its low hardness and specific gravity, often requiring careful handling and storage to prevent dehydration.
Is this mcallisterite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch mcallisterite with a known reference. Mcallisterite sits at Mohs 2.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: colorless, yellow brown, white.
- 5Look at form & habitCrystal system: trigonal. Typical habit: tabular crystals, massive, granular.
Often confused with
Mcallisterite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Colemanite is the harder of the two (Mohs 4.5 vs. 2.5).
How to tell apart: Howlite is the harder of the two (Mohs 3.5 vs. 2.5); luster reads vitreous on Mcallisterite and subvitreous, glimmering on Howlite.
Often found alongside mcallisterite
Minerals reported to co-occur with mcallisterite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mg₂(B₆O₇(OH)₆)₂·9H₂O
- Mohs hardness
- 2.5
- Density
- 1.867 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Trigonal
- Crystal habit
- Tabular crystals, massive, granular
- Cleavage
- Good on {0001} and {0112}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- A secondary mineral derived from colemanite and priceite veins in altered basalt (Furnace Creek district, California, USA); a primary mineral in a metamorphosed borate playa deposit (Tincalayu deposit, Argentina)
Where rockhounds find mcallisterite
Classic worldwide localities
- Mott colemanite prospect, Twenty Mule Team Canyon, and at the Eagle Borax Spring, Furnace Creek district, Death Valley, Inyo Co., California, USA
- Sterling Hill, Sussex Co., New Jersey, USA
- Found about 15 km northwest of Tubutama, Sonora, Mexico
- Tincalayu borax deposit, Salar del Hombre Muerto, Salta Province, Argentina
- Qinghai-Xizang Plateau, Tibet, China, not further located
- Inder borate deposit, Kazakhstan
Field-hunting tip
Where it forms: A secondary mineral derived from colemanite and priceite veins in altered basalt (Furnace Creek district, California, USA); a primary mineral in a metamorphosed borate playa deposit (Tincalayu deposit, Argentina). If you start seeing sassolite, ginorite, gypsum in float, you are in the right ground. Typical habit: tabular crystals, massive, granular. Train your eye for that shape before scanning the outcrop.
Common questions
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Find mcallisterite on the map
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