Disequilibrium reaction pathways and the twin-mediated growth of tabular forsterite during contact metamorphism of quartz-bearing dolomite

Our plain-English summary of Disequilibrium reaction pathways and the twin-mediated growth of tabular forsterite during contact metamorphism of quartz-bearing dolomite, by M.D. Acosta, L.P. Baumgartner (Contributions to Mineralogy and Petrology. Beiträge Zur Mineralogie und Petrologie, 2024).

The study

• Authors: M.D. Acosta, L.P. Baumgartner
• Journal: Contributions to Mineralogy and Petrology. Beiträge Zur Mineralogie und Petrologie
• Published: 2024;179(3):19
• Read it: PubMed · Publisher · Free full text

Abstract

The forsterite zone of the Ubehebe Peak contact aureole, Death Valley, USA consists of an outer zone of tabular/jack-straw olivine and an inner zone of subequant polyhedral olivine. Subequant polyhedral forsterite crystals close to the intrusion are small and tabular forsterite crystals farther away are larger. To investigate the formation of the two morphologies, forsterite growth experiments were conducted in cold seal pressure vessels in the CaO-MgO-SiO2-CO2-H2O system. Forsterite precipitation follows a disequilibrium reaction pathway made of three reactions: [1] tabular forsterite growth from quartz and dolomite, [2] forsterite growth from tremolite dissolution, and [3] subequant polyhedral forsterite growth from tabular forsterite dissolution. Initially, quartz reacts with dolomite to simultaneously form twinned tabular forsterite and tremolite. As quartz reacts away, forsterite precipitation continues at a slower rate through tremolite dissolution. A second generation of forsterite then precipitates on top of some tabular forsterite but has different habit and tracht. Once all the tremolite reacts away, subequant polyhedral forsterite precipitation continues at an even slower rate through dissolution of tabular forsterite. The tabular morphology of jack-straw olivine is a consequence of twin-mediated unidirectional growth; the abundance of twins being due to rapid nucleation and growth at initially high reaction affinities. Twin junctions are preferential nucleation centers for steps, so faceted growth is enhanced on {100}. This phenomenon is the twin plane re-entrant effect. Subequant polyhedral forsterite in the Ubehebe Peak inner contact aureole recrystallized and ripened from tabular forsterite. In the outer contact aureole, conditions were not conducive to recrystallization and ripening so well-developed tabular forsterite persists.

© The Author(s) 2024.

In plain English

This was a geology study, not a medical one. It looked at how the mineral forsterite, a type of olivine, forms when heat from a body of magma alters quartz-bearing dolomite rock. The authors examined rock samples from the Ubehebe Peak contact aureole in Death Valley, California, collected for an earlier study. They also ran laboratory experiments that heated crushed natural dolomite, quartz and water to 650 °C at 1.7 kbar, for times from under a minute to 30 days. In the shortest run, needle-shaped (acicular) tremolite formed along with a smaller amount of small tabular forsterite. The authors interpreted the time series as showing that tremolite then dissolved to feed forsterite overgrowths. Once the tremolite was used up, tabular forsterite dissolved while compact, roughly equal-sided crystals grew. A run that started with tremolite instead of quartz produced no tabular forsterite. In the natural rocks, crystals of both shapes were nearly pure forsterite, but the compact ones held about an order of magnitude more boron. The authors concluded that the tabular shape is a consequence of crystal twinning. The study did not examine asbestos, people or health effects.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Field and thin-section study of natural rock samples combined with laboratory crystal-growth experiments (cold seal pressure vessels, time series)
• Who was studied: Rock samples of siliceous dolomite and marble from the Ubehebe Peak contact aureole, Death Valley, California, taken from a collection described by Roselle (1997). The laboratory runs used crushed natural Inyo Mts dolomite and quartz. No people were studied.
• Exposure: No human exposure was studied. The experiments heated dolomite plus quartz (or tremolite in one run) with water to 650 °C (one run at 800 °C) at 1.7 kbar, for times from under 1 minute to 30 days.
• Main results:
  • Zero-dwell experiment (heated to 650 °C and immediately cooled): large tremolite crystals with trace tabular forsterite
  • The authors interpreted the time series as showing that, once quartz was exhausted, tremolite reacted to form forsterite overgrowths on some tabular crystals
  • After the tremolite was used up, tabular forsterite dissolved and subequant polyhedral forsterite formed
  • Experiment held at peak temperature for 30 days: only euhedral twinned subequant polyhedral forsterite and no tabular forsterite
  • The run started with tremolite instead of quartz (forMor10) produced no tabular forsterite, only stubby forsterite prisms on tremolite surfaces. Most of the tremolite was unreacted after 22 h.
  • Natural forsterite compositions ranged from Fo98.8 to Fo99.1. The microprobe found no systematic differences between tabular and subequant polyhedral crystals for the elements measured.
  • Subequant polyhedral forsterite contained about an order of magnitude more boron than tabular forsterite
  • The unit cell of tabular forsterite was slightly larger than that of subequant polyhedral forsterite. Table 1 gives unit-cell volumes of 289.093(4) Å3 for tabular and 288.827(6) Å3 for subequant polyhedral forsterite.
• The authors' conclusion: The authors concluded that the tabular shape is a consequence of twinning, which is widespread in the aureole because of high supersaturation. They also concluded that reaction overstepping or supersaturation alone can create spatial patterns in crystal shape.
• Limitations the authors state:
  • Calcium results from the microprobe were affected by secondary fluorescence and were discarded
  • The authors could not tell which twin law was responsible, so they did not assign one
  • A focused TEM study is needed to confirm the proposed enrichment at twin boundaries in tabular forsterite

Topics

Medical terms in this study, explained in our Lexicon: Forsterite, Contact Metamorphism, Dolomite, Olivine, Tremolite, Habit.

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