Holtite

Images

Formula: (Ta_0.6☐_0.4)Al₆BSi₃O₁₈
Valence: (Ta⁵⁺_0.6☐_0.4)Al₆BSi₃O₁₈
Nesosilicate (insular SiO₄ groups), holtite group, dumortierite supergroup, tantalum- and boron- bearing mineral
Crystal system: Orthorhombic
Specific gravity: 3.60 to 3.90 measured
Hardness: 8½
Streak: Light buff
Colour: Light buff to cream-buff, deep olive-buff, brown, russet
Luminescence: Fluoresces dull orange under short wave UV and bright yellow under long wave
Common impurities: Ti,Fe,As,Nb,Mn,Be,Mg,Ca,Li,Na,K,Rb,Cs
Environments

Pegmatites
Placer deposits

Holtite occurs in alluvial gravels derived from pegmatites.

Localities

At the type locality, Greenbushes Sn placers, Greenbushes tinfield, Bridgetown-Greenbushes Shire, Western Australia, holtite occurs as alluvial pebbles and as intergrown prisms and acicular coatings on stibiotantalite and as acicular replacements of tantalite (AM 57.1556).
Associated Minerals include stibiotantalite, tantalite, tourmaline, microlite and quartz (HOM).

At Szklary, Gmina Ząbkowice Śląskie, Ząbkowice Śląskie County, Lower Silesian Voivodeship, Poland, holtite is represented by three compositional varieties:
(1) Ta-bearing, which forms homogeneous crystals and cores within zoned crystals;
(2) Ti-bearing, found as small domains within the core;
(3) Nb-bearing, forming the rims of zoned crystals.
All three varieties show variable Sb+As content, reaching 19.18 wt.% Sb₂O₃ (0.87 Sb apfu) and 3.30 wt.% As₂O₃ (0.22 As apfu) in zoned Ta-bearing holtite, which constitutes the largest Sb+As content reported for the mineral.
Holtite and the As- and Sb- bearing dumortierite, which in places overgrows the youngest Nb-bearing zone, suggest the following crystallisation sequence:
Ta-bearing holtite → Ti-bearing holtite → Nb-bearing holtite → As- and Sb-bearing, (Ta,Nb,Ti)-poor dumortierite → an As- and Sb-dominant, (Ta,Nb,Ti)-free dumortierite-like mineral with (As+Sb) > Si.
The Szklary holtite crystallised from the melt along with other primary Ta-Nb-(Ti) minerals such as columbite-(Mn), tantalite-(Mn), stibiotantalite and stibiocolumbite as the availability of Ta decreased (MM 75.2.303-315).

At Vasin-Myl'k Mt, Voron'i Tundry, Murmansk Oblast, Russia, holtite occurs in pegmatites cutting amphibolites. Associated minerals include stibiotantalite, microlite and tantite (HOM).
Holtite from Vasin-Myl'k Mt - Image

Back to Minerals